Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3658 insertions, 0 deletions

diff --git a/toolkit/components/telemetry/core/TelemetryHistogram.cpp b/toolkit/components/telemetry/core/TelemetryHistogram.cpp
new file mode 100644
index 0000000000..047682285c
--- /dev/null
+++ b/toolkit/components/telemetry/core/TelemetryHistogram.cpp
@@ -0,0 +1,3658 @@
+/* -*- 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 "TelemetryHistogram.h"
+
+#include <limits>
+#include "base/histogram.h"
+#include "geckoview/streaming/GeckoViewStreamingTelemetry.h"
+#include "ipc/TelemetryIPCAccumulator.h"
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/Array.h"  // JS::GetArrayLength, JS::IsArrayObject, JS::NewArrayObject
+#include "js/GCAPI.h"
+#include "js/Object.h"  // JS::GetClass, JS::GetPrivate, JS::SetPrivate
+#include "mozilla/dom/ToJSValue.h"
+#include "mozilla/gfx/GPUProcessManager.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/JSONWriter.h"
+#include "mozilla/StartupTimeline.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/Unused.h"
+#include "nsClassHashtable.h"
+#include "nsString.h"
+#include "nsHashKeys.h"
+#include "nsITelemetry.h"
+#include "nsPrintfCString.h"
+#include "TelemetryHistogramNameMap.h"
+#include "TelemetryScalar.h"
+
+using base::BooleanHistogram;
+using base::CountHistogram;
+using base::FlagHistogram;
+using base::LinearHistogram;
+using mozilla::MakeTuple;
+using mozilla::StaticMutex;
+using mozilla::StaticMutexAutoLock;
+using mozilla::Telemetry::HistogramAccumulation;
+using mozilla::Telemetry::HistogramCount;
+using mozilla::Telemetry::HistogramID;
+using mozilla::Telemetry::HistogramIDByNameLookup;
+using mozilla::Telemetry::KeyedHistogramAccumulation;
+using mozilla::Telemetry::ProcessID;
+using mozilla::Telemetry::Common::CanRecordDataset;
+using mozilla::Telemetry::Common::CanRecordProduct;
+using mozilla::Telemetry::Common::GetCurrentProduct;
+using mozilla::Telemetry::Common::GetIDForProcessName;
+using mozilla::Telemetry::Common::GetNameForProcessID;
+using mozilla::Telemetry::Common::IsExpiredVersion;
+using mozilla::Telemetry::Common::IsInDataset;
+using mozilla::Telemetry::Common::LogToBrowserConsole;
+using mozilla::Telemetry::Common::RecordedProcessType;
+using mozilla::Telemetry::Common::StringHashSet;
+using mozilla::Telemetry::Common::SupportedProduct;
+using mozilla::Telemetry::Common::ToJSString;
+
+namespace TelemetryIPCAccumulator = mozilla::TelemetryIPCAccumulator;
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// Naming: there are two kinds of functions in this file:
+//
+// * Functions named internal_*: these can only be reached via an
+//   interface function (TelemetryHistogram::*).  They mostly expect
+//   the interface function to have acquired
+//   |gTelemetryHistogramMutex|, so they do not have to be
+//   thread-safe.  However, those internal_* functions that are
+//   reachable from internal_WrapAndReturnHistogram and
+//   internal_WrapAndReturnKeyedHistogram can sometimes be called
+//   without |gTelemetryHistogramMutex|, and so might be racey.
+//
+// * Functions named TelemetryHistogram::*.  This is the external interface.
+//   Entries and exits to these functions are serialised using
+//   |gTelemetryHistogramMutex|, except for GetKeyedHistogramSnapshots and
+//   CreateHistogramSnapshots.
+//
+// Avoiding races and deadlocks:
+//
+// All functions in the external interface (TelemetryHistogram::*) are
+// serialised using the mutex |gTelemetryHistogramMutex|.  This means
+// that the external interface is thread-safe, and many of the
+// internal_* functions can ignore thread safety.  But it also brings
+// a danger of deadlock if any function in the external interface can
+// get back to that interface.  That is, we will deadlock on any call
+// chain like this
+//
+// TelemetryHistogram::* -> .. any functions .. -> TelemetryHistogram::*
+//
+// To reduce the danger of that happening, observe the following rules:
+//
+// * No function in TelemetryHistogram::* may directly call, nor take the
+//   address of, any other function in TelemetryHistogram::*.
+//
+// * No internal function internal_* may call, nor take the address
+//   of, any function in TelemetryHistogram::*.
+//
+// internal_WrapAndReturnHistogram and
+// internal_WrapAndReturnKeyedHistogram are not protected by
+// |gTelemetryHistogramMutex| because they make calls to the JS
+// engine, but that can in turn call back to Telemetry and hence back
+// to a TelemetryHistogram:: function, in order to report GC and other
+// statistics.  This would lead to deadlock due to attempted double
+// acquisition of |gTelemetryHistogramMutex|, if the internal_* functions
+// were required to be protected by |gTelemetryHistogramMutex|.  To
+// break that cycle, we relax that requirement.  Unfortunately this
+// means that this file is not guaranteed race-free.
+
+// This is a StaticMutex rather than a plain Mutex (1) so that
+// it gets initialised in a thread-safe manner the first time
+// it is used, and (2) because it is never de-initialised, and
+// a normal Mutex would show up as a leak in BloatView.  StaticMutex
+// also has the "OffTheBooks" property, so it won't show as a leak
+// in BloatView.
+static StaticMutex gTelemetryHistogramMutex;
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE TYPES
+
+namespace {
+
+// Hardcoded probes
+//
+// The order of elements here is important to minimize the memory footprint of a
+// HistogramInfo instance.
+//
+// Any adjustements need to be reflected in gen_histogram_data.py
+struct HistogramInfo {
+  uint32_t min;
+  uint32_t max;
+  uint32_t bucketCount;
+  uint32_t name_offset;
+  uint32_t expiration_offset;
+  uint32_t label_count;
+  uint32_t key_count;
+  uint32_t store_count;
+  uint16_t label_index;
+  uint16_t key_index;
+  uint16_t store_index;
+  RecordedProcessType record_in_processes;
+  bool keyed;
+  uint8_t histogramType;
+  uint8_t dataset;
+  SupportedProduct products;
+
+  const char* name() const;
+  const char* expiration() const;
+  nsresult label_id(const char* label, uint32_t* labelId) const;
+  bool allows_key(const nsACString& key) const;
+  bool is_single_store() const;
+};
+
+// Structs used to keep information about the histograms for which a
+// snapshot should be created.
+struct HistogramSnapshotData {
+  CopyableTArray<base::Histogram::Sample> mBucketRanges;
+  CopyableTArray<base::Histogram::Count> mBucketCounts;
+  int64_t mSampleSum;  // Same type as base::Histogram::SampleSet::sum_
+};
+
+struct HistogramSnapshotInfo {
+  HistogramSnapshotData data;
+  HistogramID histogramID;
+};
+
+typedef mozilla::Vector<HistogramSnapshotInfo> HistogramSnapshotsArray;
+typedef mozilla::Vector<HistogramSnapshotsArray> HistogramProcessSnapshotsArray;
+
+// The following is used to handle snapshot information for keyed histograms.
+typedef nsDataHashtable<nsCStringHashKey, HistogramSnapshotData>
+    KeyedHistogramSnapshotData;
+
+struct KeyedHistogramSnapshotInfo {
+  KeyedHistogramSnapshotData data;
+  HistogramID histogramId;
+};
+
+typedef mozilla::Vector<KeyedHistogramSnapshotInfo>
+    KeyedHistogramSnapshotsArray;
+typedef mozilla::Vector<KeyedHistogramSnapshotsArray>
+    KeyedHistogramProcessSnapshotsArray;
+
+/**
+ * A Histogram storage engine.
+ *
+ * Takes care of recording data into multiple stores if necessary.
+ */
+class Histogram {
+ public:
+  /*
+   * Create a new histogram instance from the given info.
+   *
+   * If the histogram is already expired, this does not allocate.
+   */
+  Histogram(HistogramID histogramId, const HistogramInfo& info, bool expired);
+  ~Histogram();
+
+  /**
+   * Add a sample to this histogram in all registered stores.
+   */
+  void Add(uint32_t sample);
+
+  /**
+   * Clear the named store for this histogram.
+   */
+  void Clear(const nsACString& store);
+
+  /**
+   * Get the histogram instance from the named store.
+   */
+  bool GetHistogram(const nsACString& store, base::Histogram** h);
+
+  bool IsExpired() const { return mIsExpired; }
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf);
+
+ private:
+  // String -> Histogram*
+  typedef nsClassHashtable<nsCStringHashKey, base::Histogram> HistogramStore;
+  HistogramStore mStorage;
+
+  // A valid pointer if this histogram belongs to only the main store
+  base::Histogram* mSingleStore;
+
+  // We don't track stores for expired histograms.
+  // We just store a single flag and all other operations become a no-op.
+  bool mIsExpired;
+};
+
+class KeyedHistogram {
+ public:
+  KeyedHistogram(HistogramID id, const HistogramInfo& info, bool expired);
+  ~KeyedHistogram();
+  nsresult GetHistogram(const nsCString& aStore, const nsCString& key,
+                        base::Histogram** histogram);
+  base::Histogram* GetHistogram(const nsCString& aStore, const nsCString& key);
+  uint32_t GetHistogramType() const { return mHistogramInfo.histogramType; }
+  nsresult GetKeys(const StaticMutexAutoLock& aLock, const nsCString& store,
+                   nsTArray<nsCString>& aKeys);
+  // Note: unlike other methods, GetJSSnapshot is thread safe.
+  nsresult GetJSSnapshot(JSContext* cx, JS::Handle<JSObject*> obj,
+                         const nsACString& aStore, bool clearSubsession);
+  nsresult GetSnapshot(const StaticMutexAutoLock& aLock,
+                       const nsACString& aStore,
+                       KeyedHistogramSnapshotData& aSnapshot,
+                       bool aClearSubsession);
+
+  nsresult Add(const nsCString& key, uint32_t aSample, ProcessID aProcessType);
+  void Clear(const nsACString& aStore);
+
+  HistogramID GetHistogramID() const { return mId; }
+
+  bool IsEmpty(const nsACString& aStore) const;
+
+  bool IsExpired() const { return mIsExpired; }
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf);
+
+ private:
+  typedef nsClassHashtable<nsCStringHashKey, base::Histogram>
+      KeyedHistogramMapType;
+  typedef nsClassHashtable<nsCStringHashKey, KeyedHistogramMapType>
+      StoreMapType;
+
+  StoreMapType mStorage;
+  // A valid map if this histogram belongs to only the main store
+  KeyedHistogramMapType* mSingleStore;
+
+  const HistogramID mId;
+  const HistogramInfo& mHistogramInfo;
+  bool mIsExpired;
+};
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE STATE, SHARED BY ALL THREADS
+
+namespace {
+
+// Set to true once this global state has been initialized
+bool gInitDone = false;
+
+// Whether we are collecting the base, opt-out, Histogram data.
+bool gCanRecordBase = false;
+// Whether we are collecting the extended, opt-in, Histogram data.
+bool gCanRecordExtended = false;
+
+// The storage for actual Histogram instances.
+// We use separate ones for plain and keyed histograms.
+Histogram** gHistogramStorage;
+// Keyed histograms internally map string keys to individual Histogram
+// instances.
+KeyedHistogram** gKeyedHistogramStorage;
+
+// To simplify logic below we use a single histogram instance for all expired
+// histograms.
+Histogram* gExpiredHistogram = nullptr;
+
+// The single placeholder for expired keyed histograms.
+KeyedHistogram* gExpiredKeyedHistogram = nullptr;
+
+// This tracks whether recording is enabled for specific histograms.
+// To utilize C++ initialization rules, we invert the meaning to "disabled".
+bool gHistogramRecordingDisabled[HistogramCount] = {};
+
+// This is for gHistogramInfos, gHistogramStringTable
+#include "TelemetryHistogramData.inc"
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE CONSTANTS
+
+namespace {
+
+// List of histogram IDs which should have recording disabled initially.
+const HistogramID kRecordingInitiallyDisabledIDs[] = {
+    mozilla::Telemetry::FX_REFRESH_DRIVER_SYNC_SCROLL_FRAME_DELAY_MS,
+
+    // The array must not be empty. Leave these item here.
+    mozilla::Telemetry::TELEMETRY_TEST_COUNT_INIT_NO_RECORD,
+    mozilla::Telemetry::TELEMETRY_TEST_KEYED_COUNT_INIT_NO_RECORD};
+
+const char* TEST_HISTOGRAM_PREFIX = "TELEMETRY_TEST_";
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// The core storage access functions.
+// They wrap access to the histogram storage and lookup caches.
+
+namespace {
+
+size_t internal_KeyedHistogramStorageIndex(HistogramID aHistogramId,
+                                           ProcessID aProcessId) {
+  return aHistogramId * size_t(ProcessID::Count) + size_t(aProcessId);
+}
+
+size_t internal_HistogramStorageIndex(const StaticMutexAutoLock& aLock,
+                                      HistogramID aHistogramId,
+                                      ProcessID aProcessId) {
+  static_assert(HistogramCount < std::numeric_limits<size_t>::max() /
+                                     size_t(ProcessID::Count),
+                "Too many histograms and processes to store in a 1D array.");
+
+  return aHistogramId * size_t(ProcessID::Count) + size_t(aProcessId);
+}
+
+Histogram* internal_GetHistogramFromStorage(const StaticMutexAutoLock& aLock,
+                                            HistogramID aHistogramId,
+                                            ProcessID aProcessId) {
+  size_t index =
+      internal_HistogramStorageIndex(aLock, aHistogramId, aProcessId);
+  return gHistogramStorage[index];
+}
+
+void internal_SetHistogramInStorage(const StaticMutexAutoLock& aLock,
+                                    HistogramID aHistogramId,
+                                    ProcessID aProcessId,
+                                    Histogram* aHistogram) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Histograms are stored only in the parent process.");
+
+  size_t index =
+      internal_HistogramStorageIndex(aLock, aHistogramId, aProcessId);
+  MOZ_ASSERT(!gHistogramStorage[index],
+             "Mustn't overwrite storage without clearing it first.");
+  gHistogramStorage[index] = aHistogram;
+}
+
+KeyedHistogram* internal_GetKeyedHistogramFromStorage(HistogramID aHistogramId,
+                                                      ProcessID aProcessId) {
+  size_t index = internal_KeyedHistogramStorageIndex(aHistogramId, aProcessId);
+  return gKeyedHistogramStorage[index];
+}
+
+void internal_SetKeyedHistogramInStorage(HistogramID aHistogramId,
+                                         ProcessID aProcessId,
+                                         KeyedHistogram* aKeyedHistogram) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Keyed Histograms are stored only in the parent process.");
+
+  size_t index = internal_KeyedHistogramStorageIndex(aHistogramId, aProcessId);
+  MOZ_ASSERT(!gKeyedHistogramStorage[index],
+             "Mustn't overwrite storage without clearing it first");
+  gKeyedHistogramStorage[index] = aKeyedHistogram;
+}
+
+// Factory function for base::Histogram instances.
+base::Histogram* internal_CreateBaseHistogramInstance(const HistogramInfo& info,
+                                                      int bucketsOffset);
+
+// Factory function for histogram instances.
+Histogram* internal_CreateHistogramInstance(HistogramID histogramId);
+
+bool internal_IsHistogramEnumId(HistogramID aID) {
+  static_assert(((HistogramID)-1 > 0), "ID should be unsigned.");
+  return aID < HistogramCount;
+}
+
+// Look up a plain histogram by id.
+Histogram* internal_GetHistogramById(const StaticMutexAutoLock& aLock,
+                                     HistogramID histogramId,
+                                     ProcessID processId,
+                                     bool instantiate = true) {
+  MOZ_ASSERT(internal_IsHistogramEnumId(histogramId));
+  MOZ_ASSERT(!gHistogramInfos[histogramId].keyed);
+  MOZ_ASSERT(processId < ProcessID::Count);
+
+  Histogram* h =
+      internal_GetHistogramFromStorage(aLock, histogramId, processId);
+  if (h || !instantiate) {
+    return h;
+  }
+
+  h = internal_CreateHistogramInstance(histogramId);
+  MOZ_ASSERT(h);
+  internal_SetHistogramInStorage(aLock, histogramId, processId, h);
+
+  return h;
+}
+
+// Look up a keyed histogram by id.
+KeyedHistogram* internal_GetKeyedHistogramById(HistogramID histogramId,
+                                               ProcessID processId,
+                                               bool instantiate = true) {
+  MOZ_ASSERT(internal_IsHistogramEnumId(histogramId));
+  MOZ_ASSERT(gHistogramInfos[histogramId].keyed);
+  MOZ_ASSERT(processId < ProcessID::Count);
+
+  KeyedHistogram* kh =
+      internal_GetKeyedHistogramFromStorage(histogramId, processId);
+  if (kh || !instantiate) {
+    return kh;
+  }
+
+  const HistogramInfo& info = gHistogramInfos[histogramId];
+  const bool isExpired = IsExpiredVersion(info.expiration());
+
+  // If the keyed histogram is expired, set its storage to the expired
+  // keyed histogram.
+  if (isExpired) {
+    if (!gExpiredKeyedHistogram) {
+      // If we don't have an expired keyed histogram, create one.
+      gExpiredKeyedHistogram =
+          new KeyedHistogram(histogramId, info, true /* expired */);
+      MOZ_ASSERT(gExpiredKeyedHistogram);
+    }
+    kh = gExpiredKeyedHistogram;
+  } else {
+    kh = new KeyedHistogram(histogramId, info, false /* expired */);
+  }
+
+  internal_SetKeyedHistogramInStorage(histogramId, processId, kh);
+
+  return kh;
+}
+
+// Look up a histogram id from a histogram name.
+nsresult internal_GetHistogramIdByName(const StaticMutexAutoLock& aLock,
+                                       const nsACString& name,
+                                       HistogramID* id) {
+  const uint32_t idx = HistogramIDByNameLookup(name);
+  MOZ_ASSERT(idx < HistogramCount,
+             "Intermediate lookup should always give a valid index.");
+
+  // The lookup hashes the input and uses it as an index into the value array.
+  // Hash collisions can still happen for unknown values,
+  // therefore we check that the name matches.
+  if (name.Equals(gHistogramInfos[idx].name())) {
+    *id = HistogramID(idx);
+    return NS_OK;
+  }
+
+  return NS_ERROR_ILLEGAL_VALUE;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: Misc small helpers
+
+namespace {
+
+bool internal_CanRecordBase() { return gCanRecordBase; }
+
+bool internal_CanRecordExtended() { return gCanRecordExtended; }
+
+bool internal_AttemptedGPUProcess() {
+  // Check if it was tried to launch a process.
+  bool attemptedGPUProcess = false;
+  if (auto gpm = mozilla::gfx::GPUProcessManager::Get()) {
+    attemptedGPUProcess = gpm->AttemptedGPUProcess();
+  }
+  return attemptedGPUProcess;
+}
+
+// Note: this is completely unrelated to mozilla::IsEmpty.
+bool internal_IsEmpty(const StaticMutexAutoLock& aLock,
+                      const base::Histogram* h) {
+  return h->is_empty();
+}
+
+void internal_SetHistogramRecordingEnabled(const StaticMutexAutoLock& aLock,
+                                           HistogramID id, bool aEnabled) {
+  MOZ_ASSERT(internal_IsHistogramEnumId(id));
+  gHistogramRecordingDisabled[id] = !aEnabled;
+}
+
+bool internal_IsRecordingEnabled(HistogramID id) {
+  MOZ_ASSERT(internal_IsHistogramEnumId(id));
+  return !gHistogramRecordingDisabled[id];
+}
+
+const char* HistogramInfo::name() const {
+  return &gHistogramStringTable[this->name_offset];
+}
+
+const char* HistogramInfo::expiration() const {
+  return &gHistogramStringTable[this->expiration_offset];
+}
+
+nsresult HistogramInfo::label_id(const char* label, uint32_t* labelId) const {
+  MOZ_ASSERT(label);
+  MOZ_ASSERT(this->histogramType == nsITelemetry::HISTOGRAM_CATEGORICAL);
+  if (this->histogramType != nsITelemetry::HISTOGRAM_CATEGORICAL) {
+    return NS_ERROR_FAILURE;
+  }
+
+  for (uint32_t i = 0; i < this->label_count; ++i) {
+    // gHistogramLabelTable contains the indices of the label strings in the
+    // gHistogramStringTable.
+    // They are stored in-order and consecutively, from the offset label_index
+    // to (label_index + label_count).
+    uint32_t string_offset = gHistogramLabelTable[this->label_index + i];
+    const char* const str = &gHistogramStringTable[string_offset];
+    if (::strcmp(label, str) == 0) {
+      *labelId = i;
+      return NS_OK;
+    }
+  }
+
+  return NS_ERROR_FAILURE;
+}
+
+bool HistogramInfo::allows_key(const nsACString& key) const {
+  MOZ_ASSERT(this->keyed);
+
+  // If we didn't specify a list of allowed keys, just return true.
+  if (this->key_count == 0) {
+    return true;
+  }
+
+  // Otherwise, check if |key| is in the list of allowed keys.
+  for (uint32_t i = 0; i < this->key_count; ++i) {
+    // gHistogramKeyTable contains the indices of the key strings in the
+    // gHistogramStringTable. They are stored in-order and consecutively,
+    // from the offset key_index to (key_index + key_count).
+    uint32_t string_offset = gHistogramKeyTable[this->key_index + i];
+    const char* const str = &gHistogramStringTable[string_offset];
+    if (key.EqualsASCII(str)) {
+      return true;
+    }
+  }
+
+  // |key| was not found.
+  return false;
+}
+
+bool HistogramInfo::is_single_store() const {
+  return store_count == 1 && store_index == UINT16_MAX;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: Histogram Get, Add, Clone, Clear functions
+
+namespace {
+
+nsresult internal_CheckHistogramArguments(const HistogramInfo& info) {
+  if (info.histogramType != nsITelemetry::HISTOGRAM_BOOLEAN &&
+      info.histogramType != nsITelemetry::HISTOGRAM_FLAG &&
+      info.histogramType != nsITelemetry::HISTOGRAM_COUNT) {
+    // Sanity checks for histogram parameters.
+    if (info.min >= info.max) {
+      return NS_ERROR_ILLEGAL_VALUE;
+    }
+
+    if (info.bucketCount <= 2) {
+      return NS_ERROR_ILLEGAL_VALUE;
+    }
+
+    if (info.min < 1) {
+      return NS_ERROR_ILLEGAL_VALUE;
+    }
+  }
+
+  return NS_OK;
+}
+
+Histogram* internal_CreateHistogramInstance(HistogramID histogramId) {
+  const HistogramInfo& info = gHistogramInfos[histogramId];
+
+  if (NS_FAILED(internal_CheckHistogramArguments(info))) {
+    MOZ_ASSERT(false, "Failed histogram argument checks.");
+    return nullptr;
+  }
+
+  const bool isExpired = IsExpiredVersion(info.expiration());
+
+  if (isExpired) {
+    if (!gExpiredHistogram) {
+      gExpiredHistogram = new Histogram(histogramId, info, /* expired */ true);
+    }
+
+    return gExpiredHistogram;
+  }
+
+  Histogram* wrapper = new Histogram(histogramId, info, /* expired */ false);
+
+  return wrapper;
+}
+
+base::Histogram* internal_CreateBaseHistogramInstance(
+    const HistogramInfo& passedInfo, int bucketsOffset) {
+  if (NS_FAILED(internal_CheckHistogramArguments(passedInfo))) {
+    MOZ_ASSERT(false, "Failed histogram argument checks.");
+    return nullptr;
+  }
+
+  // We don't actually store data for expired histograms at all.
+  MOZ_ASSERT(!IsExpiredVersion(passedInfo.expiration()));
+
+  HistogramInfo info = passedInfo;
+  const int* buckets = &gHistogramBucketLowerBounds[bucketsOffset];
+
+  base::Histogram::Flags flags = base::Histogram::kNoFlags;
+  base::Histogram* h = nullptr;
+  switch (info.histogramType) {
+    case nsITelemetry::HISTOGRAM_EXPONENTIAL:
+      h = base::Histogram::FactoryGet(info.min, info.max, info.bucketCount,
+                                      flags, buckets);
+      break;
+    case nsITelemetry::HISTOGRAM_LINEAR:
+    case nsITelemetry::HISTOGRAM_CATEGORICAL:
+      h = LinearHistogram::FactoryGet(info.min, info.max, info.bucketCount,
+                                      flags, buckets);
+      break;
+    case nsITelemetry::HISTOGRAM_BOOLEAN:
+      h = BooleanHistogram::FactoryGet(flags, buckets);
+      break;
+    case nsITelemetry::HISTOGRAM_FLAG:
+      h = FlagHistogram::FactoryGet(flags, buckets);
+      break;
+    case nsITelemetry::HISTOGRAM_COUNT:
+      h = CountHistogram::FactoryGet(flags, buckets);
+      break;
+    default:
+      MOZ_ASSERT(false, "Invalid histogram type");
+      return nullptr;
+  }
+
+  return h;
+}
+
+nsresult internal_HistogramAdd(const StaticMutexAutoLock& aLock,
+                               Histogram& histogram, const HistogramID id,
+                               uint32_t value, ProcessID aProcessType) {
+  // Check if we are allowed to record the data.
+  bool canRecordDataset =
+      CanRecordDataset(gHistogramInfos[id].dataset, internal_CanRecordBase(),
+                       internal_CanRecordExtended());
+  // If `histogram` is a non-parent-process histogram, then recording-enabled
+  // has been checked in its owner process.
+  if (!canRecordDataset ||
+      (aProcessType == ProcessID::Parent && !internal_IsRecordingEnabled(id))) {
+    return NS_OK;
+  }
+
+  // Don't record if the current platform is not enabled
+  if (!CanRecordProduct(gHistogramInfos[id].products)) {
+    return NS_OK;
+  }
+
+  if (&histogram != gExpiredHistogram &&
+      GetCurrentProduct() == SupportedProduct::GeckoviewStreaming) {
+    const HistogramInfo& info = gHistogramInfos[id];
+    GeckoViewStreamingTelemetry::HistogramAccumulate(
+        nsDependentCString(info.name()),
+        info.histogramType == nsITelemetry::HISTOGRAM_CATEGORICAL, value);
+    return NS_OK;
+  }
+
+  // The internal representation of a base::Histogram's buckets uses `int`.
+  // Clamp large values of `value` to be INT_MAX so they continue to be treated
+  // as large values (instead of negative ones).
+  if (value > INT_MAX) {
+    TelemetryScalar::Add(
+        mozilla::Telemetry::ScalarID::TELEMETRY_ACCUMULATE_CLAMPED_VALUES,
+        NS_ConvertASCIItoUTF16(gHistogramInfos[id].name()), 1);
+    value = INT_MAX;
+  }
+
+  histogram.Add(value);
+
+  return NS_OK;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: Histogram reflection helpers
+
+namespace {
+
+/**
+ * Copy histograms and samples to Mozilla-friendly structures.
+ * Please note that this version does not make use of JS contexts.
+ *
+ * @param {StaticMutexAutoLock} the proof we hold the mutex.
+ * @param {Histogram} the histogram to reflect.
+ * @return {nsresult} NS_ERROR_FAILURE if we fail to allocate memory for the
+ *                    snapshot.
+ */
+nsresult internal_GetHistogramAndSamples(const StaticMutexAutoLock& aLock,
+                                         const base::Histogram* h,
+                                         HistogramSnapshotData& aSnapshot) {
+  MOZ_ASSERT(h);
+
+  // Convert the ranges of the buckets to a nsTArray.
+  const size_t bucketCount = h->bucket_count();
+  for (size_t i = 0; i < bucketCount; i++) {
+    // XXX(Bug 1631371) Check if this should use a fallible operation as it
+    // pretended earlier, or change the return type to void.
+    aSnapshot.mBucketRanges.AppendElement(h->ranges(i));
+  }
+
+  // Get a snapshot of the samples.
+  base::Histogram::SampleSet ss = h->SnapshotSample();
+
+  // Get the number of samples in each bucket.
+  for (size_t i = 0; i < bucketCount; i++) {
+    // XXX(Bug 1631371) Check if this should use a fallible operation as it
+    // pretended earlier, or change the return type to void.
+    aSnapshot.mBucketCounts.AppendElement(ss.counts(i));
+  }
+
+  // Finally, save the |sum|. We don't need to reflect declared_min,
+  // declared_max and histogram_type as they are in gHistogramInfo.
+  aSnapshot.mSampleSum = ss.sum();
+  return NS_OK;
+}
+
+/**
+ * Reflect a histogram snapshot into a JavaScript object.
+ * The returned histogram object will have the following properties:
+ *
+ *   bucket_count - Number of buckets of this histogram
+ *   histogram_type - HISTOGRAM_EXPONENTIAL, HISTOGRAM_LINEAR,
+ * HISTOGRAM_BOOLEAN, HISTOGRAM_FLAG, HISTOGRAM_COUNT, or HISTOGRAM_CATEGORICAL
+ *   sum - sum of the bucket contents
+ *   range - A 2-item array of minimum and maximum bucket size
+ *   values - Map from bucket start to the bucket's count
+ */
+nsresult internal_ReflectHistogramAndSamples(
+    JSContext* cx, JS::Handle<JSObject*> obj,
+    const HistogramInfo& aHistogramInfo,
+    const HistogramSnapshotData& aSnapshot) {
+  if (!(JS_DefineProperty(cx, obj, "bucket_count", aHistogramInfo.bucketCount,
+                          JSPROP_ENUMERATE) &&
+        JS_DefineProperty(cx, obj, "histogram_type",
+                          aHistogramInfo.histogramType, JSPROP_ENUMERATE) &&
+        JS_DefineProperty(cx, obj, "sum", double(aSnapshot.mSampleSum),
+                          JSPROP_ENUMERATE))) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Don't rely on the bucket counts from "aHistogramInfo": it may
+  // differ from the length of aSnapshot.mBucketCounts due to expired
+  // histograms.
+  const size_t count = aSnapshot.mBucketCounts.Length();
+  MOZ_ASSERT(count == aSnapshot.mBucketRanges.Length(),
+             "The number of buckets and the number of counts must match.");
+
+  // Create the "range" property and add it to the final object.
+  JS::Rooted<JSObject*> rarray(cx, JS::NewArrayObject(cx, 2));
+  if (rarray == nullptr ||
+      !JS_DefineProperty(cx, obj, "range", rarray, JSPROP_ENUMERATE)) {
+    return NS_ERROR_FAILURE;
+  }
+  // Add [min, max] into the range array
+  if (!JS_DefineElement(cx, rarray, 0, aHistogramInfo.min, JSPROP_ENUMERATE)) {
+    return NS_ERROR_FAILURE;
+  }
+  if (!JS_DefineElement(cx, rarray, 1, aHistogramInfo.max, JSPROP_ENUMERATE)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  JS::Rooted<JSObject*> values(cx, JS_NewPlainObject(cx));
+  if (values == nullptr ||
+      !JS_DefineProperty(cx, obj, "values", values, JSPROP_ENUMERATE)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  bool first = true;
+  size_t last = 0;
+
+  for (size_t i = 0; i < count; i++) {
+    auto value = aSnapshot.mBucketCounts[i];
+    if (value == 0) {
+      continue;
+    }
+
+    if (i > 0 && first) {
+      auto range = aSnapshot.mBucketRanges[i - 1];
+      if (!JS_DefineProperty(cx, values, nsPrintfCString("%d", range).get(), 0,
+                             JSPROP_ENUMERATE)) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+
+    first = false;
+    last = i + 1;
+
+    auto range = aSnapshot.mBucketRanges[i];
+    if (!JS_DefineProperty(cx, values, nsPrintfCString("%d", range).get(),
+                           value, JSPROP_ENUMERATE)) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  if (last > 0 && last < count) {
+    auto range = aSnapshot.mBucketRanges[last];
+    if (!JS_DefineProperty(cx, values, nsPrintfCString("%d", range).get(), 0,
+                           JSPROP_ENUMERATE)) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  return NS_OK;
+}
+
+bool internal_ShouldReflectHistogram(const StaticMutexAutoLock& aLock,
+                                     base::Histogram* h, HistogramID id) {
+  // Only flag histograms are serialized when they are empty.
+  // This has historical reasons, changing this will require downstream changes.
+  // The cheaper path here is to just deprecate flag histograms in favor
+  // of scalars.
+  uint32_t type = gHistogramInfos[id].histogramType;
+  if (internal_IsEmpty(aLock, h) && type != nsITelemetry::HISTOGRAM_FLAG) {
+    return false;
+  }
+
+  // Don't reflect the histogram if it's not allowed in this product.
+  if (!CanRecordProduct(gHistogramInfos[id].products)) {
+    return false;
+  }
+
+  return true;
+}
+
+/**
+ * Helper function to get a snapshot of the histograms.
+ *
+ * @param {aLock} the lock proof.
+ * @param {aStore} the name of the store to snapshot.
+ * @param {aDataset} the dataset for which the snapshot is being requested.
+ * @param {aClearSubsession} whether or not to clear the data after
+ *        taking the snapshot.
+ * @param {aIncludeGPU} whether or not to include data for the GPU.
+ * @param {aOutSnapshot} the container in which the snapshot data will be
+ *                       stored.
+ * @return {nsresult} NS_OK if the snapshot was successfully taken or
+ *         NS_ERROR_OUT_OF_MEMORY if it failed to allocate memory.
+ */
+nsresult internal_GetHistogramsSnapshot(
+    const StaticMutexAutoLock& aLock, const nsACString& aStore,
+    unsigned int aDataset, bool aClearSubsession, bool aIncludeGPU,
+    bool aFilterTest, HistogramProcessSnapshotsArray& aOutSnapshot) {
+  if (!aOutSnapshot.resize(static_cast<uint32_t>(ProcessID::Count))) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  for (uint32_t process = 0; process < static_cast<uint32_t>(ProcessID::Count);
+       ++process) {
+    HistogramSnapshotsArray& hArray = aOutSnapshot[process];
+
+    for (size_t i = 0; i < HistogramCount; ++i) {
+      const HistogramInfo& info = gHistogramInfos[i];
+      if (info.keyed) {
+        continue;
+      }
+
+      HistogramID id = HistogramID(i);
+
+      if (!CanRecordInProcess(info.record_in_processes, ProcessID(process)) ||
+          ((ProcessID(process) == ProcessID::Gpu) && !aIncludeGPU)) {
+        continue;
+      }
+
+      if (!IsInDataset(info.dataset, aDataset)) {
+        continue;
+      }
+
+      bool shouldInstantiate =
+          info.histogramType == nsITelemetry::HISTOGRAM_FLAG;
+      Histogram* w = internal_GetHistogramById(aLock, id, ProcessID(process),
+                                               shouldInstantiate);
+      if (!w || w->IsExpired()) {
+        continue;
+      }
+
+      base::Histogram* h = nullptr;
+      if (!w->GetHistogram(aStore, &h)) {
+        continue;
+      }
+
+      if (!internal_ShouldReflectHistogram(aLock, h, id)) {
+        continue;
+      }
+
+      const char* name = info.name();
+      if (aFilterTest && strncmp(TEST_HISTOGRAM_PREFIX, name,
+                                 strlen(TEST_HISTOGRAM_PREFIX)) == 0) {
+        if (aClearSubsession) {
+          h->Clear();
+        }
+        continue;
+      }
+
+      HistogramSnapshotData snapshotData;
+      if (NS_FAILED(internal_GetHistogramAndSamples(aLock, h, snapshotData))) {
+        continue;
+      }
+
+      if (!hArray.emplaceBack(HistogramSnapshotInfo{snapshotData, id})) {
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+
+      if (aClearSubsession) {
+        h->Clear();
+      }
+    }
+  }
+  return NS_OK;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: class Histogram
+
+namespace {
+
+Histogram::Histogram(HistogramID histogramId, const HistogramInfo& info,
+                     bool expired)
+    : mStorage(), mSingleStore(nullptr), mIsExpired(expired) {
+  if (IsExpired()) {
+    return;
+  }
+
+  base::Histogram* h;
+  const int bucketsOffset = gHistogramBucketLowerBoundIndex[histogramId];
+
+  if (info.is_single_store()) {
+    mSingleStore = internal_CreateBaseHistogramInstance(info, bucketsOffset);
+  } else {
+    for (uint32_t i = 0; i < info.store_count; i++) {
+      auto store = nsDependentCString(
+          &gHistogramStringTable[gHistogramStoresTable[info.store_index + i]]);
+      h = internal_CreateBaseHistogramInstance(info, bucketsOffset);
+      mStorage.Put(store, h);
+    }
+  }
+}
+
+Histogram::~Histogram() { delete mSingleStore; }
+
+void Histogram::Add(uint32_t sample) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only add to histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return;
+  }
+
+  if (IsExpired()) {
+    return;
+  }
+
+  if (mSingleStore != nullptr) {
+    mSingleStore->Add(sample);
+  } else {
+    for (auto iter = mStorage.Iter(); !iter.Done(); iter.Next()) {
+      auto& h = iter.Data();
+      h->Add(sample);
+    }
+  }
+}
+
+void Histogram::Clear(const nsACString& store) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only clear histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return;
+  }
+
+  if (mSingleStore != nullptr) {
+    if (store.EqualsASCII("main")) {
+      mSingleStore->Clear();
+    }
+  } else {
+    base::Histogram* h = nullptr;
+    bool found = GetHistogram(store, &h);
+    if (!found) {
+      return;
+    }
+    MOZ_ASSERT(h, "Should have found a valid histogram in the named store");
+
+    h->Clear();
+  }
+}
+
+bool Histogram::GetHistogram(const nsACString& store, base::Histogram** h) {
+  MOZ_ASSERT(!IsExpired());
+  if (IsExpired()) {
+    return false;
+  }
+
+  if (mSingleStore != nullptr) {
+    if (store.EqualsASCII("main")) {
+      *h = mSingleStore;
+      return true;
+    }
+
+    return false;
+  }
+
+  return mStorage.Get(store, h);
+}
+
+size_t Histogram::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) {
+  size_t n = 0;
+  n += aMallocSizeOf(this);
+  /*
+   * In theory mStorage.SizeOfExcludingThis should included the data part of the
+   * map, but the numbers seemed low, so we are only taking the shallow size and
+   * do the iteration here.
+   */
+  n += mStorage.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  for (auto iter = mStorage.Iter(); !iter.Done(); iter.Next()) {
+    auto& h = iter.Data();
+    n += h->SizeOfIncludingThis(aMallocSizeOf);
+  }
+  if (mSingleStore != nullptr) {
+    // base::Histogram doesn't have SizeOfExcludingThis, so we are overcounting
+    // the pointer here.
+    n += mSingleStore->SizeOfIncludingThis(aMallocSizeOf);
+  }
+  return n;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: class KeyedHistogram and internal_ReflectKeyedHistogram
+
+namespace {
+
+nsresult internal_ReflectKeyedHistogram(
+    const KeyedHistogramSnapshotData& aSnapshot, const HistogramInfo& info,
+    JSContext* aCx, JS::Handle<JSObject*> aObj) {
+  for (auto iter = aSnapshot.ConstIter(); !iter.Done(); iter.Next()) {
+    HistogramSnapshotData& keyData = iter.Data();
+
+    JS::RootedObject histogramSnapshot(aCx, JS_NewPlainObject(aCx));
+    if (!histogramSnapshot) {
+      return NS_ERROR_FAILURE;
+    }
+
+    if (NS_FAILED(internal_ReflectHistogramAndSamples(aCx, histogramSnapshot,
+                                                      info, keyData))) {
+      return NS_ERROR_FAILURE;
+    }
+
+    const NS_ConvertUTF8toUTF16 key(iter.Key());
+    if (!JS_DefineUCProperty(aCx, aObj, key.Data(), key.Length(),
+                             histogramSnapshot, JSPROP_ENUMERATE)) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  return NS_OK;
+}
+
+KeyedHistogram::KeyedHistogram(HistogramID id, const HistogramInfo& info,
+                               bool expired)
+    : mStorage(),
+      mSingleStore(nullptr),
+      mId(id),
+      mHistogramInfo(info),
+      mIsExpired(expired) {
+  if (IsExpired()) {
+    return;
+  }
+
+  if (info.is_single_store()) {
+    mSingleStore = new KeyedHistogramMapType;
+  } else {
+    for (uint32_t i = 0; i < info.store_count; i++) {
+      auto store = nsDependentCString(
+          &gHistogramStringTable[gHistogramStoresTable[info.store_index + i]]);
+      mStorage.Put(store, new KeyedHistogramMapType);
+    }
+  }
+}
+
+KeyedHistogram::~KeyedHistogram() { delete mSingleStore; }
+
+nsresult KeyedHistogram::GetHistogram(const nsCString& aStore,
+                                      const nsCString& key,
+                                      base::Histogram** histogram) {
+  if (IsExpired()) {
+    MOZ_ASSERT(false,
+               "KeyedHistogram::GetHistogram called on an expired histogram.");
+    return NS_ERROR_FAILURE;
+  }
+
+  KeyedHistogramMapType* histogramMap;
+  bool found;
+
+  if (mSingleStore != nullptr) {
+    histogramMap = mSingleStore;
+  } else {
+    found = mStorage.Get(aStore, &histogramMap);
+    if (!found) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  found = histogramMap->Get(key, histogram);
+  if (found) {
+    return NS_OK;
+  }
+
+  int bucketsOffset = gHistogramBucketLowerBoundIndex[mId];
+  base::Histogram* h =
+      internal_CreateBaseHistogramInstance(mHistogramInfo, bucketsOffset);
+  if (!h) {
+    return NS_ERROR_FAILURE;
+  }
+
+  h->ClearFlags(base::Histogram::kUmaTargetedHistogramFlag);
+  *histogram = h;
+
+  bool inserted = histogramMap->Put(key, h, mozilla::fallible);
+  if (MOZ_UNLIKELY(!inserted)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  return NS_OK;
+}
+
+base::Histogram* KeyedHistogram::GetHistogram(const nsCString& aStore,
+                                              const nsCString& key) {
+  base::Histogram* h = nullptr;
+  if (NS_FAILED(GetHistogram(aStore, key, &h))) {
+    return nullptr;
+  }
+  return h;
+}
+
+nsresult KeyedHistogram::Add(const nsCString& key, uint32_t sample,
+                             ProcessID aProcessType) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only add to keyed histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  bool canRecordDataset =
+      CanRecordDataset(mHistogramInfo.dataset, internal_CanRecordBase(),
+                       internal_CanRecordExtended());
+  // If `histogram` is a non-parent-process histogram, then recording-enabled
+  // has been checked in its owner process.
+  if (!canRecordDataset || (aProcessType == ProcessID::Parent &&
+                            !internal_IsRecordingEnabled(mId))) {
+    return NS_OK;
+  }
+
+  // Don't record if expired.
+  if (IsExpired()) {
+    return NS_OK;
+  }
+
+  // Don't record if the current platform is not enabled
+  if (!CanRecordProduct(gHistogramInfos[mId].products)) {
+    return NS_OK;
+  }
+
+  // The internal representation of a base::Histogram's buckets uses `int`.
+  // Clamp large values of `sample` to be INT_MAX so they continue to be treated
+  // as large values (instead of negative ones).
+  if (sample > INT_MAX) {
+    TelemetryScalar::Add(
+        mozilla::Telemetry::ScalarID::TELEMETRY_ACCUMULATE_CLAMPED_VALUES,
+        NS_ConvertASCIItoUTF16(mHistogramInfo.name()), 1);
+    sample = INT_MAX;
+  }
+
+  base::Histogram* histogram;
+  if (mSingleStore != nullptr) {
+    histogram = GetHistogram("main"_ns, key);
+    if (!histogram) {
+      MOZ_ASSERT(false, "Missing histogram in single store.");
+      return NS_ERROR_FAILURE;
+    }
+
+    histogram->Add(sample);
+  } else {
+    for (uint32_t i = 0; i < mHistogramInfo.store_count; i++) {
+      auto store = nsDependentCString(
+          &gHistogramStringTable
+              [gHistogramStoresTable[mHistogramInfo.store_index + i]]);
+      base::Histogram* histogram = GetHistogram(store, key);
+      MOZ_ASSERT(histogram);
+      if (histogram) {
+        histogram->Add(sample);
+      } else {
+        return NS_ERROR_FAILURE;
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+void KeyedHistogram::Clear(const nsACString& aStore) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only clear keyed histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return;
+  }
+
+  if (IsExpired()) {
+    return;
+  }
+
+  if (mSingleStore) {
+    if (aStore.EqualsASCII("main")) {
+      mSingleStore->Clear();
+    }
+    return;
+  }
+
+  KeyedHistogramMapType* histogramMap;
+  bool found = mStorage.Get(aStore, &histogramMap);
+  if (!found) {
+    return;
+  }
+
+  histogramMap->Clear();
+}
+
+bool KeyedHistogram::IsEmpty(const nsACString& aStore) const {
+  if (mSingleStore != nullptr) {
+    if (aStore.EqualsASCII("main")) {
+      return mSingleStore->IsEmpty();
+    }
+
+    return true;
+  }
+
+  KeyedHistogramMapType* histogramMap;
+  bool found = mStorage.Get(aStore, &histogramMap);
+  if (!found) {
+    return true;
+  }
+  return histogramMap->IsEmpty();
+}
+
+size_t KeyedHistogram::SizeOfIncludingThis(
+    mozilla::MallocSizeOf aMallocSizeOf) {
+  size_t n = 0;
+  n += aMallocSizeOf(this);
+  /*
+   * In theory mStorage.SizeOfExcludingThis should included the data part of the
+   * map, but the numbers seemed low, so we are only taking the shallow size and
+   * do the iteration here.
+   */
+  n += mStorage.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  for (auto iter = mStorage.Iter(); !iter.Done(); iter.Next()) {
+    auto& h = iter.Data();
+    n += h->SizeOfIncludingThis(aMallocSizeOf);
+  }
+  if (mSingleStore != nullptr) {
+    // base::Histogram doesn't have SizeOfExcludingThis, so we are overcounting
+    // the pointer here.
+    n += mSingleStore->SizeOfExcludingThis(aMallocSizeOf);
+  }
+  return n;
+}
+
+nsresult KeyedHistogram::GetKeys(const StaticMutexAutoLock& aLock,
+                                 const nsCString& store,
+                                 nsTArray<nsCString>& aKeys) {
+  KeyedHistogramMapType* histogramMap;
+  if (mSingleStore != nullptr) {
+    histogramMap = mSingleStore;
+  } else {
+    bool found = mStorage.Get(store, &histogramMap);
+    if (!found) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  if (!aKeys.SetCapacity(histogramMap->Count(), mozilla::fallible)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  for (auto iter = histogramMap->Iter(); !iter.Done(); iter.Next()) {
+    if (!aKeys.AppendElement(iter.Key(), mozilla::fallible)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult KeyedHistogram::GetJSSnapshot(JSContext* cx, JS::Handle<JSObject*> obj,
+                                       const nsACString& aStore,
+                                       bool clearSubsession) {
+  // Get a snapshot of the data.
+  KeyedHistogramSnapshotData dataSnapshot;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    MOZ_ASSERT(internal_IsHistogramEnumId(mId));
+
+    // Take a snapshot of the data here, protected by the lock, and then,
+    // outside of the lock protection, mirror it to a JS structure.
+    nsresult rv = GetSnapshot(locker, aStore, dataSnapshot, clearSubsession);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+  }
+
+  // Now that we have a copy of the data, mirror it to JS.
+  return internal_ReflectKeyedHistogram(dataSnapshot, gHistogramInfos[mId], cx,
+                                        obj);
+}
+
+/**
+ * Return a histogram snapshot for the named store.
+ *
+ * If getting the snapshot succeeds, NS_OK is returned and `aSnapshot` contains
+ * the snapshot data. If the histogram is not available in the named store,
+ * NS_ERROR_NO_CONTENT is returned. For other errors, NS_ERROR_FAILURE is
+ * returned.
+ */
+nsresult KeyedHistogram::GetSnapshot(const StaticMutexAutoLock& aLock,
+                                     const nsACString& aStore,
+                                     KeyedHistogramSnapshotData& aSnapshot,
+                                     bool aClearSubsession) {
+  KeyedHistogramMapType* histogramMap;
+  if (mSingleStore != nullptr) {
+    if (!aStore.EqualsASCII("main")) {
+      return NS_ERROR_NO_CONTENT;
+    }
+
+    histogramMap = mSingleStore;
+  } else {
+    bool found = mStorage.Get(aStore, &histogramMap);
+    if (!found) {
+      // Nothing in the main store is fine, it's just handled as empty
+      return NS_ERROR_NO_CONTENT;
+    }
+  }
+
+  // Snapshot every key.
+  for (auto iter = histogramMap->ConstIter(); !iter.Done(); iter.Next()) {
+    base::Histogram* keyData = iter.UserData();
+    if (!keyData) {
+      return NS_ERROR_FAILURE;
+    }
+
+    HistogramSnapshotData keySnapshot;
+    if (NS_FAILED(
+            internal_GetHistogramAndSamples(aLock, keyData, keySnapshot))) {
+      return NS_ERROR_FAILURE;
+    }
+
+    // Append to the final snapshot.
+    aSnapshot.Put(iter.Key(), std::move(keySnapshot));
+  }
+
+  if (aClearSubsession) {
+    Clear(aStore);
+  }
+
+  return NS_OK;
+}
+
+/**
+ * Helper function to get a snapshot of the keyed histograms.
+ *
+ * @param {aLock} the lock proof.
+ * @param {aDataset} the dataset for which the snapshot is being requested.
+ * @param {aClearSubsession} whether or not to clear the data after
+ *        taking the snapshot.
+ * @param {aIncludeGPU} whether or not to include data for the GPU.
+ * @param {aOutSnapshot} the container in which the snapshot data will be
+ *                       stored.
+ * @return {nsresult} NS_OK if the snapshot was successfully taken or
+ *         NS_ERROR_OUT_OF_MEMORY if it failed to allocate memory.
+ */
+nsresult internal_GetKeyedHistogramsSnapshot(
+    const StaticMutexAutoLock& aLock, const nsACString& aStore,
+    unsigned int aDataset, bool aClearSubsession, bool aIncludeGPU,
+    bool aFilterTest, KeyedHistogramProcessSnapshotsArray& aOutSnapshot) {
+  if (!aOutSnapshot.resize(static_cast<uint32_t>(ProcessID::Count))) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  for (uint32_t process = 0; process < static_cast<uint32_t>(ProcessID::Count);
+       ++process) {
+    KeyedHistogramSnapshotsArray& hArray = aOutSnapshot[process];
+
+    for (size_t i = 0; i < HistogramCount; ++i) {
+      HistogramID id = HistogramID(i);
+      const HistogramInfo& info = gHistogramInfos[id];
+      if (!info.keyed) {
+        continue;
+      }
+
+      if (!CanRecordInProcess(info.record_in_processes, ProcessID(process)) ||
+          ((ProcessID(process) == ProcessID::Gpu) && !aIncludeGPU)) {
+        continue;
+      }
+
+      if (!IsInDataset(info.dataset, aDataset)) {
+        continue;
+      }
+
+      KeyedHistogram* keyed =
+          internal_GetKeyedHistogramById(id, ProcessID(process),
+                                         /* instantiate = */ false);
+      if (!keyed || keyed->IsEmpty(aStore) || keyed->IsExpired()) {
+        continue;
+      }
+
+      const char* name = info.name();
+      if (aFilterTest && strncmp(TEST_HISTOGRAM_PREFIX, name,
+                                 strlen(TEST_HISTOGRAM_PREFIX)) == 0) {
+        if (aClearSubsession) {
+          keyed->Clear(aStore);
+        }
+        continue;
+      }
+
+      // Take a snapshot of the keyed histogram data!
+      KeyedHistogramSnapshotData snapshot;
+      if (!NS_SUCCEEDED(
+              keyed->GetSnapshot(aLock, aStore, snapshot, aClearSubsession))) {
+        return NS_ERROR_FAILURE;
+      }
+
+      if (!hArray.emplaceBack(
+              KeyedHistogramSnapshotInfo{std::move(snapshot), id})) {
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+    }
+  }
+  return NS_OK;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: thread-unsafe helpers for the external interface
+
+namespace {
+
+bool internal_RemoteAccumulate(const StaticMutexAutoLock& aLock,
+                               HistogramID aId, uint32_t aSample) {
+  if (XRE_IsParentProcess()) {
+    return false;
+  }
+
+  if (!internal_IsRecordingEnabled(aId)) {
+    return true;
+  }
+
+  TelemetryIPCAccumulator::AccumulateChildHistogram(aId, aSample);
+  return true;
+}
+
+bool internal_RemoteAccumulate(const StaticMutexAutoLock& aLock,
+                               HistogramID aId, const nsCString& aKey,
+                               uint32_t aSample) {
+  if (XRE_IsParentProcess()) {
+    return false;
+  }
+
+  if (!internal_IsRecordingEnabled(aId)) {
+    return true;
+  }
+
+  TelemetryIPCAccumulator::AccumulateChildKeyedHistogram(aId, aKey, aSample);
+  return true;
+}
+
+void internal_Accumulate(const StaticMutexAutoLock& aLock, HistogramID aId,
+                         uint32_t aSample) {
+  if (!internal_CanRecordBase() ||
+      internal_RemoteAccumulate(aLock, aId, aSample)) {
+    return;
+  }
+
+  Histogram* w = internal_GetHistogramById(aLock, aId, ProcessID::Parent);
+  MOZ_ASSERT(w);
+  internal_HistogramAdd(aLock, *w, aId, aSample, ProcessID::Parent);
+}
+
+void internal_Accumulate(const StaticMutexAutoLock& aLock, HistogramID aId,
+                         const nsCString& aKey, uint32_t aSample) {
+  if (!gInitDone || !internal_CanRecordBase() ||
+      internal_RemoteAccumulate(aLock, aId, aKey, aSample)) {
+    return;
+  }
+
+  KeyedHistogram* keyed =
+      internal_GetKeyedHistogramById(aId, ProcessID::Parent);
+  MOZ_ASSERT(keyed);
+  keyed->Add(aKey, aSample, ProcessID::Parent);
+}
+
+void internal_AccumulateChild(const StaticMutexAutoLock& aLock,
+                              ProcessID aProcessType, HistogramID aId,
+                              uint32_t aSample) {
+  if (!internal_CanRecordBase()) {
+    return;
+  }
+
+  Histogram* w = internal_GetHistogramById(aLock, aId, aProcessType);
+  if (w == nullptr) {
+    NS_WARNING("Failed GetHistogramById for CHILD");
+  } else {
+    internal_HistogramAdd(aLock, *w, aId, aSample, aProcessType);
+  }
+}
+
+void internal_AccumulateChildKeyed(const StaticMutexAutoLock& aLock,
+                                   ProcessID aProcessType, HistogramID aId,
+                                   const nsCString& aKey, uint32_t aSample) {
+  if (!gInitDone || !internal_CanRecordBase()) {
+    return;
+  }
+
+  KeyedHistogram* keyed = internal_GetKeyedHistogramById(aId, aProcessType);
+  MOZ_ASSERT(keyed);
+  keyed->Add(aKey, aSample, aProcessType);
+}
+
+void internal_ClearHistogram(const StaticMutexAutoLock& aLock, HistogramID id,
+                             const nsACString& aStore) {
+  MOZ_ASSERT(XRE_IsParentProcess());
+  if (!XRE_IsParentProcess()) {
+    return;
+  }
+
+  // Handle keyed histograms.
+  if (gHistogramInfos[id].keyed) {
+    for (uint32_t process = 0;
+         process < static_cast<uint32_t>(ProcessID::Count); ++process) {
+      KeyedHistogram* kh = internal_GetKeyedHistogramById(
+          id, static_cast<ProcessID>(process), /* instantiate = */ false);
+      if (kh) {
+        kh->Clear(aStore);
+      }
+    }
+  } else {
+    // Reset the histograms instances for all processes.
+    for (uint32_t process = 0;
+         process < static_cast<uint32_t>(ProcessID::Count); ++process) {
+      Histogram* h =
+          internal_GetHistogramById(aLock, id, static_cast<ProcessID>(process),
+                                    /* instantiate = */ false);
+      if (h) {
+        h->Clear(aStore);
+      }
+    }
+  }
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: JSHistogram_* functions
+
+// NOTE: the functions in this section:
+//
+//   internal_JSHistogram_Add
+//   internal_JSHistogram_Name
+//   internal_JSHistogram_Snapshot
+//   internal_JSHistogram_Clear
+//   internal_WrapAndReturnHistogram
+//
+// all run without protection from |gTelemetryHistogramMutex|.  If they
+// held |gTelemetryHistogramMutex|, there would be the possibility of
+// deadlock because the JS_ calls that they make may call back into the
+// TelemetryHistogram interface, hence trying to re-acquire the mutex.
+//
+// This means that these functions potentially race against threads, but
+// that seems preferable to risking deadlock.
+
+namespace {
+
+void internal_JSHistogram_finalize(JSFreeOp*, JSObject*);
+
+static const JSClassOps sJSHistogramClassOps = {nullptr, /* addProperty */
+                                                nullptr, /* delProperty */
+                                                nullptr, /* enumerate */
+                                                nullptr, /* newEnumerate */
+                                                nullptr, /* resolve */
+                                                nullptr, /* mayResolve */
+                                                internal_JSHistogram_finalize};
+
+static const JSClass sJSHistogramClass = {
+    "JSHistogram",                                     /* name */
+    JSCLASS_HAS_PRIVATE | JSCLASS_FOREGROUND_FINALIZE, /* flags */
+    &sJSHistogramClassOps};
+
+struct JSHistogramData {
+  HistogramID histogramId;
+};
+
+bool internal_JSHistogram_CoerceValue(JSContext* aCx,
+                                      JS::Handle<JS::Value> aElement,
+                                      HistogramID aId, uint32_t aHistogramType,
+                                      uint32_t& aValue) {
+  if (aElement.isString()) {
+    // Strings only allowed for categorical histograms
+    if (aHistogramType != nsITelemetry::HISTOGRAM_CATEGORICAL) {
+      LogToBrowserConsole(
+          nsIScriptError::errorFlag,
+          nsLiteralString(
+              u"String argument only allowed for categorical histogram"));
+      return false;
+    }
+
+    // Label is given by the string argument
+    nsAutoJSString label;
+    if (!label.init(aCx, aElement)) {
+      LogToBrowserConsole(nsIScriptError::errorFlag,
+                          u"Invalid string parameter"_ns);
+      return false;
+    }
+
+    // Get the label id for accumulation
+    nsresult rv = gHistogramInfos[aId].label_id(
+        NS_ConvertUTF16toUTF8(label).get(), &aValue);
+    if (NS_FAILED(rv)) {
+      nsPrintfCString msg("'%s' is an invalid string label",
+                          NS_ConvertUTF16toUTF8(label).get());
+      LogToBrowserConsole(nsIScriptError::errorFlag,
+                          NS_ConvertUTF8toUTF16(msg));
+      return false;
+    }
+  } else if (!(aElement.isNumber() || aElement.isBoolean())) {
+    LogToBrowserConsole(nsIScriptError::errorFlag, u"Argument not a number"_ns);
+    return false;
+  } else if (aElement.isNumber() && aElement.toNumber() > UINT32_MAX) {
+    // Clamp large numerical arguments to aValue's acceptable values.
+    // JS::ToUint32 will take aElement modulo 2^32 before returning it, which
+    // may result in a smaller final value.
+    aValue = UINT32_MAX;
+#ifdef DEBUG
+    LogToBrowserConsole(nsIScriptError::errorFlag,
+                        u"Clamped large numeric value"_ns);
+#endif
+  } else if (!JS::ToUint32(aCx, aElement, &aValue)) {
+    LogToBrowserConsole(nsIScriptError::errorFlag,
+                        u"Failed to convert element to UInt32"_ns);
+    return false;
+  }
+
+  // If we're here then all type checks have passed and aValue contains the
+  // coerced integer
+  return true;
+}
+
+bool internal_JSHistogram_GetValueArray(JSContext* aCx, JS::CallArgs& args,
+                                        uint32_t aHistogramType,
+                                        HistogramID aId, bool isKeyed,
+                                        nsTArray<uint32_t>& aArray) {
+  // This function populates aArray with the values extracted from args. Handles
+  // keyed and non-keyed histograms, and single and array of values. Also
+  // performs sanity checks on the arguments. Returns true upon successful
+  // population, false otherwise.
+
+  uint32_t firstArgIndex = 0;
+  if (isKeyed) {
+    firstArgIndex = 1;
+  }
+
+  // Special case of no argument (or only key) and count histogram
+  if (args.length() == firstArgIndex) {
+    if (!(aHistogramType == nsITelemetry::HISTOGRAM_COUNT)) {
+      LogToBrowserConsole(
+          nsIScriptError::errorFlag,
+          nsLiteralString(
+              u"Need at least one argument for non count type histogram"));
+      return false;
+    }
+
+    aArray.AppendElement(1);
+    return true;
+  }
+
+  if (args[firstArgIndex].isObject() && !args[firstArgIndex].isString()) {
+    JS::Rooted<JSObject*> arrayObj(aCx, &args[firstArgIndex].toObject());
+
+    bool isArray = false;
+    JS::IsArrayObject(aCx, arrayObj, &isArray);
+
+    if (!isArray) {
+      LogToBrowserConsole(
+          nsIScriptError::errorFlag,
+          nsLiteralString(
+              u"The argument to accumulate can't be a non-array object"));
+      return false;
+    }
+
+    uint32_t arrayLength = 0;
+    if (!JS::GetArrayLength(aCx, arrayObj, &arrayLength)) {
+      LogToBrowserConsole(nsIScriptError::errorFlag,
+                          u"Failed while trying to get array length"_ns);
+      return false;
+    }
+
+    for (uint32_t arrayIdx = 0; arrayIdx < arrayLength; arrayIdx++) {
+      JS::Rooted<JS::Value> element(aCx);
+
+      if (!JS_GetElement(aCx, arrayObj, arrayIdx, &element)) {
+        nsPrintfCString msg("Failed while trying to get element at index %d",
+                            arrayIdx);
+        LogToBrowserConsole(nsIScriptError::errorFlag,
+                            NS_ConvertUTF8toUTF16(msg));
+        return false;
+      }
+
+      uint32_t value = 0;
+      if (!internal_JSHistogram_CoerceValue(aCx, element, aId, aHistogramType,
+                                            value)) {
+        nsPrintfCString msg("Element at index %d failed type checks", arrayIdx);
+        LogToBrowserConsole(nsIScriptError::errorFlag,
+                            NS_ConvertUTF8toUTF16(msg));
+        return false;
+      }
+      aArray.AppendElement(value);
+    }
+
+    return true;
+  }
+
+  uint32_t value = 0;
+  if (!internal_JSHistogram_CoerceValue(aCx, args[firstArgIndex], aId,
+                                        aHistogramType, value)) {
+    return false;
+  }
+  aArray.AppendElement(value);
+  return true;
+}
+
+bool internal_JSHistogram_Add(JSContext* cx, unsigned argc, JS::Value* vp) {
+  JS::CallArgs args = CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+  MOZ_ASSERT(internal_IsHistogramEnumId(id));
+  uint32_t type = gHistogramInfos[id].histogramType;
+
+  // This function should always return |undefined| and never fail but
+  // rather report failures using the console.
+  args.rval().setUndefined();
+
+  nsTArray<uint32_t> values;
+  if (!internal_JSHistogram_GetValueArray(cx, args, type, id, false, values)) {
+    // Either GetValueArray or CoerceValue utility function will have printed a
+    // meaningful error message, so we simply return true
+    return true;
+  }
+
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    for (uint32_t aValue : values) {
+      internal_Accumulate(locker, id, aValue);
+    }
+  }
+  return true;
+}
+
+bool internal_JSHistogram_Name(JSContext* cx, unsigned argc, JS::Value* vp) {
+  JS::CallArgs args = CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+  MOZ_ASSERT(internal_IsHistogramEnumId(id));
+  const char* name = gHistogramInfos[id].name();
+
+  auto cname = NS_ConvertASCIItoUTF16(name);
+  args.rval().setString(ToJSString(cx, cname));
+
+  return true;
+}
+
+/**
+ * Extract the store name from JavaScript function arguments.
+ * The first and only argument needs to be an object with a "store" property.
+ * If no arguments are given it defaults to "main".
+ */
+nsresult internal_JS_StoreFromObjectArgument(JSContext* cx,
+                                             const JS::CallArgs& args,
+                                             nsAutoString& aStoreName) {
+  if (args.length() == 0) {
+    aStoreName.AssignLiteral("main");
+  } else if (args.length() == 1) {
+    if (!args[0].isObject()) {
+      JS_ReportErrorASCII(cx, "Expected object argument.");
+      return NS_ERROR_FAILURE;
+    }
+
+    JS::RootedValue storeValue(cx);
+    JS::RootedObject argsObject(cx, &args[0].toObject());
+    if (!JS_GetProperty(cx, argsObject, "store", &storeValue)) {
+      JS_ReportErrorASCII(cx,
+                          "Expected object argument to have property 'store'.");
+      return NS_ERROR_FAILURE;
+    }
+
+    nsAutoJSString store;
+    if (!storeValue.isString() || !store.init(cx, storeValue)) {
+      JS_ReportErrorASCII(
+          cx, "Expected object argument's 'store' property to be a string.");
+      return NS_ERROR_FAILURE;
+    }
+
+    aStoreName.Assign(store);
+  } else {
+    JS_ReportErrorASCII(cx, "Expected at most one argument.");
+    return NS_ERROR_FAILURE;
+  }
+
+  return NS_OK;
+}
+
+bool internal_JSHistogram_Snapshot(JSContext* cx, unsigned argc,
+                                   JS::Value* vp) {
+  JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+
+  if (!XRE_IsParentProcess()) {
+    JS_ReportErrorASCII(
+        cx, "Histograms can only be snapshotted in the parent process");
+    return false;
+  }
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+
+  nsAutoString storeName;
+  nsresult rv = internal_JS_StoreFromObjectArgument(cx, args, storeName);
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  HistogramSnapshotData dataSnapshot;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    MOZ_ASSERT(internal_IsHistogramEnumId(id));
+
+    // This is not good standard behavior given that we have histogram instances
+    // covering multiple processes.
+    // However, changing this requires some broader changes to callers.
+    Histogram* w = internal_GetHistogramById(locker, id, ProcessID::Parent);
+    base::Histogram* h = nullptr;
+    if (!w->GetHistogram(NS_ConvertUTF16toUTF8(storeName), &h)) {
+      // When it's not in the named store, let's skip the snapshot completely,
+      // but don't fail
+      args.rval().setUndefined();
+      return true;
+    }
+    // Take a snapshot of the data here, protected by the lock, and then,
+    // outside of the lock protection, mirror it to a JS structure
+    if (NS_FAILED(internal_GetHistogramAndSamples(locker, h, dataSnapshot))) {
+      return false;
+    }
+  }
+
+  JS::Rooted<JSObject*> snapshot(cx, JS_NewPlainObject(cx));
+  if (!snapshot) {
+    return false;
+  }
+
+  if (NS_FAILED(internal_ReflectHistogramAndSamples(
+          cx, snapshot, gHistogramInfos[id], dataSnapshot))) {
+    return false;
+  }
+
+  args.rval().setObject(*snapshot);
+  return true;
+}
+
+bool internal_JSHistogram_Clear(JSContext* cx, unsigned argc, JS::Value* vp) {
+  if (!XRE_IsParentProcess()) {
+    JS_ReportErrorASCII(cx,
+                        "Histograms can only be cleared in the parent process");
+    return false;
+  }
+
+  JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+
+  nsAutoString storeName;
+  nsresult rv = internal_JS_StoreFromObjectArgument(cx, args, storeName);
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  // This function should always return |undefined| and never fail but
+  // rather report failures using the console.
+  args.rval().setUndefined();
+
+  HistogramID id = data->histogramId;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+
+    MOZ_ASSERT(internal_IsHistogramEnumId(id));
+    internal_ClearHistogram(locker, id, NS_ConvertUTF16toUTF8(storeName));
+  }
+
+  return true;
+}
+
+// NOTE: Runs without protection from |gTelemetryHistogramMutex|.
+// See comment at the top of this section.
+nsresult internal_WrapAndReturnHistogram(HistogramID id, JSContext* cx,
+                                         JS::MutableHandle<JS::Value> ret) {
+  JS::Rooted<JSObject*> obj(cx, JS_NewObject(cx, &sJSHistogramClass));
+  if (!obj) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // The 3 functions that are wrapped up here are eventually called
+  // by the same thread that runs this function.
+  if (!(JS_DefineFunction(cx, obj, "add", internal_JSHistogram_Add, 1, 0) &&
+        JS_DefineFunction(cx, obj, "name", internal_JSHistogram_Name, 1, 0) &&
+        JS_DefineFunction(cx, obj, "snapshot", internal_JSHistogram_Snapshot, 1,
+                          0) &&
+        JS_DefineFunction(cx, obj, "clear", internal_JSHistogram_Clear, 1,
+                          0))) {
+    return NS_ERROR_FAILURE;
+  }
+
+  JSHistogramData* data = new JSHistogramData{id};
+  JS::SetPrivate(obj, data);
+  ret.setObject(*obj);
+
+  return NS_OK;
+}
+
+void internal_JSHistogram_finalize(JSFreeOp*, JSObject* obj) {
+  if (!obj || JS::GetClass(obj) != &sJSHistogramClass) {
+    MOZ_ASSERT_UNREACHABLE("Should have the right JS class.");
+    return;
+  }
+
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  delete data;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: JSKeyedHistogram_* functions
+
+// NOTE: the functions in this section:
+//
+//   internal_JSKeyedHistogram_Add
+//   internal_JSKeyedHistogram_Name
+//   internal_JSKeyedHistogram_Keys
+//   internal_JSKeyedHistogram_Snapshot
+//   internal_JSKeyedHistogram_Clear
+//   internal_WrapAndReturnKeyedHistogram
+//
+// Same comments as above, at the JSHistogram_* section, regarding
+// deadlock avoidance, apply.
+
+namespace {
+
+void internal_JSKeyedHistogram_finalize(JSFreeOp*, JSObject*);
+
+static const JSClassOps sJSKeyedHistogramClassOps = {
+    nullptr, /* addProperty */
+    nullptr, /* delProperty */
+    nullptr, /* enumerate */
+    nullptr, /* newEnumerate */
+    nullptr, /* resolve */
+    nullptr, /* mayResolve */
+    internal_JSKeyedHistogram_finalize};
+
+static const JSClass sJSKeyedHistogramClass = {
+    "JSKeyedHistogram",                                /* name */
+    JSCLASS_HAS_PRIVATE | JSCLASS_FOREGROUND_FINALIZE, /* flags */
+    &sJSKeyedHistogramClassOps};
+
+bool internal_JSKeyedHistogram_Snapshot(JSContext* cx, unsigned argc,
+                                        JS::Value* vp) {
+  if (!XRE_IsParentProcess()) {
+    JS_ReportErrorASCII(
+        cx, "Keyed histograms can only be snapshotted in the parent process");
+    return false;
+  }
+
+  JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSKeyedHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSKeyedHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+  MOZ_ASSERT(internal_IsHistogramEnumId(id));
+
+  // This function should always return |undefined| and never fail but
+  // rather report failures using the console.
+  args.rval().setUndefined();
+
+  // This is not good standard behavior given that we have histogram instances
+  // covering multiple processes.
+  // However, changing this requires some broader changes to callers.
+  KeyedHistogram* keyed = internal_GetKeyedHistogramById(
+      id, ProcessID::Parent, /* instantiate = */ true);
+  if (!keyed) {
+    JS_ReportErrorASCII(cx, "Failed to look up keyed histogram");
+    return false;
+  }
+
+  nsAutoString storeName;
+  nsresult rv;
+  rv = internal_JS_StoreFromObjectArgument(cx, args, storeName);
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  JS::RootedObject snapshot(cx, JS_NewPlainObject(cx));
+  if (!snapshot) {
+    JS_ReportErrorASCII(cx, "Failed to create object");
+    return false;
+  }
+
+  rv = keyed->GetJSSnapshot(cx, snapshot, NS_ConvertUTF16toUTF8(storeName),
+                            false);
+
+  // If the store is not available, we return nothing and don't fail
+  if (rv == NS_ERROR_NO_CONTENT) {
+    args.rval().setUndefined();
+    return true;
+  }
+
+  if (!NS_SUCCEEDED(rv)) {
+    JS_ReportErrorASCII(cx, "Failed to reflect keyed histograms");
+    return false;
+  }
+
+  args.rval().setObject(*snapshot);
+  return true;
+}
+
+bool internal_JSKeyedHistogram_Add(JSContext* cx, unsigned argc,
+                                   JS::Value* vp) {
+  JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSKeyedHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSKeyedHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+  MOZ_ASSERT(internal_IsHistogramEnumId(id));
+
+  // This function should always return |undefined| and never fail but
+  // rather report failures using the console.
+  args.rval().setUndefined();
+  if (args.length() < 1) {
+    LogToBrowserConsole(nsIScriptError::errorFlag, u"Expected one argument"_ns);
+    return true;
+  }
+
+  nsAutoJSString key;
+  if (!args[0].isString() || !key.init(cx, args[0])) {
+    LogToBrowserConsole(nsIScriptError::errorFlag, u"Not a string"_ns);
+    return true;
+  }
+
+  // Check if we're allowed to record in the provided key, for this histogram.
+  if (!gHistogramInfos[id].allows_key(NS_ConvertUTF16toUTF8(key))) {
+    nsPrintfCString msg("%s - key '%s' not allowed for this keyed histogram",
+                        gHistogramInfos[id].name(),
+                        NS_ConvertUTF16toUTF8(key).get());
+    LogToBrowserConsole(nsIScriptError::errorFlag, NS_ConvertUTF8toUTF16(msg));
+    TelemetryScalar::Add(mozilla::Telemetry::ScalarID::
+                             TELEMETRY_ACCUMULATE_UNKNOWN_HISTOGRAM_KEYS,
+                         NS_ConvertASCIItoUTF16(gHistogramInfos[id].name()), 1);
+    return true;
+  }
+
+  const uint32_t type = gHistogramInfos[id].histogramType;
+
+  nsTArray<uint32_t> values;
+  if (!internal_JSHistogram_GetValueArray(cx, args, type, id, true, values)) {
+    // Either GetValueArray or CoerceValue utility function will have printed a
+    // meaningful error message so we simple return true
+    return true;
+  }
+
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    for (uint32_t aValue : values) {
+      internal_Accumulate(locker, id, NS_ConvertUTF16toUTF8(key), aValue);
+    }
+  }
+  return true;
+}
+
+bool internal_JSKeyedHistogram_Name(JSContext* cx, unsigned argc,
+                                    JS::Value* vp) {
+  JS::CallArgs args = CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSKeyedHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSKeyedHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+  MOZ_ASSERT(internal_IsHistogramEnumId(id));
+  const char* name = gHistogramInfos[id].name();
+
+  auto cname = NS_ConvertASCIItoUTF16(name);
+  args.rval().setString(ToJSString(cx, cname));
+
+  return true;
+}
+
+bool internal_JSKeyedHistogram_Keys(JSContext* cx, unsigned argc,
+                                    JS::Value* vp) {
+  JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSKeyedHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSKeyedHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+
+  nsAutoString storeName;
+  nsresult rv = internal_JS_StoreFromObjectArgument(cx, args, storeName);
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  nsTArray<nsCString> keys;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    MOZ_ASSERT(internal_IsHistogramEnumId(id));
+
+    // This is not good standard behavior given that we have histogram instances
+    // covering multiple processes.
+    // However, changing this requires some broader changes to callers.
+    KeyedHistogram* keyed =
+        internal_GetKeyedHistogramById(id, ProcessID::Parent);
+
+    MOZ_ASSERT(keyed);
+    if (!keyed) {
+      return false;
+    }
+
+    if (NS_FAILED(
+            keyed->GetKeys(locker, NS_ConvertUTF16toUTF8(storeName), keys))) {
+      return false;
+    }
+  }
+
+  // Convert keys from nsTArray<nsCString> to JS array.
+  JS::RootedVector<JS::Value> autoKeys(cx);
+  if (!autoKeys.reserve(keys.Length())) {
+    return false;
+  }
+
+  for (const auto& key : keys) {
+    JS::RootedValue jsKey(cx);
+    jsKey.setString(ToJSString(cx, key));
+    if (!autoKeys.append(jsKey)) {
+      return false;
+    }
+  }
+
+  JS::RootedObject jsKeys(cx, JS::NewArrayObject(cx, autoKeys));
+  if (!jsKeys) {
+    return false;
+  }
+
+  args.rval().setObject(*jsKeys);
+  return true;
+}
+
+bool internal_JSKeyedHistogram_Clear(JSContext* cx, unsigned argc,
+                                     JS::Value* vp) {
+  if (!XRE_IsParentProcess()) {
+    JS_ReportErrorASCII(
+        cx, "Keyed histograms can only be cleared in the parent process");
+    return false;
+  }
+
+  JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+
+  if (!args.thisv().isObject() ||
+      JS::GetClass(&args.thisv().toObject()) != &sJSKeyedHistogramClass) {
+    JS_ReportErrorASCII(cx, "Wrong JS class, expected JSKeyedHistogram class");
+    return false;
+  }
+
+  JSObject* obj = &args.thisv().toObject();
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  HistogramID id = data->histogramId;
+
+  // This function should always return |undefined| and never fail but
+  // rather report failures using the console.
+  args.rval().setUndefined();
+
+  nsAutoString storeName;
+  nsresult rv = internal_JS_StoreFromObjectArgument(cx, args, storeName);
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  KeyedHistogram* keyed = nullptr;
+  {
+    MOZ_ASSERT(internal_IsHistogramEnumId(id));
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+
+    // This is not good standard behavior given that we have histogram instances
+    // covering multiple processes.
+    // However, changing this requires some broader changes to callers.
+    keyed = internal_GetKeyedHistogramById(id, ProcessID::Parent,
+                                           /* instantiate = */ false);
+
+    if (!keyed) {
+      return true;
+    }
+
+    keyed->Clear(NS_ConvertUTF16toUTF8(storeName));
+  }
+
+  return true;
+}
+
+// NOTE: Runs without protection from |gTelemetryHistogramMutex|.
+// See comment at the top of this section.
+nsresult internal_WrapAndReturnKeyedHistogram(
+    HistogramID id, JSContext* cx, JS::MutableHandle<JS::Value> ret) {
+  JS::Rooted<JSObject*> obj(cx, JS_NewObject(cx, &sJSKeyedHistogramClass));
+  if (!obj) return NS_ERROR_FAILURE;
+  // The 6 functions that are wrapped up here are eventually called
+  // by the same thread that runs this function.
+  if (!(JS_DefineFunction(cx, obj, "add", internal_JSKeyedHistogram_Add, 2,
+                          0) &&
+        JS_DefineFunction(cx, obj, "name", internal_JSKeyedHistogram_Name, 1,
+                          0) &&
+        JS_DefineFunction(cx, obj, "snapshot",
+                          internal_JSKeyedHistogram_Snapshot, 1, 0) &&
+        JS_DefineFunction(cx, obj, "keys", internal_JSKeyedHistogram_Keys, 1,
+                          0) &&
+        JS_DefineFunction(cx, obj, "clear", internal_JSKeyedHistogram_Clear, 1,
+                          0))) {
+    return NS_ERROR_FAILURE;
+  }
+
+  JSHistogramData* data = new JSHistogramData{id};
+  JS::SetPrivate(obj, data);
+  ret.setObject(*obj);
+
+  return NS_OK;
+}
+
+void internal_JSKeyedHistogram_finalize(JSFreeOp*, JSObject* obj) {
+  if (!obj || JS::GetClass(obj) != &sJSKeyedHistogramClass) {
+    MOZ_ASSERT_UNREACHABLE("Should have the right JS class.");
+    return;
+  }
+
+  JSHistogramData* data = static_cast<JSHistogramData*>(JS::GetPrivate(obj));
+  MOZ_ASSERT(data);
+  delete data;
+}
+
+}  // namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// EXTERNALLY VISIBLE FUNCTIONS in namespace TelemetryHistogram::
+
+// All of these functions are actually in namespace TelemetryHistogram::,
+// but the ::TelemetryHistogram prefix is given explicitly.  This is
+// because it is critical to see which calls from these functions are
+// to another function in this interface.  Mis-identifying "inwards
+// calls" from "calls to another function in this interface" will lead
+// to deadlocking and/or races.  See comments at the top of the file
+// for further (important!) details.
+
+void TelemetryHistogram::InitializeGlobalState(bool canRecordBase,
+                                               bool canRecordExtended) {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  MOZ_ASSERT(!gInitDone,
+             "TelemetryHistogram::InitializeGlobalState "
+             "may only be called once");
+
+  gCanRecordBase = canRecordBase;
+  gCanRecordExtended = canRecordExtended;
+
+  if (XRE_IsParentProcess()) {
+    gHistogramStorage =
+        new Histogram* [HistogramCount * size_t(ProcessID::Count)] {};
+    gKeyedHistogramStorage =
+        new KeyedHistogram* [HistogramCount * size_t(ProcessID::Count)] {};
+  }
+
+  // Some Telemetry histograms depend on the value of C++ constants and hardcode
+  // their values in Histograms.json.
+  // We add static asserts here for those values to match so that future changes
+  // don't go unnoticed.
+  // clang-format off
+  static_assert((uint32_t(JS::GCReason::NUM_TELEMETRY_REASONS) + 1) ==
+      gHistogramInfos[mozilla::Telemetry::GC_MINOR_REASON].bucketCount &&
+      (uint32_t(JS::GCReason::NUM_TELEMETRY_REASONS) + 1) ==
+      gHistogramInfos[mozilla::Telemetry::GC_MINOR_REASON_LONG].bucketCount &&
+      (uint32_t(JS::GCReason::NUM_TELEMETRY_REASONS) + 1) ==
+      gHistogramInfos[mozilla::Telemetry::GC_REASON_2].bucketCount,
+      "NUM_TELEMETRY_REASONS is assumed to be a fixed value in Histograms.json."
+      " If this was an intentional change, update the n_values for the "
+      "following in Histograms.json: GC_MINOR_REASON, GC_MINOR_REASON_LONG, "
+      "GC_REASON_2");
+
+  // clang-format on
+
+  gInitDone = true;
+}
+
+void TelemetryHistogram::DeInitializeGlobalState() {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  gCanRecordBase = false;
+  gCanRecordExtended = false;
+  gInitDone = false;
+
+  // FactoryGet `new`s Histograms for us, but requires us to manually delete.
+  if (XRE_IsParentProcess()) {
+    for (size_t i = 0; i < HistogramCount * size_t(ProcessID::Count); ++i) {
+      if (gKeyedHistogramStorage[i] != gExpiredKeyedHistogram) {
+        delete gKeyedHistogramStorage[i];
+      }
+      if (gHistogramStorage[i] != gExpiredHistogram) {
+        delete gHistogramStorage[i];
+      }
+    }
+    delete[] gHistogramStorage;
+    delete[] gKeyedHistogramStorage;
+  }
+  delete gExpiredHistogram;
+  gExpiredHistogram = nullptr;
+  delete gExpiredKeyedHistogram;
+  gExpiredKeyedHistogram = nullptr;
+}
+
+#ifdef DEBUG
+bool TelemetryHistogram::GlobalStateHasBeenInitialized() {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  return gInitDone;
+}
+#endif
+
+bool TelemetryHistogram::CanRecordBase() {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  return internal_CanRecordBase();
+}
+
+void TelemetryHistogram::SetCanRecordBase(bool b) {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  gCanRecordBase = b;
+}
+
+bool TelemetryHistogram::CanRecordExtended() {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  return internal_CanRecordExtended();
+}
+
+void TelemetryHistogram::SetCanRecordExtended(bool b) {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  gCanRecordExtended = b;
+}
+
+void TelemetryHistogram::InitHistogramRecordingEnabled() {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  auto processType = XRE_GetProcessType();
+  for (size_t i = 0; i < HistogramCount; ++i) {
+    const HistogramInfo& h = gHistogramInfos[i];
+    mozilla::Telemetry::HistogramID id = mozilla::Telemetry::HistogramID(i);
+    bool canRecordInProcess =
+        CanRecordInProcess(h.record_in_processes, processType);
+    internal_SetHistogramRecordingEnabled(locker, id, canRecordInProcess);
+  }
+
+  for (auto recordingInitiallyDisabledID : kRecordingInitiallyDisabledIDs) {
+    internal_SetHistogramRecordingEnabled(locker, recordingInitiallyDisabledID,
+                                          false);
+  }
+}
+
+void TelemetryHistogram::SetHistogramRecordingEnabled(HistogramID aID,
+                                                      bool aEnabled) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(aID))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+    return;
+  }
+
+  const HistogramInfo& h = gHistogramInfos[aID];
+  if (!CanRecordInProcess(h.record_in_processes, XRE_GetProcessType())) {
+    // Don't permit record_in_process-disabled recording to be re-enabled.
+    return;
+  }
+
+  if (!CanRecordProduct(h.products)) {
+    // Don't permit products-disabled recording to be re-enabled.
+    return;
+  }
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  internal_SetHistogramRecordingEnabled(locker, aID, aEnabled);
+}
+
+nsresult TelemetryHistogram::SetHistogramRecordingEnabled(
+    const nsACString& name, bool aEnabled) {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  HistogramID id;
+  if (NS_FAILED(internal_GetHistogramIdByName(locker, name, &id))) {
+    return NS_ERROR_FAILURE;
+  }
+
+  const HistogramInfo& hi = gHistogramInfos[id];
+  if (CanRecordInProcess(hi.record_in_processes, XRE_GetProcessType())) {
+    internal_SetHistogramRecordingEnabled(locker, id, aEnabled);
+  }
+  return NS_OK;
+}
+
+void TelemetryHistogram::Accumulate(HistogramID aID, uint32_t aSample) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(aID))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+    return;
+  }
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  internal_Accumulate(locker, aID, aSample);
+}
+
+void TelemetryHistogram::Accumulate(HistogramID aID,
+                                    const nsTArray<uint32_t>& aSamples) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(aID))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+    return;
+  }
+
+  MOZ_ASSERT(!gHistogramInfos[aID].keyed,
+             "Cannot accumulate into a keyed histogram. No key given.");
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  for (uint32_t sample : aSamples) {
+    internal_Accumulate(locker, aID, sample);
+  }
+}
+
+void TelemetryHistogram::Accumulate(HistogramID aID, const nsCString& aKey,
+                                    uint32_t aSample) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(aID))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+    return;
+  }
+
+  // Check if we're allowed to record in the provided key, for this histogram.
+  if (!gHistogramInfos[aID].allows_key(aKey)) {
+    nsPrintfCString msg("%s - key '%s' not allowed for this keyed histogram",
+                        gHistogramInfos[aID].name(), aKey.get());
+    LogToBrowserConsole(nsIScriptError::errorFlag, NS_ConvertUTF8toUTF16(msg));
+    TelemetryScalar::Add(mozilla::Telemetry::ScalarID::
+                             TELEMETRY_ACCUMULATE_UNKNOWN_HISTOGRAM_KEYS,
+                         NS_ConvertASCIItoUTF16(gHistogramInfos[aID].name()),
+                         1);
+    return;
+  }
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  internal_Accumulate(locker, aID, aKey, aSample);
+}
+
+void TelemetryHistogram::Accumulate(HistogramID aID, const nsCString& aKey,
+                                    const nsTArray<uint32_t>& aSamples) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(aID))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids");
+    return;
+  }
+
+  // Check that this histogram is keyed
+  MOZ_ASSERT(gHistogramInfos[aID].keyed,
+             "Cannot accumulate into a non-keyed histogram using a key.");
+
+  // Check if we're allowed to record in the provided key, for this histogram.
+  if (!gHistogramInfos[aID].allows_key(aKey)) {
+    nsPrintfCString msg("%s - key '%s' not allowed for this keyed histogram",
+                        gHistogramInfos[aID].name(), aKey.get());
+    LogToBrowserConsole(nsIScriptError::errorFlag, NS_ConvertUTF8toUTF16(msg));
+    TelemetryScalar::Add(mozilla::Telemetry::ScalarID::
+                             TELEMETRY_ACCUMULATE_UNKNOWN_HISTOGRAM_KEYS,
+                         NS_ConvertASCIItoUTF16(gHistogramInfos[aID].name()),
+                         1);
+    return;
+  }
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  for (uint32_t sample : aSamples) {
+    internal_Accumulate(locker, aID, aKey, sample);
+  }
+}
+
+nsresult TelemetryHistogram::Accumulate(const char* name, uint32_t sample) {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  if (!internal_CanRecordBase()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  HistogramID id;
+  nsresult rv =
+      internal_GetHistogramIdByName(locker, nsDependentCString(name), &id);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  internal_Accumulate(locker, id, sample);
+  return NS_OK;
+}
+
+nsresult TelemetryHistogram::Accumulate(const char* name, const nsCString& key,
+                                        uint32_t sample) {
+  bool keyNotAllowed = false;
+
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    if (!internal_CanRecordBase()) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    HistogramID id;
+    nsresult rv =
+        internal_GetHistogramIdByName(locker, nsDependentCString(name), &id);
+    if (NS_SUCCEEDED(rv)) {
+      // Check if we're allowed to record in the provided key, for this
+      // histogram.
+      if (gHistogramInfos[id].allows_key(key)) {
+        internal_Accumulate(locker, id, key, sample);
+        return NS_OK;
+      }
+      // We're holding |gTelemetryHistogramMutex|, so we can't print a message
+      // here.
+      keyNotAllowed = true;
+    }
+  }
+
+  if (keyNotAllowed) {
+    LogToBrowserConsole(nsIScriptError::errorFlag,
+                        u"Key not allowed for this keyed histogram"_ns);
+    TelemetryScalar::Add(mozilla::Telemetry::ScalarID::
+                             TELEMETRY_ACCUMULATE_UNKNOWN_HISTOGRAM_KEYS,
+                         NS_ConvertASCIItoUTF16(name), 1);
+  }
+  return NS_ERROR_FAILURE;
+}
+
+void TelemetryHistogram::AccumulateCategorical(HistogramID aId,
+                                               const nsCString& label) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(aId))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+    return;
+  }
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  if (!internal_CanRecordBase()) {
+    return;
+  }
+  uint32_t labelId = 0;
+  if (NS_FAILED(gHistogramInfos[aId].label_id(label.get(), &labelId))) {
+    return;
+  }
+  internal_Accumulate(locker, aId, labelId);
+}
+
+void TelemetryHistogram::AccumulateCategorical(
+    HistogramID aId, const nsTArray<nsCString>& aLabels) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(aId))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+    return;
+  }
+
+  if (!internal_CanRecordBase()) {
+    return;
+  }
+
+  // We use two loops, one for getting label_ids and another one for actually
+  // accumulating the values. This ensures that in the case of an invalid label
+  // in the array, no values are accumulated. In any call to this API, either
+  // all or (in case of error) none of the values will be accumulated.
+
+  nsTArray<uint32_t> intSamples(aLabels.Length());
+  for (const nsCString& label : aLabels) {
+    uint32_t labelId = 0;
+    if (NS_FAILED(gHistogramInfos[aId].label_id(label.get(), &labelId))) {
+      return;
+    }
+    intSamples.AppendElement(labelId);
+  }
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+
+  for (uint32_t sample : intSamples) {
+    internal_Accumulate(locker, aId, sample);
+  }
+}
+
+void TelemetryHistogram::AccumulateChild(
+    ProcessID aProcessType,
+    const nsTArray<HistogramAccumulation>& aAccumulations) {
+  MOZ_ASSERT(XRE_IsParentProcess());
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  if (!internal_CanRecordBase()) {
+    return;
+  }
+  for (uint32_t i = 0; i < aAccumulations.Length(); ++i) {
+    if (NS_WARN_IF(!internal_IsHistogramEnumId(aAccumulations[i].mId))) {
+      MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+      continue;
+    }
+    internal_AccumulateChild(locker, aProcessType, aAccumulations[i].mId,
+                             aAccumulations[i].mSample);
+  }
+}
+
+void TelemetryHistogram::AccumulateChildKeyed(
+    ProcessID aProcessType,
+    const nsTArray<KeyedHistogramAccumulation>& aAccumulations) {
+  MOZ_ASSERT(XRE_IsParentProcess());
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  if (!internal_CanRecordBase()) {
+    return;
+  }
+  for (uint32_t i = 0; i < aAccumulations.Length(); ++i) {
+    if (NS_WARN_IF(!internal_IsHistogramEnumId(aAccumulations[i].mId))) {
+      MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+      continue;
+    }
+    internal_AccumulateChildKeyed(locker, aProcessType, aAccumulations[i].mId,
+                                  aAccumulations[i].mKey,
+                                  aAccumulations[i].mSample);
+  }
+}
+
+nsresult TelemetryHistogram::GetAllStores(StringHashSet& set) {
+  for (uint32_t storeIdx : gHistogramStoresTable) {
+    const char* name = &gHistogramStringTable[storeIdx];
+    nsAutoCString store;
+    store.AssignASCII(name);
+    if (!set.PutEntry(store)) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+  return NS_OK;
+}
+
+nsresult TelemetryHistogram::GetCategoricalHistogramLabels(
+    JSContext* aCx, JS::MutableHandle<JS::Value> aResult) {
+  JS::Rooted<JSObject*> root_obj(aCx, JS_NewPlainObject(aCx));
+  if (!root_obj) {
+    return NS_ERROR_FAILURE;
+  }
+  aResult.setObject(*root_obj);
+
+  for (const HistogramInfo& info : gHistogramInfos) {
+    if (info.histogramType != nsITelemetry::HISTOGRAM_CATEGORICAL) {
+      continue;
+    }
+
+    const char* name = info.name();
+    JS::RootedObject labels(aCx, JS::NewArrayObject(aCx, info.label_count));
+    if (!labels) {
+      return NS_ERROR_FAILURE;
+    }
+
+    if (!JS_DefineProperty(aCx, root_obj, name, labels, JSPROP_ENUMERATE)) {
+      return NS_ERROR_FAILURE;
+    }
+
+    for (uint32_t i = 0; i < info.label_count; ++i) {
+      uint32_t string_offset = gHistogramLabelTable[info.label_index + i];
+      const char* const label = &gHistogramStringTable[string_offset];
+      auto clabel = NS_ConvertASCIItoUTF16(label);
+      JS::Rooted<JS::Value> value(aCx);
+      value.setString(ToJSString(aCx, clabel));
+      if (!JS_DefineElement(aCx, labels, i, value, JSPROP_ENUMERATE)) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult TelemetryHistogram::GetHistogramById(
+    const nsACString& name, JSContext* cx, JS::MutableHandle<JS::Value> ret) {
+  HistogramID id;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    nsresult rv = internal_GetHistogramIdByName(locker, name, &id);
+    if (NS_FAILED(rv)) {
+      return NS_ERROR_FAILURE;
+    }
+
+    if (gHistogramInfos[id].keyed) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+  // Runs without protection from |gTelemetryHistogramMutex|
+  return internal_WrapAndReturnHistogram(id, cx, ret);
+}
+
+nsresult TelemetryHistogram::GetKeyedHistogramById(
+    const nsACString& name, JSContext* cx, JS::MutableHandle<JS::Value> ret) {
+  HistogramID id;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    nsresult rv = internal_GetHistogramIdByName(locker, name, &id);
+    if (NS_FAILED(rv)) {
+      return NS_ERROR_FAILURE;
+    }
+
+    if (!gHistogramInfos[id].keyed) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+  // Runs without protection from |gTelemetryHistogramMutex|
+  return internal_WrapAndReturnKeyedHistogram(id, cx, ret);
+}
+
+const char* TelemetryHistogram::GetHistogramName(HistogramID id) {
+  if (NS_WARN_IF(!internal_IsHistogramEnumId(id))) {
+    MOZ_ASSERT_UNREACHABLE("Histogram usage requires valid ids.");
+    return nullptr;
+  }
+
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+  const HistogramInfo& h = gHistogramInfos[id];
+  return h.name();
+}
+
+nsresult TelemetryHistogram::CreateHistogramSnapshots(
+    JSContext* aCx, JS::MutableHandleValue aResult, const nsACString& aStore,
+    unsigned int aDataset, bool aClearSubsession, bool aFilterTest) {
+  if (!XRE_IsParentProcess()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Runs without protection from |gTelemetryHistogramMutex|
+  JS::Rooted<JSObject*> root_obj(aCx, JS_NewPlainObject(aCx));
+  if (!root_obj) {
+    return NS_ERROR_FAILURE;
+  }
+  aResult.setObject(*root_obj);
+
+  // Include the GPU process in histogram snapshots only if we actually tried
+  // to launch a process for it.
+  bool includeGPUProcess = internal_AttemptedGPUProcess();
+
+  HistogramProcessSnapshotsArray processHistArray;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    nsresult rv = internal_GetHistogramsSnapshot(
+        locker, aStore, aDataset, aClearSubsession, includeGPUProcess,
+        aFilterTest, processHistArray);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+  }
+
+  // Make the JS calls on the stashed histograms for every process
+  for (uint32_t process = 0; process < processHistArray.length(); ++process) {
+    JS::Rooted<JSObject*> processObject(aCx, JS_NewPlainObject(aCx));
+    if (!processObject) {
+      return NS_ERROR_FAILURE;
+    }
+    if (!JS_DefineProperty(aCx, root_obj,
+                           GetNameForProcessID(ProcessID(process)),
+                           processObject, JSPROP_ENUMERATE)) {
+      return NS_ERROR_FAILURE;
+    }
+
+    for (const HistogramSnapshotInfo& hData : processHistArray[process]) {
+      HistogramID id = hData.histogramID;
+
+      JS::Rooted<JSObject*> hobj(aCx, JS_NewPlainObject(aCx));
+      if (!hobj) {
+        return NS_ERROR_FAILURE;
+      }
+
+      if (NS_FAILED(internal_ReflectHistogramAndSamples(
+              aCx, hobj, gHistogramInfos[id], hData.data))) {
+        return NS_ERROR_FAILURE;
+      }
+
+      if (!JS_DefineProperty(aCx, processObject, gHistogramInfos[id].name(),
+                             hobj, JSPROP_ENUMERATE)) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+  }
+  return NS_OK;
+}
+
+nsresult TelemetryHistogram::GetKeyedHistogramSnapshots(
+    JSContext* aCx, JS::MutableHandleValue aResult, const nsACString& aStore,
+    unsigned int aDataset, bool aClearSubsession, bool aFilterTest) {
+  if (!XRE_IsParentProcess()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Runs without protection from |gTelemetryHistogramMutex|
+  JS::Rooted<JSObject*> obj(aCx, JS_NewPlainObject(aCx));
+  if (!obj) {
+    return NS_ERROR_FAILURE;
+  }
+  aResult.setObject(*obj);
+
+  // Include the GPU process in histogram snapshots only if we actually tried
+  // to launch a process for it.
+  bool includeGPUProcess = internal_AttemptedGPUProcess();
+
+  // Get a snapshot of all the data while holding the mutex.
+  KeyedHistogramProcessSnapshotsArray processHistArray;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    nsresult rv = internal_GetKeyedHistogramsSnapshot(
+        locker, aStore, aDataset, aClearSubsession, includeGPUProcess,
+        aFilterTest, processHistArray);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+  }
+
+  // Mirror the snapshot data to JS, now that we released the mutex.
+  for (uint32_t process = 0; process < processHistArray.length(); ++process) {
+    JS::Rooted<JSObject*> processObject(aCx, JS_NewPlainObject(aCx));
+    if (!processObject) {
+      return NS_ERROR_FAILURE;
+    }
+    if (!JS_DefineProperty(aCx, obj, GetNameForProcessID(ProcessID(process)),
+                           processObject, JSPROP_ENUMERATE)) {
+      return NS_ERROR_FAILURE;
+    }
+
+    for (const KeyedHistogramSnapshotInfo& hData : processHistArray[process]) {
+      const HistogramInfo& info = gHistogramInfos[hData.histogramId];
+
+      JS::RootedObject snapshot(aCx, JS_NewPlainObject(aCx));
+      if (!snapshot) {
+        return NS_ERROR_FAILURE;
+      }
+
+      if (!NS_SUCCEEDED(internal_ReflectKeyedHistogram(hData.data, info, aCx,
+                                                       snapshot))) {
+        return NS_ERROR_FAILURE;
+      }
+
+      if (!JS_DefineProperty(aCx, processObject, info.name(), snapshot,
+                             JSPROP_ENUMERATE)) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+  }
+  return NS_OK;
+}
+
+size_t TelemetryHistogram::GetHistogramSizesOfIncludingThis(
+    mozilla::MallocSizeOf aMallocSizeOf) {
+  StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+
+  size_t n = 0;
+
+  // If we allocated the array, let's count the number of pointers in there and
+  // each entry's size.
+  if (gKeyedHistogramStorage) {
+    n += HistogramCount * size_t(ProcessID::Count) * sizeof(KeyedHistogram*);
+    for (size_t i = 0; i < HistogramCount * size_t(ProcessID::Count); ++i) {
+      if (gKeyedHistogramStorage[i] &&
+          gKeyedHistogramStorage[i] != gExpiredKeyedHistogram) {
+        n += gKeyedHistogramStorage[i]->SizeOfIncludingThis(aMallocSizeOf);
+      }
+    }
+  }
+
+  // If we allocated the array, let's count the number of pointers in there.
+  if (gHistogramStorage) {
+    n += HistogramCount * size_t(ProcessID::Count) * sizeof(Histogram*);
+    for (size_t i = 0; i < HistogramCount * size_t(ProcessID::Count); ++i) {
+      if (gHistogramStorage[i] && gHistogramStorage[i] != gExpiredHistogram) {
+        n += gHistogramStorage[i]->SizeOfIncludingThis(aMallocSizeOf);
+      }
+    }
+  }
+
+  // We only allocate the expired (keyed) histogram once.
+  if (gExpiredKeyedHistogram) {
+    n += gExpiredKeyedHistogram->SizeOfIncludingThis(aMallocSizeOf);
+  }
+
+  if (gExpiredHistogram) {
+    n += gExpiredHistogram->SizeOfIncludingThis(aMallocSizeOf);
+  }
+
+  return n;
+}
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PRIVATE: GeckoView specific helpers
+
+namespace base {
+class PersistedSampleSet : public base::Histogram::SampleSet {
+ public:
+  explicit PersistedSampleSet(const nsTArray<base::Histogram::Count>& aCounts,
+                              int64_t aSampleSum);
+};
+
+PersistedSampleSet::PersistedSampleSet(
+    const nsTArray<base::Histogram::Count>& aCounts, int64_t aSampleSum) {
+  // Initialize the data in the base class. See Histogram::SampleSet
+  // for the fields documentation.
+  const size_t numCounts = aCounts.Length();
+  counts_.SetLength(numCounts);
+
+  for (size_t i = 0; i < numCounts; i++) {
+    counts_[i] = aCounts[i];
+    redundant_count_ += aCounts[i];
+  }
+  sum_ = aSampleSum;
+};
+}  // namespace base
+
+namespace {
+/**
+ * Helper function to write histogram properties to JSON.
+ * Please note that this needs to be called between
+ * StartObjectProperty/EndObject calls that mark the histogram's
+ * JSON creation.
+ */
+void internal_ReflectHistogramToJSON(const HistogramSnapshotData& aSnapshot,
+                                     mozilla::JSONWriter& aWriter) {
+  aWriter.IntProperty("sum", aSnapshot.mSampleSum);
+
+  // Fill the "counts" property.
+  aWriter.StartArrayProperty("counts");
+  for (size_t i = 0; i < aSnapshot.mBucketCounts.Length(); i++) {
+    aWriter.IntElement(aSnapshot.mBucketCounts[i]);
+  }
+  aWriter.EndArray();
+}
+
+bool internal_CanRecordHistogram(const HistogramID id, ProcessID aProcessType) {
+  // Check if we are allowed to record the data.
+  if (!CanRecordDataset(gHistogramInfos[id].dataset, internal_CanRecordBase(),
+                        internal_CanRecordExtended())) {
+    return false;
+  }
+
+  // Check if we're allowed to record in the given process.
+  if (aProcessType == ProcessID::Parent && !internal_IsRecordingEnabled(id)) {
+    return false;
+  }
+
+  if (aProcessType != ProcessID::Parent &&
+      !CanRecordInProcess(gHistogramInfos[id].record_in_processes,
+                          aProcessType)) {
+    return false;
+  }
+
+  // Don't record if the current platform is not enabled
+  if (!CanRecordProduct(gHistogramInfos[id].products)) {
+    return false;
+  }
+
+  return true;
+}
+
+nsresult internal_ParseHistogramData(
+    JSContext* aCx, JS::HandleId aEntryId, JS::HandleObject aContainerObj,
+    nsACString& aOutName, nsTArray<base::Histogram::Count>& aOutCountArray,
+    int64_t& aOutSum) {
+  // Get the histogram name.
+  nsAutoJSString histogramName;
+  if (!histogramName.init(aCx, aEntryId)) {
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  CopyUTF16toUTF8(histogramName, aOutName);
+
+  // Get the data for this histogram.
+  JS::RootedValue histogramData(aCx);
+  if (!JS_GetPropertyById(aCx, aContainerObj, aEntryId, &histogramData)) {
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  if (!histogramData.isObject()) {
+    // base::Histogram data need to be an object. If that's not the case, skip
+    // it and try to load the rest of the data.
+    return NS_ERROR_FAILURE;
+  }
+
+  // Get the "sum" property.
+  JS::RootedValue sumValue(aCx);
+  JS::RootedObject histogramObj(aCx, &histogramData.toObject());
+  if (!JS_GetProperty(aCx, histogramObj, "sum", &sumValue)) {
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  if (!JS::ToInt64(aCx, sumValue, &aOutSum)) {
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  // Get the "counts" array.
+  JS::RootedValue countsArray(aCx);
+  bool countsIsArray = false;
+  if (!JS_GetProperty(aCx, histogramObj, "counts", &countsArray) ||
+      !JS::IsArrayObject(aCx, countsArray, &countsIsArray)) {
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  if (!countsIsArray) {
+    // The "counts" property needs to be an array. If this is not the case,
+    // skip this histogram.
+    return NS_ERROR_FAILURE;
+  }
+
+  // Get the length of the array.
+  uint32_t countsLen = 0;
+  JS::RootedObject countsArrayObj(aCx, &countsArray.toObject());
+  if (!JS::GetArrayLength(aCx, countsArrayObj, &countsLen)) {
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  // Parse the "counts" in the array.
+  for (uint32_t arrayIdx = 0; arrayIdx < countsLen; arrayIdx++) {
+    JS::RootedValue elementValue(aCx);
+    int countAsInt = 0;
+    if (!JS_GetElement(aCx, countsArrayObj, arrayIdx, &elementValue) ||
+        !JS::ToInt32(aCx, elementValue, &countAsInt)) {
+      JS_ClearPendingException(aCx);
+      return NS_ERROR_FAILURE;
+    }
+    aOutCountArray.AppendElement(countAsInt);
+  }
+
+  return NS_OK;
+}
+
+}  // Anonymous namespace
+
+////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////
+//
+// PUBLIC: GeckoView serialization/deserialization functions.
+
+nsresult TelemetryHistogram::SerializeHistograms(mozilla::JSONWriter& aWriter) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only save histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Include the GPU process in histogram snapshots only if we actually tried
+  // to launch a process for it.
+  bool includeGPUProcess = internal_AttemptedGPUProcess();
+
+  // Take a snapshot of the histograms.
+  HistogramProcessSnapshotsArray processHistArray;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    // We always request the "opt-in"/"prerelease" dataset: we internally
+    // record the right subset, so this will only return "prerelease" if
+    // it was recorded.
+    if (NS_FAILED(internal_GetHistogramsSnapshot(
+            locker, "main"_ns, nsITelemetry::DATASET_PRERELEASE_CHANNELS,
+            false /* aClearSubsession */, includeGPUProcess,
+            false /* aFilterTest */, processHistArray))) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  // Make the JSON calls on the stashed histograms for every process
+  for (uint32_t process = 0; process < processHistArray.length(); ++process) {
+    aWriter.StartObjectProperty(
+        mozilla::MakeStringSpan(GetNameForProcessID(ProcessID(process))));
+
+    for (const HistogramSnapshotInfo& hData : processHistArray[process]) {
+      HistogramID id = hData.histogramID;
+
+      aWriter.StartObjectProperty(
+          mozilla::MakeStringSpan(gHistogramInfos[id].name()));
+      internal_ReflectHistogramToJSON(hData.data, aWriter);
+      aWriter.EndObject();
+    }
+    aWriter.EndObject();
+  }
+
+  return NS_OK;
+}
+
+nsresult TelemetryHistogram::SerializeKeyedHistograms(
+    mozilla::JSONWriter& aWriter) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only save keyed histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Include the GPU process in histogram snapshots only if we actually tried
+  // to launch a process for it.
+  bool includeGPUProcess = internal_AttemptedGPUProcess();
+
+  // Take a snapshot of the keyed histograms.
+  KeyedHistogramProcessSnapshotsArray processHistArray;
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+    // We always request the "opt-in"/"prerelease" dataset: we internally
+    // record the right subset, so this will only return "prerelease" if
+    // it was recorded.
+    if (NS_FAILED(internal_GetKeyedHistogramsSnapshot(
+            locker, "main"_ns, nsITelemetry::DATASET_PRERELEASE_CHANNELS,
+            false /* aClearSubsession */, includeGPUProcess,
+            false /* aFilterTest */, processHistArray))) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  // Serialize the keyed histograms for every process.
+  for (uint32_t process = 0; process < processHistArray.length(); ++process) {
+    aWriter.StartObjectProperty(
+        mozilla::MakeStringSpan(GetNameForProcessID(ProcessID(process))));
+
+    const KeyedHistogramSnapshotsArray& hArray = processHistArray[process];
+    for (size_t i = 0; i < hArray.length(); ++i) {
+      const KeyedHistogramSnapshotInfo& hData = hArray[i];
+      HistogramID id = hData.histogramId;
+      const HistogramInfo& info = gHistogramInfos[id];
+
+      aWriter.StartObjectProperty(mozilla::MakeStringSpan(info.name()));
+
+      // Each key is a new object with a "sum" and a "counts" property.
+      for (auto iter = hData.data.ConstIter(); !iter.Done(); iter.Next()) {
+        HistogramSnapshotData& keyData = iter.Data();
+        aWriter.StartObjectProperty(PromiseFlatCString(iter.Key()));
+        internal_ReflectHistogramToJSON(keyData, aWriter);
+        aWriter.EndObject();
+      }
+
+      aWriter.EndObject();
+    }
+    aWriter.EndObject();
+  }
+
+  return NS_OK;
+}
+
+nsresult TelemetryHistogram::DeserializeHistograms(JSContext* aCx,
+                                                   JS::HandleValue aData) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only load histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Telemetry is disabled. This should never happen, but let's leave this check
+  // for consistency with other histogram updates routines.
+  if (!internal_CanRecordBase()) {
+    return NS_OK;
+  }
+
+  typedef mozilla::Tuple<nsCString, nsTArray<base::Histogram::Count>, int64_t>
+      PersistedHistogramTuple;
+  typedef mozilla::Vector<PersistedHistogramTuple> PersistedHistogramArray;
+  typedef mozilla::Vector<PersistedHistogramArray> PersistedHistogramStorage;
+
+  // Before updating the histograms, we need to get the data out of the JS
+  // wrappers. We can't hold the histogram mutex while handling JS stuff.
+  // Build a <histogram name, value> map.
+  JS::RootedObject histogramDataObj(aCx, &aData.toObject());
+  JS::Rooted<JS::IdVector> processes(aCx, JS::IdVector(aCx));
+  if (!JS_Enumerate(aCx, histogramDataObj, &processes)) {
+    // We can't even enumerate the processes in the loaded data, so
+    // there is nothing we could recover from the persistence file. Bail out.
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  // Make sure we have enough storage for all the processes.
+  PersistedHistogramStorage histogramsToUpdate;
+  if (!histogramsToUpdate.resize(static_cast<uint32_t>(ProcessID::Count))) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  // The following block of code attempts to extract as much data as possible
+  // from the serialized JSON, even in case of light data corruptions: if, for
+  // example, the data for a single process is corrupted or is in an unexpected
+  // form, we press on and attempt to load the data for the other processes.
+  JS::RootedId process(aCx);
+  for (auto& processVal : processes) {
+    // This is required as JS API calls require an Handle<jsid> and not a
+    // plain jsid.
+    process = processVal;
+    // Get the process name.
+    nsAutoJSString processNameJS;
+    if (!processNameJS.init(aCx, process)) {
+      JS_ClearPendingException(aCx);
+      continue;
+    }
+
+    // Make sure it's valid. Note that this is safe to call outside
+    // of a locked section.
+    NS_ConvertUTF16toUTF8 processName(processNameJS);
+    ProcessID processID = GetIDForProcessName(processName.get());
+    if (processID == ProcessID::Count) {
+      NS_WARNING(
+          nsPrintfCString("Failed to get process ID for %s", processName.get())
+              .get());
+      continue;
+    }
+
+    // And its probes.
+    JS::RootedValue processData(aCx);
+    if (!JS_GetPropertyById(aCx, histogramDataObj, process, &processData)) {
+      JS_ClearPendingException(aCx);
+      continue;
+    }
+
+    if (!processData.isObject()) {
+      // |processData| should be an object containing histograms. If this is
+      // not the case, silently skip and try to load the data for the other
+      // processes.
+      continue;
+    }
+
+    // Iterate through each histogram.
+    JS::RootedObject processDataObj(aCx, &processData.toObject());
+    JS::Rooted<JS::IdVector> histograms(aCx, JS::IdVector(aCx));
+    if (!JS_Enumerate(aCx, processDataObj, &histograms)) {
+      JS_ClearPendingException(aCx);
+      continue;
+    }
+
+    // Get a reference to the deserialized data for this process.
+    PersistedHistogramArray& deserializedProcessData =
+        histogramsToUpdate[static_cast<uint32_t>(processID)];
+
+    JS::RootedId histogram(aCx);
+    for (auto& histogramVal : histograms) {
+      histogram = histogramVal;
+
+      int64_t sum = 0;
+      nsTArray<base::Histogram::Count> deserializedCounts;
+      nsCString histogramName;
+      if (NS_FAILED(internal_ParseHistogramData(aCx, histogram, processDataObj,
+                                                histogramName,
+                                                deserializedCounts, sum))) {
+        continue;
+      }
+
+      // Finally append the deserialized data to the storage.
+      if (!deserializedProcessData.emplaceBack(MakeTuple(
+              std::move(histogramName), std::move(deserializedCounts), sum))) {
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+    }
+  }
+
+  // Update the histogram storage.
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+
+    for (uint32_t process = 0; process < histogramsToUpdate.length();
+         ++process) {
+      PersistedHistogramArray& processArray = histogramsToUpdate[process];
+
+      for (auto& histogramData : processArray) {
+        // Attempt to get the corresponding ID for the deserialized histogram
+        // name.
+        HistogramID id;
+        if (NS_FAILED(internal_GetHistogramIdByName(
+                locker, mozilla::Get<0>(histogramData), &id))) {
+          continue;
+        }
+
+        ProcessID procID = static_cast<ProcessID>(process);
+        if (!internal_CanRecordHistogram(id, procID)) {
+          // We're not allowed to record this, so don't try to restore it.
+          continue;
+        }
+
+        // Get the Histogram instance: this will instantiate it if it doesn't
+        // exist.
+        Histogram* w = internal_GetHistogramById(locker, id, procID);
+        MOZ_ASSERT(w);
+
+        if (!w || w->IsExpired()) {
+          continue;
+        }
+
+        base::Histogram* h = nullptr;
+        constexpr auto store = "main"_ns;
+        if (!w->GetHistogram(store, &h)) {
+          continue;
+        }
+        MOZ_ASSERT(h);
+
+        if (!h) {
+          // Don't restore expired histograms.
+          continue;
+        }
+
+        // Make sure that histogram counts have matching sizes. If not,
+        // |AddSampleSet| will fail and crash.
+        size_t numCounts = mozilla::Get<1>(histogramData).Length();
+        if (h->bucket_count() != numCounts) {
+          MOZ_ASSERT(false,
+                     "The number of restored buckets does not match with the "
+                     "on in the definition");
+          continue;
+        }
+
+        // Update the data for the histogram.
+        h->AddSampleSet(
+            base::PersistedSampleSet(std::move(mozilla::Get<1>(histogramData)),
+                                     mozilla::Get<2>(histogramData)));
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult TelemetryHistogram::DeserializeKeyedHistograms(JSContext* aCx,
+                                                        JS::HandleValue aData) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Only load keyed histograms in the parent process");
+  if (!XRE_IsParentProcess()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Telemetry is disabled. This should never happen, but let's leave this check
+  // for consistency with other histogram updates routines.
+  if (!internal_CanRecordBase()) {
+    return NS_OK;
+  }
+
+  typedef mozilla::Tuple<nsCString, nsCString, nsTArray<base::Histogram::Count>,
+                         int64_t>
+      PersistedKeyedHistogramTuple;
+  typedef mozilla::Vector<PersistedKeyedHistogramTuple>
+      PersistedKeyedHistogramArray;
+  typedef mozilla::Vector<PersistedKeyedHistogramArray>
+      PersistedKeyedHistogramStorage;
+
+  // Before updating the histograms, we need to get the data out of the JS
+  // wrappers. We can't hold the histogram mutex while handling JS stuff.
+  // Build a <histogram name, value> map.
+  JS::RootedObject histogramDataObj(aCx, &aData.toObject());
+  JS::Rooted<JS::IdVector> processes(aCx, JS::IdVector(aCx));
+  if (!JS_Enumerate(aCx, histogramDataObj, &processes)) {
+    // We can't even enumerate the processes in the loaded data, so
+    // there is nothing we could recover from the persistence file. Bail out.
+    JS_ClearPendingException(aCx);
+    return NS_ERROR_FAILURE;
+  }
+
+  // Make sure we have enough storage for all the processes.
+  PersistedKeyedHistogramStorage histogramsToUpdate;
+  if (!histogramsToUpdate.resize(static_cast<uint32_t>(ProcessID::Count))) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  // The following block of code attempts to extract as much data as possible
+  // from the serialized JSON, even in case of light data corruptions: if, for
+  // example, the data for a single process is corrupted or is in an unexpected
+  // form, we press on and attempt to load the data for the other processes.
+  JS::RootedId process(aCx);
+  for (auto& processVal : processes) {
+    // This is required as JS API calls require an Handle<jsid> and not a
+    // plain jsid.
+    process = processVal;
+    // Get the process name.
+    nsAutoJSString processNameJS;
+    if (!processNameJS.init(aCx, process)) {
+      JS_ClearPendingException(aCx);
+      continue;
+    }
+
+    // Make sure it's valid. Note that this is safe to call outside
+    // of a locked section.
+    NS_ConvertUTF16toUTF8 processName(processNameJS);
+    ProcessID processID = GetIDForProcessName(processName.get());
+    if (processID == ProcessID::Count) {
+      NS_WARNING(
+          nsPrintfCString("Failed to get process ID for %s", processName.get())
+              .get());
+      continue;
+    }
+
+    // And its probes.
+    JS::RootedValue processData(aCx);
+    if (!JS_GetPropertyById(aCx, histogramDataObj, process, &processData)) {
+      JS_ClearPendingException(aCx);
+      continue;
+    }
+
+    if (!processData.isObject()) {
+      // |processData| should be an object containing histograms. If this is
+      // not the case, silently skip and try to load the data for the other
+      // processes.
+      continue;
+    }
+
+    // Iterate through each keyed histogram.
+    JS::RootedObject processDataObj(aCx, &processData.toObject());
+    JS::Rooted<JS::IdVector> histograms(aCx, JS::IdVector(aCx));
+    if (!JS_Enumerate(aCx, processDataObj, &histograms)) {
+      JS_ClearPendingException(aCx);
+      continue;
+    }
+
+    // Get a reference to the deserialized data for this process.
+    PersistedKeyedHistogramArray& deserializedProcessData =
+        histogramsToUpdate[static_cast<uint32_t>(processID)];
+
+    JS::RootedId histogram(aCx);
+    for (auto& histogramVal : histograms) {
+      histogram = histogramVal;
+      // Get the histogram name.
+      nsAutoJSString histogramName;
+      if (!histogramName.init(aCx, histogram)) {
+        JS_ClearPendingException(aCx);
+        continue;
+      }
+
+      // Get the data for this histogram.
+      JS::RootedValue histogramData(aCx);
+      if (!JS_GetPropertyById(aCx, processDataObj, histogram, &histogramData)) {
+        JS_ClearPendingException(aCx);
+        continue;
+      }
+
+      // Iterate through each key in the histogram.
+      JS::RootedObject keysDataObj(aCx, &histogramData.toObject());
+      JS::Rooted<JS::IdVector> keys(aCx, JS::IdVector(aCx));
+      if (!JS_Enumerate(aCx, keysDataObj, &keys)) {
+        JS_ClearPendingException(aCx);
+        continue;
+      }
+
+      JS::RootedId key(aCx);
+      for (auto& keyVal : keys) {
+        key = keyVal;
+
+        int64_t sum = 0;
+        nsTArray<base::Histogram::Count> deserializedCounts;
+        nsCString keyName;
+        if (NS_FAILED(internal_ParseHistogramData(
+                aCx, key, keysDataObj, keyName, deserializedCounts, sum))) {
+          continue;
+        }
+
+        // Finally append the deserialized data to the storage.
+        if (!deserializedProcessData.emplaceBack(MakeTuple(
+                nsCString(NS_ConvertUTF16toUTF8(histogramName)),
+                std::move(keyName), std::move(deserializedCounts), sum))) {
+          return NS_ERROR_OUT_OF_MEMORY;
+        }
+      }
+    }
+  }
+
+  // Update the keyed histogram storage.
+  {
+    StaticMutexAutoLock locker(gTelemetryHistogramMutex);
+
+    for (uint32_t process = 0; process < histogramsToUpdate.length();
+         ++process) {
+      PersistedKeyedHistogramArray& processArray = histogramsToUpdate[process];
+
+      for (auto& histogramData : processArray) {
+        // Attempt to get the corresponding ID for the deserialized histogram
+        // name.
+        HistogramID id;
+        if (NS_FAILED(internal_GetHistogramIdByName(
+                locker, mozilla::Get<0>(histogramData), &id))) {
+          continue;
+        }
+
+        ProcessID procID = static_cast<ProcessID>(process);
+        if (!internal_CanRecordHistogram(id, procID)) {
+          // We're not allowed to record this, so don't try to restore it.
+          continue;
+        }
+
+        KeyedHistogram* keyed = internal_GetKeyedHistogramById(id, procID);
+        MOZ_ASSERT(keyed);
+
+        if (!keyed || keyed->IsExpired()) {
+          // Don't restore if we don't have a destination storage or the
+          // histogram is expired.
+          continue;
+        }
+
+        // Get data for the key we're looking for.
+        base::Histogram* h = nullptr;
+        if (NS_FAILED(keyed->GetHistogram(
+                "main"_ns, mozilla::Get<1>(histogramData), &h))) {
+          continue;
+        }
+        MOZ_ASSERT(h);
+
+        if (!h) {
+          // Don't restore if we don't have a destination storage.
+          continue;
+        }
+
+        // Make sure that histogram counts have matching sizes. If not,
+        // |AddSampleSet| will fail and crash.
+        size_t numCounts = mozilla::Get<2>(histogramData).Length();
+        if (h->bucket_count() != numCounts) {
+          MOZ_ASSERT(false,
+                     "The number of restored buckets does not match with the "
+                     "on in the definition");
+          continue;
+        }
+
+        // Update the data for the histogram.
+        h->AddSampleSet(
+            base::PersistedSampleSet(std::move(mozilla::Get<2>(histogramData)),
+                                     mozilla::Get<3>(histogramData)));
+      }
+    }
+  }
+
+  return NS_OK;
+}