Add m-esr52 at 52.6.0

11 files changed, 2055 insertions, 0 deletions

diff --git a/startupcache/StartupCache.cpp b/startupcache/StartupCache.cpp
new file mode 100644
index 0000000000..371f4795c0
--- /dev/null
+++ b/startupcache/StartupCache.cpp
@@ -0,0 +1,835 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "prio.h"
+#include "PLDHashTable.h"
+#include "nsXPCOMStrings.h"
+#include "mozilla/IOInterposer.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/scache/StartupCache.h"
+
+#include "nsAutoPtr.h"
+#include "nsClassHashtable.h"
+#include "nsComponentManagerUtils.h"
+#include "nsDirectoryServiceUtils.h"
+#include "nsIClassInfo.h"
+#include "nsIFile.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "nsIOutputStream.h"
+#include "nsIStartupCache.h"
+#include "nsIStorageStream.h"
+#include "nsIStreamBufferAccess.h"
+#include "nsIStringStream.h"
+#include "nsISupports.h"
+#include "nsITimer.h"
+#include "nsIZipWriter.h"
+#include "nsIZipReader.h"
+#include "nsWeakReference.h"
+#include "nsZipArchive.h"
+#include "mozilla/Omnijar.h"
+#include "prenv.h"
+#include "mozilla/Telemetry.h"
+#include "nsThreadUtils.h"
+#include "nsXULAppAPI.h"
+#include "nsIProtocolHandler.h"
+
+#ifdef IS_BIG_ENDIAN
+#define SC_ENDIAN "big"
+#else
+#define SC_ENDIAN "little"
+#endif
+
+#if PR_BYTES_PER_WORD == 4
+#define SC_WORDSIZE "4"
+#else
+#define SC_WORDSIZE "8"
+#endif
+
+namespace mozilla {
+namespace scache {
+
+MOZ_DEFINE_MALLOC_SIZE_OF(StartupCacheMallocSizeOf)
+
+NS_IMETHODIMP
+StartupCache::CollectReports(nsIHandleReportCallback* aHandleReport,
+                             nsISupports* aData, bool aAnonymize)
+{
+  MOZ_COLLECT_REPORT(
+    "explicit/startup-cache/mapping", KIND_NONHEAP, UNITS_BYTES,
+    SizeOfMapping(),
+    "Memory used to hold the mapping of the startup cache from file. "
+    "This memory is likely to be swapped out shortly after start-up.");
+
+  MOZ_COLLECT_REPORT(
+    "explicit/startup-cache/data", KIND_HEAP, UNITS_BYTES,
+    HeapSizeOfIncludingThis(StartupCacheMallocSizeOf),
+    "Memory used by the startup cache for things other than the file mapping.");
+
+  return NS_OK;
+}
+
+#define STARTUP_CACHE_NAME "startupCache." SC_WORDSIZE "." SC_ENDIAN
+
+StartupCache*
+StartupCache::GetSingleton()
+{
+  if (!gStartupCache) {
+    if (!XRE_IsParentProcess()) {
+      return nullptr;
+    }
+#ifdef MOZ_DISABLE_STARTUPCACHE
+    return nullptr;
+#else
+    StartupCache::InitSingleton();
+#endif
+  }
+
+  return StartupCache::gStartupCache;
+}
+
+void
+StartupCache::DeleteSingleton()
+{
+  StartupCache::gStartupCache = nullptr;
+}
+
+nsresult
+StartupCache::InitSingleton()
+{
+  nsresult rv;
+  StartupCache::gStartupCache = new StartupCache();
+
+  rv = StartupCache::gStartupCache->Init();
+  if (NS_FAILED(rv)) {
+    StartupCache::gStartupCache = nullptr;
+  }
+  return rv;
+}
+
+StaticRefPtr<StartupCache> StartupCache::gStartupCache;
+bool StartupCache::gShutdownInitiated;
+bool StartupCache::gIgnoreDiskCache;
+enum StartupCache::TelemetrifyAge StartupCache::gPostFlushAgeAction = StartupCache::IGNORE_AGE;
+
+NS_IMPL_ISUPPORTS(StartupCache, nsIMemoryReporter)
+
+StartupCache::StartupCache()
+  : mArchive(nullptr), mStartupWriteInitiated(false), mWriteThread(nullptr)
+{ }
+
+StartupCache::~StartupCache()
+{
+  if (mTimer) {
+    mTimer->Cancel();
+  }
+
+  // Generally, the in-memory table should be empty here,
+  // but an early shutdown means either mTimer didn't run
+  // or the write thread is still running.
+  WaitOnWriteThread();
+
+  // If we shutdown quickly timer wont have fired. Instead of writing
+  // it on the main thread and block the shutdown we simply wont update
+  // the startup cache. Always do this if the file doesn't exist since
+  // we use it part of the package step.
+  if (!mArchive) {
+    WriteToDisk();
+  }
+
+  UnregisterWeakMemoryReporter(this);
+}
+
+nsresult
+StartupCache::Init()
+{
+  // workaround for bug 653936
+  nsCOMPtr<nsIProtocolHandler> jarInitializer(do_GetService(NS_NETWORK_PROTOCOL_CONTRACTID_PREFIX "jar"));
+
+  nsresult rv;
+
+  // This allows to override the startup cache filename
+  // which is useful from xpcshell, when there is no ProfLDS directory to keep cache in.
+  char *env = PR_GetEnv("MOZ_STARTUP_CACHE");
+  if (env) {
+    rv = NS_NewLocalFile(NS_ConvertUTF8toUTF16(env), false, getter_AddRefs(mFile));
+  } else {
+    nsCOMPtr<nsIFile> file;
+    rv = NS_GetSpecialDirectory("ProfLDS",
+                                getter_AddRefs(file));
+    if (NS_FAILED(rv)) {
+      // return silently, this will fail in mochitests's xpcshell process.
+      return rv;
+    }
+
+    nsCOMPtr<nsIFile> profDir;
+    NS_GetSpecialDirectory("ProfDS", getter_AddRefs(profDir));
+    if (profDir) {
+      bool same;
+      if (NS_SUCCEEDED(profDir->Equals(file, &same)) && !same) {
+        // We no longer store the startup cache in the main profile
+        // directory, so we should cleanup the old one.
+        if (NS_SUCCEEDED(
+              profDir->AppendNative(NS_LITERAL_CSTRING("startupCache")))) {
+          profDir->Remove(true);
+        }
+      }
+    }
+
+    rv = file->AppendNative(NS_LITERAL_CSTRING("startupCache"));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    // Try to create the directory if it's not there yet
+    rv = file->Create(nsIFile::DIRECTORY_TYPE, 0777);
+    if (NS_FAILED(rv) && rv != NS_ERROR_FILE_ALREADY_EXISTS)
+      return rv;
+
+    rv = file->AppendNative(NS_LITERAL_CSTRING(STARTUP_CACHE_NAME));
+
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    mFile = do_QueryInterface(file);
+  }
+
+  NS_ENSURE_TRUE(mFile, NS_ERROR_UNEXPECTED);
+
+  mObserverService = do_GetService("@mozilla.org/observer-service;1");
+
+  if (!mObserverService) {
+    NS_WARNING("Could not get observerService.");
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  mListener = new StartupCacheListener();
+  rv = mObserverService->AddObserver(mListener, NS_XPCOM_SHUTDOWN_OBSERVER_ID,
+                                     false);
+  NS_ENSURE_SUCCESS(rv, rv);
+  rv = mObserverService->AddObserver(mListener, "startupcache-invalidate",
+                                     false);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = LoadArchive(RECORD_AGE);
+
+  // Sometimes we don't have a cache yet, that's ok.
+  // If it's corrupted, just remove it and start over.
+  if (gIgnoreDiskCache || (NS_FAILED(rv) && rv != NS_ERROR_FILE_NOT_FOUND)) {
+    NS_WARNING("Failed to load startupcache file correctly, removing!");
+    InvalidateCache();
+  }
+
+  RegisterWeakMemoryReporter(this);
+
+  return NS_OK;
+}
+
+/**
+ * LoadArchive can be called from the main thread or while reloading cache on write thread.
+ */
+nsresult
+StartupCache::LoadArchive(enum TelemetrifyAge flag)
+{
+  if (gIgnoreDiskCache)
+    return NS_ERROR_FAILURE;
+
+  bool exists;
+  mArchive = nullptr;
+  nsresult rv = mFile->Exists(&exists);
+  if (NS_FAILED(rv) || !exists)
+    return NS_ERROR_FILE_NOT_FOUND;
+  
+  mArchive = new nsZipArchive();
+  rv = mArchive->OpenArchive(mFile);
+  if (NS_FAILED(rv) || flag == IGNORE_AGE)
+    return rv;
+
+  nsCString comment;
+  if (!mArchive->GetComment(comment)) {
+    return rv;
+  }
+
+  const char *data;
+  size_t len = NS_CStringGetData(comment, &data);
+  PRTime creationStamp;
+  // We might not have a comment if the startup cache file was created
+  // before we started recording creation times in the comment.
+  if (len == sizeof(creationStamp)) {
+    memcpy(&creationStamp, data, len);
+    PRTime current = PR_Now();
+    int64_t diff = current - creationStamp;
+
+    // We can't use AccumulateTimeDelta here because we have no way of
+    // reifying a TimeStamp from creationStamp.
+    int64_t usec_per_hour = PR_USEC_PER_SEC * int64_t(3600);
+    int64_t hour_diff = (diff + usec_per_hour - 1) / usec_per_hour;
+    mozilla::Telemetry::Accumulate(Telemetry::STARTUP_CACHE_AGE_HOURS,
+                                   hour_diff);
+  }
+
+  return rv;
+}
+
+namespace {
+
+nsresult
+GetBufferFromZipArchive(nsZipArchive *zip, bool doCRC, const char* id,
+                        UniquePtr<char[]>* outbuf, uint32_t* length)
+{
+  if (!zip)
+    return NS_ERROR_NOT_AVAILABLE;
+
+  nsZipItemPtr<char> zipItem(zip, id, doCRC);
+  if (!zipItem)
+    return NS_ERROR_NOT_AVAILABLE;
+
+  *outbuf = zipItem.Forget();
+  *length = zipItem.Length();
+  return NS_OK;
+}
+
+} /* anonymous namespace */
+
+// NOTE: this will not find a new entry until it has been written to disk!
+// Consumer should take ownership of the resulting buffer.
+nsresult
+StartupCache::GetBuffer(const char* id, UniquePtr<char[]>* outbuf, uint32_t* length) 
+{
+  PROFILER_LABEL_FUNC(js::ProfileEntry::Category::OTHER);
+
+  NS_ASSERTION(NS_IsMainThread(), "Startup cache only available on main thread");
+
+  WaitOnWriteThread();
+  if (!mStartupWriteInitiated) {
+    CacheEntry* entry; 
+    nsDependentCString idStr(id);
+    mTable.Get(idStr, &entry);
+    if (entry) {
+      *outbuf = MakeUnique<char[]>(entry->size);
+      memcpy(outbuf->get(), entry->data.get(), entry->size);
+      *length = entry->size;
+      return NS_OK;
+    }
+  }
+
+  nsresult rv = GetBufferFromZipArchive(mArchive, true, id, outbuf, length);
+  if (NS_SUCCEEDED(rv))
+    return rv;
+
+  RefPtr<nsZipArchive> omnijar = mozilla::Omnijar::GetReader(mozilla::Omnijar::APP);
+  // no need to checksum omnijarred entries
+  rv = GetBufferFromZipArchive(omnijar, false, id, outbuf, length);
+  if (NS_SUCCEEDED(rv))
+    return rv;
+
+  omnijar = mozilla::Omnijar::GetReader(mozilla::Omnijar::GRE);
+  // no need to checksum omnijarred entries
+  return GetBufferFromZipArchive(omnijar, false, id, outbuf, length);
+}
+
+// Makes a copy of the buffer, client retains ownership of inbuf.
+nsresult
+StartupCache::PutBuffer(const char* id, const char* inbuf, uint32_t len) 
+{
+  NS_ASSERTION(NS_IsMainThread(), "Startup cache only available on main thread");
+  WaitOnWriteThread();
+  if (StartupCache::gShutdownInitiated) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  auto data = MakeUnique<char[]>(len);
+  memcpy(data.get(), inbuf, len);
+
+  nsCString idStr(id);
+  // Cache it for now, we'll write all together later.
+  CacheEntry* entry; 
+  
+  if (mTable.Get(idStr)) {
+    NS_WARNING("Existing entry in StartupCache.");
+    // Double-caching is undesirable but not an error.
+    return NS_OK;
+  }
+
+#ifdef DEBUG
+  if (mArchive) {
+    nsZipItem* zipItem = mArchive->GetItem(id);
+    NS_ASSERTION(zipItem == nullptr, "Existing entry in disk StartupCache.");
+  }
+#endif
+
+  entry = new CacheEntry(Move(data), len);
+  mTable.Put(idStr, entry);
+  mPendingWrites.AppendElement(idStr);
+  return ResetStartupWriteTimer();
+}
+
+size_t
+StartupCache::SizeOfMapping()
+{
+    return mArchive ? mArchive->SizeOfMapping() : 0;
+}
+
+size_t
+StartupCache::HeapSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+{
+    // This function could measure more members, but they haven't been found by
+    // DMD to be significant.  They can be added later if necessary.
+
+    size_t n = aMallocSizeOf(this);
+
+    n += mTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
+    for (auto iter = mTable.ConstIter(); !iter.Done(); iter.Next()) {
+        n += iter.Data()->SizeOfIncludingThis(aMallocSizeOf);
+    }
+
+    n += mPendingWrites.ShallowSizeOfExcludingThis(aMallocSizeOf);
+
+    return n;
+}
+
+struct CacheWriteHolder
+{
+  nsCOMPtr<nsIZipWriter> writer;
+  nsCOMPtr<nsIStringInputStream> stream;
+  PRTime time;
+};
+
+static void
+CacheCloseHelper(const nsACString& key, const CacheEntry* data,
+                 const CacheWriteHolder* holder)
+{
+  MOZ_ASSERT(data); // assert key was found in mTable.
+
+  nsresult rv;
+  nsIStringInputStream* stream = holder->stream;
+  nsIZipWriter* writer = holder->writer;
+
+  stream->ShareData(data->data.get(), data->size);
+
+#ifdef DEBUG
+  bool hasEntry;
+  rv = writer->HasEntry(key, &hasEntry);
+  NS_ASSERTION(NS_SUCCEEDED(rv) && hasEntry == false, 
+               "Existing entry in disk StartupCache.");
+#endif
+  rv = writer->AddEntryStream(key, holder->time, true, stream, false);
+
+  if (NS_FAILED(rv)) {
+    NS_WARNING("cache entry deleted but not written to disk.");
+  }
+}
+
+
+/** 
+ * WriteToDisk writes the cache out to disk. Callers of WriteToDisk need to call WaitOnWriteThread
+ * to make sure there isn't a write happening on another thread
+ */
+void
+StartupCache::WriteToDisk() 
+{
+  nsresult rv;
+  mStartupWriteInitiated = true;
+
+  if (mTable.Count() == 0)
+    return;
+
+  nsCOMPtr<nsIZipWriter> zipW = do_CreateInstance("@mozilla.org/zipwriter;1");
+  if (!zipW)
+    return;
+
+  rv = zipW->Open(mFile, PR_RDWR | PR_CREATE_FILE);
+  if (NS_FAILED(rv)) {
+    NS_WARNING("could not open zipfile for write");
+    return;
+  } 
+
+  // If we didn't have an mArchive member, that means that we failed to
+  // open the startup cache for reading.  Therefore, we need to record
+  // the time of creation in a zipfile comment; this will be useful for
+  // Telemetry statistics.
+  PRTime now = PR_Now();
+  if (!mArchive) {
+    nsCString comment;
+    comment.Assign((char *)&now, sizeof(now));
+    zipW->SetComment(comment);
+  }
+
+  nsCOMPtr<nsIStringInputStream> stream 
+    = do_CreateInstance("@mozilla.org/io/string-input-stream;1", &rv);
+  if (NS_FAILED(rv)) {
+    NS_WARNING("Couldn't create string input stream.");
+    return;
+  }
+
+  CacheWriteHolder holder;
+  holder.stream = stream;
+  holder.writer = zipW;
+  holder.time = now;
+
+  for (auto key = mPendingWrites.begin(); key != mPendingWrites.end(); key++) {
+    CacheCloseHelper(*key, mTable.Get(*key), &holder);
+  }
+  mPendingWrites.Clear();
+  mTable.Clear();
+
+  // Close the archive so Windows doesn't choke.
+  mArchive = nullptr;
+  zipW->Close();
+
+  // We succesfully wrote the archive to disk; mark the disk file as trusted
+  gIgnoreDiskCache = false;
+
+  // Our reader's view of the archive is outdated now, reload it.
+  LoadArchive(gPostFlushAgeAction);
+  
+  return;
+}
+
+void
+StartupCache::InvalidateCache() 
+{
+  WaitOnWriteThread();
+  mPendingWrites.Clear();
+  mTable.Clear();
+  mArchive = nullptr;
+  nsresult rv = mFile->Remove(false);
+  if (NS_FAILED(rv) && rv != NS_ERROR_FILE_TARGET_DOES_NOT_EXIST &&
+      rv != NS_ERROR_FILE_NOT_FOUND) {
+    gIgnoreDiskCache = true;
+    mozilla::Telemetry::Accumulate(Telemetry::STARTUP_CACHE_INVALID, true);
+    return;
+  }
+  gIgnoreDiskCache = false;
+  LoadArchive(gPostFlushAgeAction);
+}
+
+void
+StartupCache::IgnoreDiskCache()
+{
+  gIgnoreDiskCache = true;
+  if (gStartupCache)
+    gStartupCache->InvalidateCache();
+}
+
+/*
+ * WaitOnWriteThread() is called from a main thread to wait for the worker
+ * thread to finish. However since the same code is used in the worker thread and
+ * main thread, the worker thread can also call WaitOnWriteThread() which is a no-op.
+ */
+void
+StartupCache::WaitOnWriteThread()
+{
+  NS_ASSERTION(NS_IsMainThread(), "Startup cache should only wait for io thread on main thread");
+  if (!mWriteThread || mWriteThread == PR_GetCurrentThread())
+    return;
+
+  PR_JoinThread(mWriteThread);
+  mWriteThread = nullptr;
+}
+
+void
+StartupCache::ThreadedWrite(void *aClosure)
+{
+  PR_SetCurrentThreadName("StartupCache");
+  mozilla::IOInterposer::RegisterCurrentThread();
+  /*
+   * It is safe to use the pointer passed in aClosure to reference the
+   * StartupCache object because the thread's lifetime is tightly coupled to
+   * the lifetime of the StartupCache object; this thread is joined in the
+   * StartupCache destructor, guaranteeing that this function runs if and only
+   * if the StartupCache object is valid.
+   */
+  StartupCache* startupCacheObj = static_cast<StartupCache*>(aClosure);
+  startupCacheObj->WriteToDisk();
+  mozilla::IOInterposer::UnregisterCurrentThread();
+}
+
+/*
+ * The write-thread is spawned on a timeout(which is reset with every write). This
+ * can avoid a slow shutdown. After writing out the cache, the zipreader is
+ * reloaded on the worker thread.
+ */
+void
+StartupCache::WriteTimeout(nsITimer *aTimer, void *aClosure)
+{
+  /*
+   * It is safe to use the pointer passed in aClosure to reference the
+   * StartupCache object because the timer's lifetime is tightly coupled to
+   * the lifetime of the StartupCache object; this timer is canceled in the
+   * StartupCache destructor, guaranteeing that this function runs if and only
+   * if the StartupCache object is valid.
+   */
+  StartupCache* startupCacheObj = static_cast<StartupCache*>(aClosure);
+  startupCacheObj->mWriteThread = PR_CreateThread(PR_USER_THREAD,
+                                                  StartupCache::ThreadedWrite,
+                                                  startupCacheObj,
+                                                  PR_PRIORITY_NORMAL,
+                                                  PR_GLOBAL_THREAD,
+                                                  PR_JOINABLE_THREAD,
+                                                  0);
+}
+
+// We don't want to refcount StartupCache, so we'll just
+// hold a ref to this and pass it to observerService instead.
+NS_IMPL_ISUPPORTS(StartupCacheListener, nsIObserver)
+
+nsresult
+StartupCacheListener::Observe(nsISupports *subject, const char* topic, const char16_t* data)
+{
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc)
+    return NS_OK;
+
+  if (strcmp(topic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0) {
+    // Do not leave the thread running past xpcom shutdown
+    sc->WaitOnWriteThread();
+    StartupCache::gShutdownInitiated = true;
+  } else if (strcmp(topic, "startupcache-invalidate") == 0) {
+    sc->InvalidateCache();
+  }
+  return NS_OK;
+} 
+
+nsresult
+StartupCache::GetDebugObjectOutputStream(nsIObjectOutputStream* aStream,
+                                         nsIObjectOutputStream** aOutStream) 
+{
+  NS_ENSURE_ARG_POINTER(aStream);
+#ifdef DEBUG
+  StartupCacheDebugOutputStream* stream
+    = new StartupCacheDebugOutputStream(aStream, &mWriteObjectMap);
+  NS_ADDREF(*aOutStream = stream);
+#else
+  NS_ADDREF(*aOutStream = aStream);
+#endif
+  
+  return NS_OK;
+}
+
+nsresult
+StartupCache::ResetStartupWriteTimer()
+{
+  mStartupWriteInitiated = false;
+  nsresult rv;
+  if (!mTimer)
+    mTimer = do_CreateInstance("@mozilla.org/timer;1", &rv);
+  else
+    rv = mTimer->Cancel();
+  NS_ENSURE_SUCCESS(rv, rv);
+  // Wait for 10 seconds, then write out the cache.
+  mTimer->InitWithFuncCallback(StartupCache::WriteTimeout, this, 60000,
+                               nsITimer::TYPE_ONE_SHOT);
+  return NS_OK;
+}
+
+nsresult
+StartupCache::RecordAgesAlways()
+{
+  gPostFlushAgeAction = RECORD_AGE;
+  return NS_OK;
+}
+
+// StartupCacheDebugOutputStream implementation
+#ifdef DEBUG
+NS_IMPL_ISUPPORTS(StartupCacheDebugOutputStream, nsIObjectOutputStream, 
+                  nsIBinaryOutputStream, nsIOutputStream)
+
+bool
+StartupCacheDebugOutputStream::CheckReferences(nsISupports* aObject)
+{
+  nsresult rv;
+  
+  nsCOMPtr<nsIClassInfo> classInfo = do_QueryInterface(aObject);
+  if (!classInfo) {
+    NS_ERROR("aObject must implement nsIClassInfo");
+    return false;
+  }
+  
+  uint32_t flags;
+  rv = classInfo->GetFlags(&flags);
+  NS_ENSURE_SUCCESS(rv, false);
+  if (flags & nsIClassInfo::SINGLETON)
+    return true;
+  
+  nsISupportsHashKey* key = mObjectMap->GetEntry(aObject);
+  if (key) {
+    NS_ERROR("non-singleton aObject is referenced multiple times in this" 
+                  "serialization, we don't support that.");
+    return false;
+  }
+
+  mObjectMap->PutEntry(aObject);
+  return true;
+}
+
+// nsIObjectOutputStream implementation
+nsresult
+StartupCacheDebugOutputStream::WriteObject(nsISupports* aObject, bool aIsStrongRef)
+{
+  nsCOMPtr<nsISupports> rootObject(do_QueryInterface(aObject));
+  
+  NS_ASSERTION(rootObject.get() == aObject,
+               "bad call to WriteObject -- call WriteCompoundObject!");
+  bool check = CheckReferences(aObject);
+  NS_ENSURE_TRUE(check, NS_ERROR_FAILURE);
+  return mBinaryStream->WriteObject(aObject, aIsStrongRef);
+}
+
+nsresult
+StartupCacheDebugOutputStream::WriteSingleRefObject(nsISupports* aObject)
+{
+  nsCOMPtr<nsISupports> rootObject(do_QueryInterface(aObject));
+  
+  NS_ASSERTION(rootObject.get() == aObject,
+               "bad call to WriteSingleRefObject -- call WriteCompoundObject!");
+  bool check = CheckReferences(aObject);
+  NS_ENSURE_TRUE(check, NS_ERROR_FAILURE);
+  return mBinaryStream->WriteSingleRefObject(aObject);
+}
+
+nsresult
+StartupCacheDebugOutputStream::WriteCompoundObject(nsISupports* aObject,
+                                                const nsIID& aIID,
+                                                bool aIsStrongRef)
+{
+  nsCOMPtr<nsISupports> rootObject(do_QueryInterface(aObject));
+  
+  nsCOMPtr<nsISupports> roundtrip;
+  rootObject->QueryInterface(aIID, getter_AddRefs(roundtrip));
+  NS_ASSERTION(roundtrip.get() == aObject,
+               "bad aggregation or multiple inheritance detected by call to "
+               "WriteCompoundObject!");
+
+  bool check = CheckReferences(aObject);
+  NS_ENSURE_TRUE(check, NS_ERROR_FAILURE);
+  return mBinaryStream->WriteCompoundObject(aObject, aIID, aIsStrongRef);
+}
+
+nsresult
+StartupCacheDebugOutputStream::WriteID(nsID const& aID) 
+{
+  return mBinaryStream->WriteID(aID);
+}
+
+char*
+StartupCacheDebugOutputStream::GetBuffer(uint32_t aLength, uint32_t aAlignMask)
+{
+  return mBinaryStream->GetBuffer(aLength, aAlignMask);
+}
+
+void
+StartupCacheDebugOutputStream::PutBuffer(char* aBuffer, uint32_t aLength)
+{
+  mBinaryStream->PutBuffer(aBuffer, aLength);
+}
+#endif //DEBUG
+
+StartupCacheWrapper* StartupCacheWrapper::gStartupCacheWrapper = nullptr;
+
+NS_IMPL_ISUPPORTS(StartupCacheWrapper, nsIStartupCache)
+
+StartupCacheWrapper::~StartupCacheWrapper()
+{
+  MOZ_ASSERT(gStartupCacheWrapper == this);
+  gStartupCacheWrapper = nullptr;
+}
+
+StartupCacheWrapper* StartupCacheWrapper::GetSingleton() 
+{
+  if (!gStartupCacheWrapper)
+    gStartupCacheWrapper = new StartupCacheWrapper();
+
+  NS_ADDREF(gStartupCacheWrapper);
+  return gStartupCacheWrapper;
+}
+
+nsresult 
+StartupCacheWrapper::GetBuffer(const char* id, char** outbuf, uint32_t* length) 
+{
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  UniquePtr<char[]> buf;
+  nsresult rv = sc->GetBuffer(id, &buf, length);
+  *outbuf = buf.release();
+  return rv;
+}
+
+nsresult
+StartupCacheWrapper::PutBuffer(const char* id, const char* inbuf, uint32_t length) 
+{
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  return sc->PutBuffer(id, inbuf, length);
+}
+
+nsresult
+StartupCacheWrapper::InvalidateCache() 
+{
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  sc->InvalidateCache();
+  return NS_OK;
+}
+
+nsresult
+StartupCacheWrapper::IgnoreDiskCache()
+{
+  StartupCache::IgnoreDiskCache();
+  return NS_OK;
+}
+
+nsresult 
+StartupCacheWrapper::GetDebugObjectOutputStream(nsIObjectOutputStream* stream,
+                                                nsIObjectOutputStream** outStream) 
+{
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  return sc->GetDebugObjectOutputStream(stream, outStream);
+}
+
+nsresult
+StartupCacheWrapper::StartupWriteComplete(bool *complete)
+{
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  sc->WaitOnWriteThread();
+  *complete = sc->mStartupWriteInitiated && sc->mTable.Count() == 0;
+  return NS_OK;
+}
+
+nsresult
+StartupCacheWrapper::ResetStartupWriteTimer()
+{
+  StartupCache* sc = StartupCache::GetSingleton();
+  return sc ? sc->ResetStartupWriteTimer() : NS_ERROR_NOT_INITIALIZED;
+}
+
+nsresult
+StartupCacheWrapper::GetObserver(nsIObserver** obv) {
+  StartupCache* sc = StartupCache::GetSingleton();
+  if (!sc) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  NS_ADDREF(*obv = sc->mListener);
+  return NS_OK;
+}
+
+nsresult
+StartupCacheWrapper::RecordAgesAlways() {
+  StartupCache *sc = StartupCache::GetSingleton();
+  return sc ? sc->RecordAgesAlways() : NS_ERROR_NOT_INITIALIZED;
+}
+
+} // namespace scache
+} // namespace mozilla
diff --git a/startupcache/StartupCache.h b/startupcache/StartupCache.h
new file mode 100644
index 0000000000..be428c004e
--- /dev/null
+++ b/startupcache/StartupCache.h
@@ -0,0 +1,230 @@
+/* -*-  Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef StartupCache_h_
+#define StartupCache_h_
+
+#include "nsClassHashtable.h"
+#include "nsComponentManagerUtils.h"
+#include "nsTArray.h"
+#include "nsZipArchive.h"
+#include "nsIStartupCache.h"
+#include "nsITimer.h"
+#include "nsIMemoryReporter.h"
+#include "nsIObserverService.h"
+#include "nsIObserver.h"
+#include "nsIOutputStream.h"
+#include "nsIFile.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/UniquePtr.h"
+
+/**
+ * The StartupCache is a persistent cache of simple key-value pairs,
+ * where the keys are null-terminated c-strings and the values are 
+ * arbitrary data, passed as a (char*, size) tuple. 
+ *
+ * Clients should use the GetSingleton() static method to access the cache. It 
+ * will be available from the end of XPCOM init (NS_InitXPCOM3 in XPCOMInit.cpp), 
+ * until XPCOM shutdown begins. The GetSingleton() method will return null if the cache
+ * is unavailable. The cache is only provided for libxul builds --
+ * it will fail to link in non-libxul builds. The XPCOM interface is provided
+ * only to allow compiled-code tests; clients should avoid using it.
+ *
+ * The API provided is very simple: GetBuffer() returns a buffer that was previously
+ * stored in the cache (if any), and PutBuffer() inserts a buffer into the cache.
+ * GetBuffer returns a new buffer, and the caller must take ownership of it.
+ * PutBuffer will assert if the client attempts to insert a buffer with the same name as
+ * an existing entry. The cache makes a copy of the passed-in buffer, so client
+ * retains ownership.
+ *
+ * InvalidateCache() may be called if a client suspects data corruption 
+ * or wishes to invalidate for any other reason. This will remove all existing cache data.
+ * Additionally, the static method IgnoreDiskCache() can be called if it is
+ * believed that the on-disk cache file is itself corrupt. This call implicitly
+ * calls InvalidateCache (if the singleton has been initialized) to ensure any
+ * data already read from disk is discarded. The cache will not load data from
+ * the disk file until a successful write occurs.
+ *
+ * Finally, getDebugObjectOutputStream() allows debug code to wrap an objectstream
+ * with a debug objectstream, to check for multiply-referenced objects. These will
+ * generally fail to deserialize correctly, unless they are stateless singletons or the 
+ * client maintains their own object data map for deserialization.
+ *
+ * Writes before the final-ui-startup notification are placed in an intermediate
+ * cache in memory, then written out to disk at a later time, to get writes off the
+ * startup path. In any case, clients should not rely on being able to GetBuffer()
+ * data that is written to the cache, since it may not have been written to disk or
+ * another client may have invalidated the cache. In other words, it should be used as
+ * a cache only, and not a reliable persistent store.
+ *
+ * Some utility functions are provided in StartupCacheUtils. These functions wrap the
+ * buffers into object streams, which may be useful for serializing objects. Note
+ * the above caution about multiply-referenced objects, though -- the streams are just
+ * as 'dumb' as the underlying buffers about multiply-referenced objects. They just
+ * provide some convenience in writing out data.
+ */
+
+namespace mozilla {
+
+namespace scache {
+
+struct CacheEntry
+{
+  UniquePtr<char[]> data;
+  uint32_t size;
+
+  CacheEntry() : size(0) { }
+
+  // Takes possession of buf
+  CacheEntry(UniquePtr<char[]> buf, uint32_t len) : data(Move(buf)), size(len) { }
+
+  ~CacheEntry()
+  {
+  }
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) {
+    return mallocSizeOf(this) + mallocSizeOf(data.get());
+  }
+};
+
+// We don't want to refcount StartupCache, and ObserverService wants to
+// refcount its listeners, so we'll let it refcount this instead.
+class StartupCacheListener final : public nsIObserver
+{
+  ~StartupCacheListener() {}
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+};
+
+class StartupCache : public nsIMemoryReporter
+{
+
+friend class StartupCacheListener;
+friend class StartupCacheWrapper;
+
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIMEMORYREPORTER
+
+  // StartupCache methods. See above comments for a more detailed description.
+
+  // Returns a buffer that was previously stored, caller takes ownership.
+  nsresult GetBuffer(const char* id, UniquePtr<char[]>* outbuf, uint32_t* length);
+
+  // Stores a buffer. Caller keeps ownership, we make a copy.
+  nsresult PutBuffer(const char* id, const char* inbuf, uint32_t length);
+
+  // Removes the cache file.
+  void InvalidateCache();
+
+  // Signal that data should not be loaded from the cache file
+  static void IgnoreDiskCache();
+
+  // In DEBUG builds, returns a stream that will attempt to check for
+  // and disallow multiple writes of the same object.
+  nsresult GetDebugObjectOutputStream(nsIObjectOutputStream* aStream,
+                                      nsIObjectOutputStream** outStream);
+
+  nsresult RecordAgesAlways();
+
+  static StartupCache* GetSingleton();
+  static void DeleteSingleton();
+
+  // This measures all the heap memory used by the StartupCache, i.e. it
+  // excludes the mapping.
+  size_t HeapSizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+
+  size_t SizeOfMapping();
+
+private:
+  StartupCache();
+  virtual ~StartupCache();
+
+  enum TelemetrifyAge {
+    IGNORE_AGE = 0,
+    RECORD_AGE = 1
+  };
+  static enum TelemetrifyAge gPostFlushAgeAction;
+
+  nsresult LoadArchive(enum TelemetrifyAge flag);
+  nsresult Init();
+  void WriteToDisk();
+  nsresult ResetStartupWriteTimer();
+  void WaitOnWriteThread();
+
+  static nsresult InitSingleton();
+  static void WriteTimeout(nsITimer *aTimer, void *aClosure);
+  static void ThreadedWrite(void *aClosure);
+
+  nsClassHashtable<nsCStringHashKey, CacheEntry> mTable;
+  nsTArray<nsCString> mPendingWrites;
+  RefPtr<nsZipArchive> mArchive;
+  nsCOMPtr<nsIFile> mFile;
+
+  nsCOMPtr<nsIObserverService> mObserverService;
+  RefPtr<StartupCacheListener> mListener;
+  nsCOMPtr<nsITimer> mTimer;
+
+  bool mStartupWriteInitiated;
+
+  static StaticRefPtr<StartupCache> gStartupCache;
+  static bool gShutdownInitiated;
+  static bool gIgnoreDiskCache;
+  PRThread *mWriteThread;
+#ifdef DEBUG
+  nsTHashtable<nsISupportsHashKey> mWriteObjectMap;
+#endif
+};
+
+// This debug outputstream attempts to detect if clients are writing multiple
+// references to the same object. We only support that if that object
+// is a singleton.
+#ifdef DEBUG
+class StartupCacheDebugOutputStream final
+  : public nsIObjectOutputStream
+{
+  ~StartupCacheDebugOutputStream() {}
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBJECTOUTPUTSTREAM
+
+  StartupCacheDebugOutputStream (nsIObjectOutputStream* binaryStream,
+                                   nsTHashtable<nsISupportsHashKey>* objectMap)
+  : mBinaryStream(binaryStream), mObjectMap(objectMap) { }
+  
+  NS_FORWARD_SAFE_NSIBINARYOUTPUTSTREAM(mBinaryStream)
+  NS_FORWARD_SAFE_NSIOUTPUTSTREAM(mBinaryStream)
+  
+  bool CheckReferences(nsISupports* aObject);
+  
+  nsCOMPtr<nsIObjectOutputStream> mBinaryStream;
+  nsTHashtable<nsISupportsHashKey> *mObjectMap;
+};
+#endif // DEBUG
+
+// XPCOM wrapper interface provided for tests only.
+#define NS_STARTUPCACHE_CID \
+      {0xae4505a9, 0x87ab, 0x477c, \
+      {0xb5, 0x77, 0xf9, 0x23, 0x57, 0xed, 0xa8, 0x84}}
+// contract id: "@mozilla.org/startupcache/cache;1"
+
+class StartupCacheWrapper final
+  : public nsIStartupCache
+{
+  ~StartupCacheWrapper();
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSISTARTUPCACHE
+
+  static StartupCacheWrapper* GetSingleton();
+  static StartupCacheWrapper *gStartupCacheWrapper;
+};
+
+} // namespace scache
+} // namespace mozilla
+
+#endif //StartupCache_h_
diff --git a/startupcache/StartupCacheModule.cpp b/startupcache/StartupCacheModule.cpp
new file mode 100644
index 0000000000..aa9f08e330
--- /dev/null
+++ b/startupcache/StartupCacheModule.cpp
@@ -0,0 +1,45 @@
+/* -*-  Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <string.h>
+
+#include "nscore.h"
+
+#include "nsID.h"
+#include "nsIComponentManager.h"
+#include "nsIServiceManager.h"
+#include "nsCOMPtr.h"
+#include "nsIModule.h"
+#include "mozilla/ModuleUtils.h"
+#include "mozilla/scache/StartupCache.h"
+
+using namespace mozilla::scache;
+
+// XXX Need help with guard for ENABLE_TEST
+NS_GENERIC_FACTORY_SINGLETON_CONSTRUCTOR(StartupCacheWrapper,
+                                         StartupCacheWrapper::GetSingleton)
+NS_DEFINE_NAMED_CID(NS_STARTUPCACHE_CID);
+
+static const mozilla::Module::CIDEntry kStartupCacheCIDs[] = {
+    { &kNS_STARTUPCACHE_CID, false, nullptr, StartupCacheWrapperConstructor },
+    { nullptr }
+};
+
+static const mozilla::Module::ContractIDEntry kStartupCacheContracts[] = {
+    { "@mozilla.org/startupcache/cache;1", &kNS_STARTUPCACHE_CID },
+    { nullptr }
+};
+
+static const mozilla::Module kStartupCacheModule = {
+    mozilla::Module::kVersion,
+    kStartupCacheCIDs,
+    kStartupCacheContracts,
+    nullptr,
+    nullptr,
+    nullptr,
+    nullptr
+};
+
+NSMODULE_DEFN(StartupCacheModule) = &kStartupCacheModule;
diff --git a/startupcache/StartupCacheUtils.cpp b/startupcache/StartupCacheUtils.cpp
new file mode 100644
index 0000000000..19a6b4e1b9
--- /dev/null
+++ b/startupcache/StartupCacheUtils.cpp
@@ -0,0 +1,253 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsCOMPtr.h"
+#include "nsIInputStream.h"
+#include "nsIStringStream.h"
+#include "nsNetUtil.h"
+#include "nsIFileURL.h"
+#include "nsIJARURI.h"
+#include "nsIResProtocolHandler.h"
+#include "nsIChromeRegistry.h"
+#include "nsAutoPtr.h"
+#include "StartupCacheUtils.h"
+#include "mozilla/scache/StartupCache.h"
+#include "mozilla/Omnijar.h"
+
+namespace mozilla {
+namespace scache {
+
+NS_EXPORT nsresult
+NewObjectInputStreamFromBuffer(UniquePtr<char[]> buffer, uint32_t len, 
+                               nsIObjectInputStream** stream)
+{
+  nsCOMPtr<nsIStringInputStream> stringStream =
+    do_CreateInstance("@mozilla.org/io/string-input-stream;1");
+  NS_ENSURE_TRUE(stringStream, NS_ERROR_FAILURE);
+
+  nsCOMPtr<nsIObjectInputStream> objectInput =
+    do_CreateInstance("@mozilla.org/binaryinputstream;1");
+  NS_ENSURE_TRUE(objectInput, NS_ERROR_FAILURE);
+
+  stringStream->AdoptData(buffer.release(), len);
+  objectInput->SetInputStream(stringStream);
+
+  objectInput.forget(stream);
+  return NS_OK;
+}
+
+NS_EXPORT nsresult
+NewObjectOutputWrappedStorageStream(nsIObjectOutputStream **wrapperStream,
+                                    nsIStorageStream** stream,
+                                    bool wantDebugStream)
+{
+  nsCOMPtr<nsIStorageStream> storageStream;
+
+  nsresult rv = NS_NewStorageStream(256, UINT32_MAX, getter_AddRefs(storageStream));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIObjectOutputStream> objectOutput
+    = do_CreateInstance("@mozilla.org/binaryoutputstream;1");
+  nsCOMPtr<nsIOutputStream> outputStream
+    = do_QueryInterface(storageStream);
+  
+  objectOutput->SetOutputStream(outputStream);
+  
+#ifdef DEBUG
+  if (wantDebugStream) {
+    // Wrap in debug stream to detect unsupported writes of 
+    // multiply-referenced non-singleton objects
+    StartupCache* sc = StartupCache::GetSingleton();
+    NS_ENSURE_TRUE(sc, NS_ERROR_UNEXPECTED);
+    nsCOMPtr<nsIObjectOutputStream> debugStream;
+    sc->GetDebugObjectOutputStream(objectOutput, getter_AddRefs(debugStream));
+    debugStream.forget(wrapperStream);
+  } else {
+    objectOutput.forget(wrapperStream);
+  }
+#else
+  objectOutput.forget(wrapperStream);
+#endif
+  
+  storageStream.forget(stream);
+  return NS_OK;
+}
+
+NS_EXPORT nsresult
+NewBufferFromStorageStream(nsIStorageStream *storageStream, 
+                           UniquePtr<char[]>* buffer, uint32_t* len) 
+{
+  nsresult rv;
+  nsCOMPtr<nsIInputStream> inputStream;
+  rv = storageStream->NewInputStream(0, getter_AddRefs(inputStream));
+  NS_ENSURE_SUCCESS(rv, rv);
+  
+  uint64_t avail64;
+  rv = inputStream->Available(&avail64);
+  NS_ENSURE_SUCCESS(rv, rv);
+  NS_ENSURE_TRUE(avail64 <= UINT32_MAX, NS_ERROR_FILE_TOO_BIG);
+
+  uint32_t avail = (uint32_t)avail64;
+  auto temp = MakeUnique<char[]>(avail);
+  uint32_t read;
+  rv = inputStream->Read(temp.get(), avail, &read);
+  if (NS_SUCCEEDED(rv) && avail != read)
+    rv = NS_ERROR_UNEXPECTED;
+  
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  
+  *len = avail;
+  *buffer = Move(temp);
+  return NS_OK;
+}
+
+static const char baseName[2][5] = { "gre/", "app/" };
+
+static inline bool
+canonicalizeBase(nsAutoCString &spec,
+                 nsACString &out)
+{
+    nsAutoCString greBase, appBase;
+    nsresult rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::GRE, greBase);
+    if (NS_FAILED(rv) || !greBase.Length())
+        return false;
+
+    rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::APP, appBase);
+    if (NS_FAILED(rv))
+        return false;
+
+    bool underGre = !greBase.Compare(spec.get(), false, greBase.Length());
+    bool underApp = appBase.Length() &&
+                    !appBase.Compare(spec.get(), false, appBase.Length());
+
+    if (!underGre && !underApp)
+        return false;
+
+    /**
+     * At this point, if both underGre and underApp are true, it can be one
+     * of the two following cases:
+     * - the GRE directory points to a subdirectory of the APP directory,
+     *   meaning spec points under GRE.
+     * - the APP directory points to a subdirectory of the GRE directory,
+     *   meaning spec points under APP.
+     * Checking the GRE and APP path length is enough to know in which case
+     * we are.
+     */
+    if (underGre && underApp && greBase.Length() < appBase.Length())
+        underGre = false;
+
+    out.AppendLiteral("/resource/");
+    out.Append(baseName[underGre ? mozilla::Omnijar::GRE : mozilla::Omnijar::APP]);
+    out.Append(Substring(spec, underGre ? greBase.Length() : appBase.Length()));
+    return true;
+}
+
+/**
+ * PathifyURI transforms uris into useful zip paths
+ * to make it easier to manipulate startup cache entries
+ * using standard zip tools.
+ * Transformations applied:
+ *  * resource:// URIs are resolved to their corresponding file/jar URI to
+ *    canonicalize resources URIs other than gre and app.
+ *  * Paths under GRE or APP directory have their base path replaced with
+ *    resource/gre or resource/app to avoid depending on install location.
+ *  * jar:file:///path/to/file.jar!/sub/path urls are replaced with
+ *    /path/to/file.jar/sub/path
+ *
+ *  The result is appended to the string passed in. Adding a prefix before
+ *  calling is recommended to avoid colliding with other cache users.
+ *
+ * For example, in the js loader (string is prefixed with jsloader by caller):
+ *  resource://gre/modules/XPCOMUtils.jsm or
+ *  file://$GRE_DIR/modules/XPCOMUtils.jsm or
+ *  jar:file://$GRE_DIR/omni.jar!/modules/XPCOMUtils.jsm becomes
+ *     jsloader/resource/gre/modules/XPCOMUtils.jsm
+ *  file://$PROFILE_DIR/extensions/{uuid}/components/component.js becomes
+ *     jsloader/$PROFILE_DIR/extensions/%7Buuid%7D/components/component.js
+ *  jar:file://$PROFILE_DIR/extensions/some.xpi!/components/component.js becomes
+ *     jsloader/$PROFILE_DIR/extensions/some.xpi/components/component.js
+ */
+NS_EXPORT nsresult
+PathifyURI(nsIURI *in, nsACString &out)
+{
+    bool equals;
+    nsresult rv;
+    nsCOMPtr<nsIURI> uri = in;
+    nsAutoCString spec;
+
+    // Resolve resource:// URIs. At the end of this if/else block, we
+    // have both spec and uri variables identifying the same URI.
+    if (NS_SUCCEEDED(in->SchemeIs("resource", &equals)) && equals) {
+        nsCOMPtr<nsIIOService> ioService = do_GetIOService(&rv);
+        NS_ENSURE_SUCCESS(rv, rv);
+
+        nsCOMPtr<nsIProtocolHandler> ph;
+        rv = ioService->GetProtocolHandler("resource", getter_AddRefs(ph));
+        NS_ENSURE_SUCCESS(rv, rv);
+
+        nsCOMPtr<nsIResProtocolHandler> irph(do_QueryInterface(ph, &rv));
+        NS_ENSURE_SUCCESS(rv, rv);
+
+        rv = irph->ResolveURI(in, spec);
+        NS_ENSURE_SUCCESS(rv, rv);
+
+        rv = ioService->NewURI(spec, nullptr, nullptr, getter_AddRefs(uri));
+        NS_ENSURE_SUCCESS(rv, rv);
+    } else {
+        if (NS_SUCCEEDED(in->SchemeIs("chrome", &equals)) && equals) {
+            nsCOMPtr<nsIChromeRegistry> chromeReg =
+                mozilla::services::GetChromeRegistryService();
+            if (!chromeReg)
+                return NS_ERROR_UNEXPECTED;
+
+            rv = chromeReg->ConvertChromeURL(in, getter_AddRefs(uri));
+            NS_ENSURE_SUCCESS(rv, rv);
+        }
+
+        rv = uri->GetSpec(spec);
+        NS_ENSURE_SUCCESS(rv, rv);
+    }
+
+    if (!canonicalizeBase(spec, out)) {
+        if (NS_SUCCEEDED(uri->SchemeIs("file", &equals)) && equals) {
+            nsCOMPtr<nsIFileURL> baseFileURL;
+            baseFileURL = do_QueryInterface(uri, &rv);
+            NS_ENSURE_SUCCESS(rv, rv);
+
+            nsAutoCString path;
+            rv = baseFileURL->GetPath(path);
+            NS_ENSURE_SUCCESS(rv, rv);
+
+            out.Append(path);
+        } else if (NS_SUCCEEDED(uri->SchemeIs("jar", &equals)) && equals) {
+            nsCOMPtr<nsIJARURI> jarURI = do_QueryInterface(uri, &rv);
+            NS_ENSURE_SUCCESS(rv, rv);
+
+            nsCOMPtr<nsIURI> jarFileURI;
+            rv = jarURI->GetJARFile(getter_AddRefs(jarFileURI));
+            NS_ENSURE_SUCCESS(rv, rv);
+
+            rv = PathifyURI(jarFileURI, out);
+            NS_ENSURE_SUCCESS(rv, rv);
+
+            nsAutoCString path;
+            rv = jarURI->GetJAREntry(path);
+            NS_ENSURE_SUCCESS(rv, rv);
+            out.Append('/');
+            out.Append(path);
+        } else { // Very unlikely
+            rv = uri->GetSpec(spec);
+            NS_ENSURE_SUCCESS(rv, rv);
+
+            out.Append('/');
+            out.Append(spec);
+        }
+    }
+    return NS_OK;
+}
+
+} // namespace scache
+} // namespace mozilla
diff --git a/startupcache/StartupCacheUtils.h b/startupcache/StartupCacheUtils.h
new file mode 100644
index 0000000000..7a4da25434
--- /dev/null
+++ b/startupcache/StartupCacheUtils.h
@@ -0,0 +1,43 @@
+/* -*-  Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef nsStartupCacheUtils_h_
+#define nsStartupCacheUtils_h_
+
+#include "nsIStorageStream.h"
+#include "nsIObjectInputStream.h"
+#include "nsIObjectOutputStream.h"
+#include "mozilla/UniquePtr.h"
+
+namespace mozilla {
+namespace scache {
+
+NS_EXPORT nsresult
+NewObjectInputStreamFromBuffer(UniquePtr<char[]> buffer, uint32_t len, 
+                               nsIObjectInputStream** stream);
+
+// We can't retrieve the wrapped stream from the objectOutputStream later,
+// so we return it here. We give callers in debug builds the option 
+// to wrap the outputstream in a debug stream, which will detect if
+// non-singleton objects are written out multiple times during a serialization.
+// This could cause them to be deserialized incorrectly (as multiple copies
+// instead of references).
+NS_EXPORT nsresult
+NewObjectOutputWrappedStorageStream(nsIObjectOutputStream **wrapperStream,
+                                    nsIStorageStream** stream,
+                                    bool wantDebugStream);
+
+// Creates a buffer for storing the stream into the cache. The buffer is
+// allocated with 'new []'.  After calling this function, the caller would
+// typically call nsIStartupCache::PutBuffer with the returned buffer.
+NS_EXPORT nsresult
+NewBufferFromStorageStream(nsIStorageStream *storageStream, 
+                           UniquePtr<char[]>* buffer, uint32_t* len);
+
+NS_EXPORT nsresult
+PathifyURI(nsIURI *in, nsACString &out);
+} // namespace scache
+} // namespace mozilla
+
+#endif //nsStartupCacheUtils_h_
diff --git a/startupcache/moz.build b/startupcache/moz.build
new file mode 100644
index 0000000000..24068cb7e8
--- /dev/null
+++ b/startupcache/moz.build
@@ -0,0 +1,28 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+if not CONFIG['MOZ_B2G']:
+    TEST_DIRS += ['test']
+
+XPIDL_SOURCES += [
+    'nsIStartupCache.idl',
+]
+
+XPIDL_MODULE = 'startupcache'
+
+EXPORTS.mozilla.scache += [
+    'StartupCache.h',
+    'StartupCacheUtils.h',
+]
+
+# These files cannot be built in unified mode because they rely on plarena.h.
+SOURCES += [
+    'StartupCache.cpp',
+    'StartupCacheModule.cpp',
+    'StartupCacheUtils.cpp',
+]
+
+FINAL_LIBRARY = 'xul'
diff --git a/startupcache/nsIStartupCache.idl b/startupcache/nsIStartupCache.idl
new file mode 100644
index 0000000000..b03dcb3ccd
--- /dev/null
+++ b/startupcache/nsIStartupCache.idl
@@ -0,0 +1,65 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIInputStream.idl"
+#include "nsISupports.idl"
+#include "nsIObserver.idl"
+#include "nsIObjectOutputStream.idl"
+
+%{C++
+#include "mozilla/UniquePtr.h"
+%}
+
+[uuid(25957820-90a1-428c-8739-b0845d3cc534)]
+interface nsIStartupCache : nsISupports
+{
+
+  /** This interface is provided for testing purposes only, basically
+   *  just to solve link vagaries. See docs in StartupCache.h
+   *  GetBuffer, PutBuffer, and InvalidateCache act as described 
+   *  in that file. */
+
+  uint32_t getBuffer(in string aID, out charPtr aBuffer);
+%{C++
+  /* A more convenient interface for using from C++.  */
+  nsresult GetBuffer(const char* id, mozilla::UniquePtr<char[]>* outbuf, uint32_t* length)
+  {
+    char* buf;
+    nsresult rv = GetBuffer(id, &buf, length);
+    NS_ENSURE_SUCCESS(rv, rv);
+    outbuf->reset(buf);
+    return rv;
+  }
+%}
+
+  void putBuffer(in string aID, in string aBuffer, 
+                            in uint32_t aLength);
+ 
+  void invalidateCache();
+  
+  void ignoreDiskCache();
+
+  /** In debug builds, wraps this object output stream with a stream that will 
+   *  detect and prevent the write of a multiply-referenced non-singleton object 
+   *  during serialization. In non-debug, returns an add-ref'd pointer to
+   *  original stream, unwrapped. */
+  nsIObjectOutputStream getDebugObjectOutputStream(in nsIObjectOutputStream aStream);
+
+  /* Allows clients to check whether the one-time writeout after startup 
+   * has finished yet, and also to set this variable as needed (so test
+   * code can fire mulitple startup writes if needed).
+   */
+  boolean startupWriteComplete();
+  void resetStartupWriteTimer();
+
+  /* Instruct clients to always post cache ages to Telemetry, even in
+     cases where it would not normally make sense.  */
+  void recordAgesAlways();
+
+  /* Allows clients to simulate the behavior of ObserverService. */
+  readonly attribute nsIObserver observer;
+};
+
diff --git a/startupcache/test/TestStartupCache.cpp b/startupcache/test/TestStartupCache.cpp
new file mode 100644
index 0000000000..a16c2de727
--- /dev/null
+++ b/startupcache/test/TestStartupCache.cpp
@@ -0,0 +1,480 @@
+/* -*-  Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "TestHarness.h"
+
+#include "nsThreadUtils.h"
+#include "nsIClassInfo.h"
+#include "nsIOutputStream.h"
+#include "nsIObserver.h"
+#include "nsISerializable.h"
+#include "nsISupports.h"
+#include "nsIStartupCache.h"
+#include "nsIStringStream.h"
+#include "nsIStorageStream.h"
+#include "nsIObjectInputStream.h"
+#include "nsIObjectOutputStream.h"
+#include "nsIURI.h"
+#include "nsStringAPI.h"
+#include "nsIPrefBranch.h"
+#include "nsIPrefService.h"
+#include "nsIXPConnect.h"
+#include "prio.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/UniquePtr.h"
+
+using namespace JS;
+
+namespace mozilla {
+namespace scache {
+
+NS_IMPORT nsresult
+NewObjectInputStreamFromBuffer(UniquePtr<char[]> buffer, uint32_t len, 
+                               nsIObjectInputStream** stream);
+
+// We can't retrieve the wrapped stream from the objectOutputStream later,
+// so we return it here.
+NS_IMPORT nsresult
+NewObjectOutputWrappedStorageStream(nsIObjectOutputStream **wrapperStream,
+                                    nsIStorageStream** stream);
+
+NS_IMPORT nsresult
+NewBufferFromStorageStream(nsIStorageStream *storageStream, 
+                           UniquePtr<char[]>* buffer, uint32_t* len);
+} // namespace scache
+} // namespace mozilla
+
+using namespace mozilla::scache;
+using mozilla::UniquePtr;
+
+#define NS_ENSURE_STR_MATCH(str1, str2, testname)  \
+PR_BEGIN_MACRO                                     \
+if (0 != strcmp(str1, str2)) {                     \
+  fail("failed " testname);                        \
+  return NS_ERROR_FAILURE;                         \
+}                                                  \
+passed("passed " testname);                        \
+PR_END_MACRO
+
+nsresult
+WaitForStartupTimer() {
+  nsresult rv;
+  nsCOMPtr<nsIStartupCache> sc
+    = do_GetService("@mozilla.org/startupcache/cache;1");
+  PR_Sleep(10 * PR_TicksPerSecond());
+  
+  bool complete;
+  while (true) {
+    
+    NS_ProcessPendingEvents(nullptr);
+    rv = sc->StartupWriteComplete(&complete);
+    if (NS_FAILED(rv) || complete)
+      break;
+    PR_Sleep(1 * PR_TicksPerSecond());
+  }
+  return rv;
+}
+
+nsresult
+TestStartupWriteRead() {
+  nsresult rv;
+  nsCOMPtr<nsIStartupCache> sc
+    = do_GetService("@mozilla.org/startupcache/cache;1", &rv);
+  if (!sc) {
+    fail("didn't get a pointer...");
+    return NS_ERROR_FAILURE;
+  } else {
+    passed("got a pointer?");
+  }
+  sc->InvalidateCache();
+  
+  const char* buf = "Market opportunities for BeardBook";
+  const char* id = "id";
+  UniquePtr<char[]> outbuf;  
+  uint32_t len;
+  
+  rv = sc->PutBuffer(id, buf, strlen(buf) + 1);
+  NS_ENSURE_SUCCESS(rv, rv);
+  
+  rv = sc->GetBuffer(id, &outbuf, &len);
+  NS_ENSURE_SUCCESS(rv, rv);
+  NS_ENSURE_STR_MATCH(buf, outbuf.get(), "pre-write read");
+
+  rv = sc->ResetStartupWriteTimer();
+  rv = WaitForStartupTimer();
+  NS_ENSURE_SUCCESS(rv, rv);
+  
+  rv = sc->GetBuffer(id, &outbuf, &len);
+  NS_ENSURE_SUCCESS(rv, rv);
+  NS_ENSURE_STR_MATCH(buf, outbuf.get(), "simple write/read");
+
+  return NS_OK;
+}
+
+nsresult
+TestWriteInvalidateRead() {
+  nsresult rv;
+  const char* buf = "BeardBook competitive analysis";
+  const char* id = "id";
+  UniquePtr<char[]> outbuf;
+  uint32_t len;
+  nsCOMPtr<nsIStartupCache> sc
+    = do_GetService("@mozilla.org/startupcache/cache;1", &rv);
+  sc->InvalidateCache();
+
+  rv = sc->PutBuffer(id, buf, strlen(buf) + 1);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  sc->InvalidateCache();
+
+  rv = sc->GetBuffer(id, &outbuf, &len);
+  if (rv == NS_ERROR_NOT_AVAILABLE) {
+    passed("buffer not available after invalidate");
+  } else if (NS_SUCCEEDED(rv)) {
+    fail("GetBuffer succeeded unexpectedly after invalidate");
+    return NS_ERROR_UNEXPECTED;
+  } else {
+    fail("GetBuffer gave an unexpected failure, expected NOT_AVAILABLE");
+    return rv;
+  }
+
+  sc->InvalidateCache();
+  return NS_OK;
+}
+
+nsresult
+TestWriteObject() {
+  nsresult rv;
+
+  nsCOMPtr<nsIURI> obj
+    = do_CreateInstance("@mozilla.org/network/simple-uri;1");
+  if (!obj) {
+    fail("did not create object in test write object");
+    return NS_ERROR_UNEXPECTED;
+  }
+  NS_NAMED_LITERAL_CSTRING(spec, "http://www.mozilla.org");
+  rv = obj->SetSpec(spec);
+  NS_ENSURE_SUCCESS(rv, rv);
+  nsCOMPtr<nsIStartupCache> sc = do_GetService("@mozilla.org/startupcache/cache;1", &rv);
+
+  sc->InvalidateCache();
+  
+  // Create an object stream. Usually this is done with
+  // NewObjectOutputWrappedStorageStream, but that uses
+  // StartupCache::GetSingleton in debug builds, and we
+  // don't have access to that here. Obviously.
+  const char* id = "id";
+  nsCOMPtr<nsIStorageStream> storageStream
+    = do_CreateInstance("@mozilla.org/storagestream;1");
+  NS_ENSURE_ARG_POINTER(storageStream);
+  
+  rv = storageStream->Init(256, (uint32_t) -1);
+  NS_ENSURE_SUCCESS(rv, rv);
+  
+  nsCOMPtr<nsIObjectOutputStream> objectOutput
+    = do_CreateInstance("@mozilla.org/binaryoutputstream;1");
+  if (!objectOutput)
+    return NS_ERROR_OUT_OF_MEMORY;
+  
+  nsCOMPtr<nsIOutputStream> outputStream
+    = do_QueryInterface(storageStream);
+  
+  rv = objectOutput->SetOutputStream(outputStream);
+
+  if (NS_FAILED(rv)) {
+    fail("failed to create output stream");
+    return rv;
+  }
+  nsCOMPtr<nsISupports> objQI(do_QueryInterface(obj));
+  rv = objectOutput->WriteObject(objQI, true);
+  if (NS_FAILED(rv)) {
+    fail("failed to write object");
+    return rv;
+  }
+
+  UniquePtr<char[]> buf;
+  uint32_t len;
+  NewBufferFromStorageStream(storageStream, &buf, &len);
+
+  // Since this is a post-startup write, it should be written and
+  // available.
+  rv = sc->PutBuffer(id, buf.get(), len);
+  if (NS_FAILED(rv)) {
+    fail("failed to insert input stream");
+    return rv;
+  }
+    
+  UniquePtr<char[]> buf2;
+  uint32_t len2;
+  nsCOMPtr<nsIObjectInputStream> objectInput;
+  rv = sc->GetBuffer(id, &buf2, &len2);
+  if (NS_FAILED(rv)) {
+    fail("failed to retrieve buffer");
+    return rv;
+  }
+
+  rv = NewObjectInputStreamFromBuffer(Move(buf2), len2,
+                                      getter_AddRefs(objectInput));
+  if (NS_FAILED(rv)) {
+    fail("failed to created input stream");
+    return rv;
+  }  
+
+  nsCOMPtr<nsISupports> deserialized;
+  rv = objectInput->ReadObject(true, getter_AddRefs(deserialized));
+  if (NS_FAILED(rv)) {
+    fail("failed to read object");
+    return rv;
+  }
+  
+  bool match = false;
+  nsCOMPtr<nsIURI> uri(do_QueryInterface(deserialized));
+  if (uri) {
+    nsCString outSpec;
+    rv = uri->GetSpec(outSpec);
+    if (NS_FAILED(rv)) {
+      fail("failed to get spec");
+      return rv;
+    }
+    match = outSpec.Equals(spec);
+  }
+  if (!match) {
+    fail("deserialized object has incorrect information");
+    return rv;
+  }
+  
+  passed("write object");
+  return NS_OK;
+}
+
+nsresult
+LockCacheFile(bool protect, nsIFile* profileDir) {
+  NS_ENSURE_ARG(profileDir);
+
+  nsCOMPtr<nsIFile> startupCache;
+  profileDir->Clone(getter_AddRefs(startupCache));
+  NS_ENSURE_STATE(startupCache);
+  startupCache->AppendNative(NS_LITERAL_CSTRING("startupCache"));
+
+  nsresult rv;
+#ifndef XP_WIN
+  static uint32_t oldPermissions;
+#else
+  static PRFileDesc* fd = nullptr;
+#endif
+
+  // To prevent deletion of the startupcache file, we change the containing
+  // directory's permissions on Linux/Mac, and hold the file open on Windows
+  if (protect) {
+#ifndef XP_WIN
+    rv = startupCache->GetPermissions(&oldPermissions);
+    NS_ENSURE_SUCCESS(rv, rv);
+    rv = startupCache->SetPermissions(0555);
+    NS_ENSURE_SUCCESS(rv, rv);
+#else
+    // Filename logic from StartupCache.cpp
+    #ifdef IS_BIG_ENDIAN
+    #define SC_ENDIAN "big"
+    #else
+    #define SC_ENDIAN "little"
+    #endif
+
+    #if PR_BYTES_PER_WORD == 4
+    #define SC_WORDSIZE "4"
+    #else
+    #define SC_WORDSIZE "8"
+    #endif
+    char sStartupCacheName[] = "startupCache." SC_WORDSIZE "." SC_ENDIAN;
+    startupCache->AppendNative(NS_LITERAL_CSTRING(sStartupCacheName));
+
+    rv = startupCache->OpenNSPRFileDesc(PR_RDONLY, 0, &fd);
+    NS_ENSURE_SUCCESS(rv, rv);
+#endif
+  } else {
+#ifndef XP_WIN
+    rv = startupCache->SetPermissions(oldPermissions);
+    NS_ENSURE_SUCCESS(rv, rv);
+#else
+   PR_Close(fd);
+#endif
+  }
+
+  return NS_OK;
+}
+
+nsresult
+TestIgnoreDiskCache(nsIFile* profileDir) {
+  nsresult rv;
+  nsCOMPtr<nsIStartupCache> sc
+    = do_GetService("@mozilla.org/startupcache/cache;1", &rv);
+  sc->InvalidateCache();
+  
+  const char* buf = "Get a Beardbook app for your smartphone";
+  const char* id = "id";
+  UniquePtr<char[]> outbuf;
+  uint32_t len;
+  
+  rv = sc->PutBuffer(id, buf, strlen(buf) + 1);
+  NS_ENSURE_SUCCESS(rv, rv);
+  rv = sc->ResetStartupWriteTimer();
+  rv = WaitForStartupTimer();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Prevent StartupCache::InvalidateCache from deleting the disk file
+  rv = LockCacheFile(true, profileDir);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  sc->IgnoreDiskCache();
+
+  rv = sc->GetBuffer(id, &outbuf, &len);
+
+  nsresult r = LockCacheFile(false, profileDir);
+  NS_ENSURE_SUCCESS(r, r);
+
+  if (rv == NS_ERROR_NOT_AVAILABLE) {
+    passed("buffer not available after ignoring disk cache");
+  } else if (NS_SUCCEEDED(rv)) {
+    fail("GetBuffer succeeded unexpectedly after ignoring disk cache");
+    return NS_ERROR_UNEXPECTED;
+  } else {
+    fail("GetBuffer gave an unexpected failure, expected NOT_AVAILABLE");
+    return rv;
+  }
+
+  sc->InvalidateCache();
+  return NS_OK;
+}
+
+nsresult
+TestEarlyShutdown() {
+  nsresult rv;
+  nsCOMPtr<nsIStartupCache> sc
+    = do_GetService("@mozilla.org/startupcache/cache;1", &rv);
+  sc->InvalidateCache();
+
+  const char* buf = "Find your soul beardmate on BeardBook";
+  const char* id = "id";
+  uint32_t len;
+  UniquePtr<char[]> outbuf;
+  
+  sc->ResetStartupWriteTimer();
+  rv = sc->PutBuffer(id, buf, strlen(buf) + 1);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIObserver> obs;
+  sc->GetObserver(getter_AddRefs(obs));
+  obs->Observe(nullptr, "xpcom-shutdown", nullptr);
+  rv = WaitForStartupTimer();
+  NS_ENSURE_SUCCESS(rv, rv);
+  
+  rv = sc->GetBuffer(id, &outbuf, &len);
+
+  if (NS_SUCCEEDED(rv)) {
+    passed("GetBuffer succeeded after early shutdown");
+  } else {
+    fail("GetBuffer failed after early shutdown");
+    return rv;
+  }
+
+  const char* other_id = "other_id";
+  rv = sc->PutBuffer(other_id, buf, strlen(buf) + 1);
+
+  if (rv == NS_ERROR_NOT_AVAILABLE) {
+    passed("PutBuffer not available after early shutdown");
+  } else if (NS_SUCCEEDED(rv)) {
+    fail("PutBuffer succeeded unexpectedly after early shutdown");
+    return NS_ERROR_UNEXPECTED;
+  } else {
+    fail("PutBuffer gave an unexpected failure, expected NOT_AVAILABLE");
+    return rv;
+  }
+ 
+  return NS_OK;
+}
+
+int main(int argc, char** argv)
+{
+  ScopedXPCOM xpcom("Startup Cache");
+  if (xpcom.failed())
+    return 1;
+
+  nsCOMPtr<nsIPrefBranch> prefs = do_GetService(NS_PREFSERVICE_CONTRACTID);
+  if (!prefs) {
+    fail("prefs");
+    return 1;
+  }
+  prefs->SetIntPref("hangmonitor.timeout", 0);
+
+  int rv = 0;
+  nsresult scrv;
+
+  // Register TestStartupCacheTelemetry
+  nsCOMPtr<nsIFile> manifest;
+  scrv = NS_GetSpecialDirectory(NS_GRE_DIR,
+                                getter_AddRefs(manifest));
+  if (NS_FAILED(scrv)) {
+    fail("NS_XPCOM_CURRENT_PROCESS_DIR");
+    return 1;
+  }
+
+#ifdef XP_MACOSX
+  nsCOMPtr<nsIFile> tempManifest;
+  manifest->Clone(getter_AddRefs(tempManifest));
+  manifest->AppendNative(
+    NS_LITERAL_CSTRING("TestStartupCacheTelemetry.manifest"));
+  bool exists;
+  manifest->Exists(&exists);
+  if (!exists) {
+    // Workaround for bug 1080338 in mozharness.
+    manifest = tempManifest.forget();
+    manifest->SetNativeLeafName(NS_LITERAL_CSTRING("MacOS"));
+    manifest->AppendNative(
+      NS_LITERAL_CSTRING("TestStartupCacheTelemetry.manifest"));
+  }
+#else
+  manifest->AppendNative(
+    NS_LITERAL_CSTRING("TestStartupCacheTelemetry.manifest"));
+#endif
+
+  XRE_AddManifestLocation(NS_APP_LOCATION, manifest);
+
+  nsCOMPtr<nsIObserver> telemetryThing =
+    do_GetService("@mozilla.org/testing/startup-cache-telemetry.js");
+  if (!telemetryThing) {
+    fail("telemetryThing");
+    return 1;
+  }
+  scrv = telemetryThing->Observe(nullptr, "save-initial", nullptr);
+  if (NS_FAILED(scrv)) {
+    fail("save-initial");
+    rv = 1;
+  }
+
+  nsCOMPtr<nsIStartupCache> sc 
+    = do_GetService("@mozilla.org/startupcache/cache;1", &scrv);
+  if (NS_FAILED(scrv))
+    rv = 1;
+  else
+    sc->RecordAgesAlways();
+  if (NS_FAILED(TestStartupWriteRead()))
+    rv = 1;
+  if (NS_FAILED(TestWriteInvalidateRead()))
+    rv = 1;
+  if (NS_FAILED(TestWriteObject()))
+    rv = 1;
+  nsCOMPtr<nsIFile> profileDir = xpcom.GetProfileDirectory();
+  if (NS_FAILED(TestIgnoreDiskCache(profileDir)))
+    rv = 1;
+  if (NS_FAILED(TestEarlyShutdown()))
+    rv = 1;
+
+  scrv = telemetryThing->Observe(nullptr, "save-initial", nullptr);
+  if (NS_FAILED(scrv)) {
+    fail("check-final");
+    rv = 1;
+  }
+
+  return rv;
+}
diff --git a/startupcache/test/TestStartupCacheTelemetry.js b/startupcache/test/TestStartupCacheTelemetry.js
new file mode 100644
index 0000000000..7a570187fd
--- /dev/null
+++ b/startupcache/test/TestStartupCacheTelemetry.js
@@ -0,0 +1,60 @@
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+function shouldHaveChanged(a, b)
+{
+  if (a.length != b.length) {
+    throw Error("TEST-UNEXPECTED-FAIL: telemetry count array size changed");
+  }
+
+  for (let i = 0; i < a.length; ++i) {
+    if (a[i] == b[i]) {
+      continue;
+    }
+    return; // something was different, that's all that matters
+  }
+  throw Error("TEST-UNEXPECTED-FAIL: telemetry data didn't change");
+}
+
+function TestStartupCacheTelemetry() { }
+
+TestStartupCacheTelemetry.prototype = {
+  classID: Components.ID("{73cbeffd-d6c7-42f0-aaf3-f176430dcfc8}"),
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
+
+  saveInitial: function() {
+    let t = Services.telemetry;
+    this._age = t.getHistogramById("STARTUP_CACHE_AGE_HOURS").snapshot.counts;
+    this._invalid = t.getHistogramById("STARTUP_CACHE_INVALID").snapshot.counts;
+  },
+
+  checkFinal: function() {
+    let t = Services.telemetry;
+    let newAge = t.getHistogramById("STARTUP_CACHE_AGE_HOURS").snapshot.counts;
+    shouldHaveChanged(this._age, newAge);
+
+    let newInvalid = t.getHistogramById("STARTUP_CACHE_INVALID").snapshot.counts;
+    shouldHaveChanged(this._invalid, newInvalid);
+  },
+
+  observe: function(subject, topic, data) {
+    switch (topic) {
+    case "save-initial":
+      this.saveInitial();
+      break;
+
+    case "check-final":
+      this.checkFinal();
+      break;
+
+    default:
+      throw Error("BADDOG, NO MILKBONE FOR YOU");
+    }
+  },
+};
+
+this.NSGetFactory = XPCOMUtils.generateNSGetFactory([TestStartupCacheTelemetry]);
diff --git a/startupcache/test/TestStartupCacheTelemetry.manifest b/startupcache/test/TestStartupCacheTelemetry.manifest
new file mode 100644
index 0000000000..b288a72928
--- /dev/null
+++ b/startupcache/test/TestStartupCacheTelemetry.manifest
@@ -0,0 +1,2 @@
+component {73cbeffd-d6c7-42f0-aaf3-f176430dcfc8} TestStartupCacheTelemetry.js
+contract @mozilla.org/testing/startup-cache-telemetry.js {73cbeffd-d6c7-42f0-aaf3-f176430dcfc8}
diff --git a/startupcache/test/moz.build b/startupcache/test/moz.build
new file mode 100644
index 0000000000..5a9e6a61e0
--- /dev/null
+++ b/startupcache/test/moz.build
@@ -0,0 +1,14 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+GeckoCppUnitTests([
+    'TestStartupCache',
+])
+
+EXTRA_COMPONENTS += [
+    'TestStartupCacheTelemetry.js',
+    'TestStartupCacheTelemetry.manifest',
+]