Adding upstream version 86.0.1.upstream/86.0.1upstream

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

1 files changed, 1872 insertions, 0 deletions

diff --git a/mozglue/baseprofiler/public/ProfileChunkedBuffer.h b/mozglue/baseprofiler/public/ProfileChunkedBuffer.h
new file mode 100644
index 0000000000..d4d55eafcb
--- /dev/null
+++ b/mozglue/baseprofiler/public/ProfileChunkedBuffer.h
@@ -0,0 +1,1872 @@
+/* -*- Mode: C++; tab-width: 2; 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/. */
+
+#ifndef ProfileChunkedBuffer_h
+#define ProfileChunkedBuffer_h
+
+#include "mozilla/BaseProfilerDetail.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/ProfileBufferChunkManager.h"
+#include "mozilla/ProfileBufferChunkManagerSingle.h"
+#include "mozilla/ProfileBufferEntrySerialization.h"
+#include "mozilla/RefCounted.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Unused.h"
+
+#include <cstdio>
+#include <utility>
+
+namespace mozilla {
+
+namespace detail {
+
+// Internal accessor pointing at a position inside a chunk.
+// It can handle two groups of chunks (typically the extant chunks stored in
+// the store manager, and the current chunk).
+// The main operations are:
+// - ReadEntrySize() to read an entry size, 0 means failure.
+// - operator+=(Length) to skip a number of bytes.
+// - EntryReader() creates an entry reader at the current position for a given
+//   size (it may fail with an empty reader), and skips the entry.
+// Note that there is no "past-the-end" position -- as soon as InChunkPointer
+// reaches the end, it becomes effectively null.
+class InChunkPointer {
+ public:
+  using Byte = ProfileBufferChunk::Byte;
+  using Length = ProfileBufferChunk::Length;
+
+  // Nullptr-like InChunkPointer, may be used as end iterator.
+  InChunkPointer()
+      : mChunk(nullptr), mNextChunkGroup(nullptr), mOffsetInChunk(0) {}
+
+  // InChunkPointer over one or two chunk groups, pointing at the given
+  // block index (if still in range).
+  // This constructor should only be used with *trusted* block index values!
+  InChunkPointer(const ProfileBufferChunk* aChunk,
+                 const ProfileBufferChunk* aNextChunkGroup,
+                 ProfileBufferBlockIndex aBlockIndex)
+      : mChunk(aChunk), mNextChunkGroup(aNextChunkGroup) {
+    if (mChunk) {
+      mOffsetInChunk = mChunk->OffsetFirstBlock();
+      Adjust();
+    } else if (mNextChunkGroup) {
+      mChunk = mNextChunkGroup;
+      mNextChunkGroup = nullptr;
+      mOffsetInChunk = mChunk->OffsetFirstBlock();
+      Adjust();
+    } else {
+      mOffsetInChunk = 0;
+    }
+
+    // Try to advance to given position.
+    if (!AdvanceToGlobalRangePosition(aBlockIndex)) {
+      // Block does not exist anymore (or block doesn't look valid), reset the
+      // in-chunk pointer.
+      mChunk = nullptr;
+      mNextChunkGroup = nullptr;
+    }
+  }
+
+  // InChunkPointer over one or two chunk groups, will start at the first
+  // block (if any). This may be slow, so avoid using it too much.
+  InChunkPointer(const ProfileBufferChunk* aChunk,
+                 const ProfileBufferChunk* aNextChunkGroup,
+                 ProfileBufferIndex aIndex = ProfileBufferIndex(0))
+      : mChunk(aChunk), mNextChunkGroup(aNextChunkGroup) {
+    if (mChunk) {
+      mOffsetInChunk = mChunk->OffsetFirstBlock();
+      Adjust();
+    } else if (mNextChunkGroup) {
+      mChunk = mNextChunkGroup;
+      mNextChunkGroup = nullptr;
+      mOffsetInChunk = mChunk->OffsetFirstBlock();
+      Adjust();
+    } else {
+      mOffsetInChunk = 0;
+    }
+
+    // Try to advance to given position.
+    if (!AdvanceToGlobalRangePosition(aIndex)) {
+      // Block does not exist anymore, reset the in-chunk pointer.
+      mChunk = nullptr;
+      mNextChunkGroup = nullptr;
+    }
+  }
+
+  // Compute the current position in the global range.
+  // 0 if null (including if we're reached the end).
+  [[nodiscard]] ProfileBufferIndex GlobalRangePosition() const {
+    if (IsNull()) {
+      return 0;
+    }
+    return mChunk->RangeStart() + mOffsetInChunk;
+  }
+
+  // Move InChunkPointer forward to the block at the given global block
+  // position, which is assumed to be valid exactly -- but it may be obsolete.
+  // 0 stays where it is (if valid already).
+  // MOZ_ASSERTs if the index is invalid.
+  [[nodiscard]] bool AdvanceToGlobalRangePosition(
+      ProfileBufferBlockIndex aBlockIndex) {
+    if (IsNull()) {
+      // Pointer is null already. (Not asserting because it's acceptable.)
+      return false;
+    }
+    if (!aBlockIndex) {
+      // Special null position, just stay where we are.
+      return ShouldPointAtValidBlock();
+    }
+    if (aBlockIndex.ConvertToProfileBufferIndex() < GlobalRangePosition()) {
+      // Past the requested position, stay where we are (assuming the current
+      // position was valid).
+      return ShouldPointAtValidBlock();
+    }
+    for (;;) {
+      if (aBlockIndex.ConvertToProfileBufferIndex() <
+          mChunk->RangeStart() + mChunk->OffsetPastLastBlock()) {
+        // Target position is in this chunk's written space, move to it.
+        mOffsetInChunk =
+            aBlockIndex.ConvertToProfileBufferIndex() - mChunk->RangeStart();
+        return ShouldPointAtValidBlock();
+      }
+      // Position is after this chunk, try next chunk.
+      GoToNextChunk();
+      if (IsNull()) {
+        return false;
+      }
+      // Skip whatever block tail there is, we don't allow pointing in the
+      // middle of a block.
+      mOffsetInChunk = mChunk->OffsetFirstBlock();
+      if (aBlockIndex.ConvertToProfileBufferIndex() < GlobalRangePosition()) {
+        // Past the requested position, meaning that the given position was in-
+        // between blocks -> Failure.
+        MOZ_ASSERT(false, "AdvanceToGlobalRangePosition - In-between blocks");
+        return false;
+      }
+    }
+  }
+
+  // Move InChunkPointer forward to the block at or after the given global
+  // range position.
+  // 0 stays where it is (if valid already).
+  [[nodiscard]] bool AdvanceToGlobalRangePosition(
+      ProfileBufferIndex aPosition) {
+    if (aPosition == 0) {
+      // Special position '0', just stay where we are.
+      // Success if this position is already valid.
+      return !IsNull();
+    }
+    for (;;) {
+      ProfileBufferIndex currentPosition = GlobalRangePosition();
+      if (currentPosition == 0) {
+        // Pointer is null.
+        return false;
+      }
+      if (aPosition <= currentPosition) {
+        // At or past the requested position, stay where we are.
+        return true;
+      }
+      if (aPosition < mChunk->RangeStart() + mChunk->OffsetPastLastBlock()) {
+        // Target position is in this chunk's written space, move to it.
+        for (;;) {
+          // Skip the current block.
+          mOffsetInChunk += ReadEntrySize();
+          if (mOffsetInChunk >= mChunk->OffsetPastLastBlock()) {
+            // Reached the end of the chunk, this can happen for the last
+            // block, let's just continue to the next chunk.
+            break;
+          }
+          if (aPosition <= mChunk->RangeStart() + mOffsetInChunk) {
+            // We're at or after the position, return at this block position.
+            return true;
+          }
+        }
+      }
+      // Position is after this chunk, try next chunk.
+      GoToNextChunk();
+      if (IsNull()) {
+        return false;
+      }
+      // Skip whatever block tail there is, we don't allow pointing in the
+      // middle of a block.
+      mOffsetInChunk = mChunk->OffsetFirstBlock();
+    }
+  }
+
+  [[nodiscard]] Byte ReadByte() {
+    MOZ_ASSERT(!IsNull());
+    MOZ_ASSERT(mOffsetInChunk < mChunk->OffsetPastLastBlock());
+    Byte byte = mChunk->ByteAt(mOffsetInChunk);
+    if (MOZ_UNLIKELY(++mOffsetInChunk == mChunk->OffsetPastLastBlock())) {
+      Adjust();
+    }
+    return byte;
+  }
+
+  // Read and skip a ULEB128-encoded size.
+  // 0 means failure (0-byte entries are not allowed.)
+  // Note that this doesn't guarantee that there are actually that many bytes
+  // available to read! (EntryReader() below may gracefully fail.)
+  [[nodiscard]] Length ReadEntrySize() {
+    ULEB128Reader<Length> reader;
+    if (IsNull()) {
+      return 0;
+    }
+    for (;;) {
+      const bool isComplete = reader.FeedByteIsComplete(ReadByte());
+      if (MOZ_UNLIKELY(IsNull())) {
+        // End of chunks, so there's no actual entry after this anyway.
+        return 0;
+      }
+      if (MOZ_LIKELY(isComplete)) {
+        if (MOZ_UNLIKELY(reader.Value() > mChunk->BufferBytes())) {
+          // Don't allow entries larger than a chunk.
+          return 0;
+        }
+        return reader.Value();
+      }
+    }
+  }
+
+  InChunkPointer& operator+=(Length aLength) {
+    MOZ_ASSERT(!IsNull());
+    mOffsetInChunk += aLength;
+    Adjust();
+    return *this;
+  }
+
+  [[nodiscard]] ProfileBufferEntryReader EntryReader(Length aLength) {
+    if (IsNull() || aLength == 0) {
+      return ProfileBufferEntryReader();
+    }
+
+    MOZ_ASSERT(mOffsetInChunk < mChunk->OffsetPastLastBlock());
+
+    // We should be pointing at the entry, past the entry size.
+    const ProfileBufferIndex entryIndex = GlobalRangePosition();
+    // Verify that there's enough space before for the size (starting at index
+    // 1 at least).
+    MOZ_ASSERT(entryIndex >= 1u + ULEB128Size(aLength));
+
+    const Length remaining = mChunk->OffsetPastLastBlock() - mOffsetInChunk;
+    Span<const Byte> mem0 = mChunk->BufferSpan();
+    mem0 = mem0.From(mOffsetInChunk);
+    if (aLength <= remaining) {
+      // Move to the end of this block, which could make this null if we have
+      // reached the end of all buffers.
+      *this += aLength;
+      return ProfileBufferEntryReader(
+          mem0.To(aLength),
+          // Block starts before the entry size.
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+              entryIndex - ULEB128Size(aLength)),
+          // Block ends right after the entry (could be null for last entry).
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+              GlobalRangePosition()));
+    }
+
+    // We need to go to the next chunk for the 2nd part of this block.
+    GoToNextChunk();
+    if (IsNull()) {
+      return ProfileBufferEntryReader();
+    }
+
+    Span<const Byte> mem1 = mChunk->BufferSpan();
+    const Length tail = aLength - remaining;
+    MOZ_ASSERT(tail <= mChunk->BufferBytes());
+    MOZ_ASSERT(tail == mChunk->OffsetFirstBlock());
+    // We are in the correct chunk, move the offset to the end of the block.
+    mOffsetInChunk = tail;
+    // And adjust as needed, which could make this null if we have reached the
+    // end of all buffers.
+    Adjust();
+    return ProfileBufferEntryReader(
+        mem0, mem1.To(tail),
+        // Block starts before the entry size.
+        ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+            entryIndex - ULEB128Size(aLength)),
+        // Block ends right after the entry (could be null for last entry).
+        ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+            GlobalRangePosition()));
+  }
+
+  [[nodiscard]] bool IsNull() const { return !mChunk; }
+
+  [[nodiscard]] bool operator==(const InChunkPointer& aOther) const {
+    if (IsNull() || aOther.IsNull()) {
+      return IsNull() && aOther.IsNull();
+    }
+    return mChunk == aOther.mChunk && mOffsetInChunk == aOther.mOffsetInChunk;
+  }
+
+  [[nodiscard]] bool operator!=(const InChunkPointer& aOther) const {
+    return !(*this == aOther);
+  }
+
+  [[nodiscard]] Byte operator*() const {
+    MOZ_ASSERT(!IsNull());
+    MOZ_ASSERT(mOffsetInChunk < mChunk->OffsetPastLastBlock());
+    return mChunk->ByteAt(mOffsetInChunk);
+  }
+
+  InChunkPointer& operator++() {
+    MOZ_ASSERT(!IsNull());
+    MOZ_ASSERT(mOffsetInChunk < mChunk->OffsetPastLastBlock());
+    if (MOZ_UNLIKELY(++mOffsetInChunk == mChunk->OffsetPastLastBlock())) {
+      mOffsetInChunk = 0;
+      GoToNextChunk();
+      Adjust();
+    }
+    return *this;
+  }
+
+ private:
+  void GoToNextChunk() {
+    MOZ_ASSERT(!IsNull());
+    const ProfileBufferIndex expectedNextRangeStart =
+        mChunk->RangeStart() + mChunk->BufferBytes();
+
+    mChunk = mChunk->GetNext();
+    if (!mChunk) {
+      // Reached the end of the current chunk group, try the next one (which
+      // may be null too, especially on the 2nd try).
+      mChunk = mNextChunkGroup;
+      mNextChunkGroup = nullptr;
+    }
+
+    if (mChunk && mChunk->RangeStart() == 0) {
+      // Reached a chunk without a valid (non-null) range start, assume there
+      // are only unused chunks from here on.
+      mChunk = nullptr;
+    }
+
+    MOZ_ASSERT(!mChunk || mChunk->RangeStart() == expectedNextRangeStart,
+               "We don't handle discontinuous buffers (yet)");
+    // Non-DEBUG fallback: Stop reading past discontinuities.
+    // (They should be rare, only happening on temporary OOMs.)
+    // TODO: Handle discontinuities (by skipping over incomplete blocks).
+    if (mChunk && mChunk->RangeStart() != expectedNextRangeStart) {
+      mChunk = nullptr;
+    }
+  }
+
+  // We want `InChunkPointer` to always point at a valid byte (or be null).
+  // After some operations, `mOffsetInChunk` may point past the end of the
+  // current `mChunk`, in which case we need to adjust our position to be inside
+  // the appropriate chunk. E.g., if we're 10 bytes after the end of the current
+  // chunk, we should end up at offset 10 in the next chunk.
+  // Note that we may "fall off" the last chunk and make this `InChunkPointer`
+  // effectively null.
+  void Adjust() {
+    while (mChunk && mOffsetInChunk >= mChunk->OffsetPastLastBlock()) {
+      // TODO: Try to adjust offset between chunks relative to mRangeStart
+      // differences. But we don't handle discontinuities yet.
+      if (mOffsetInChunk < mChunk->BufferBytes()) {
+        mOffsetInChunk -= mChunk->BufferBytes();
+      } else {
+        mOffsetInChunk -= mChunk->OffsetPastLastBlock();
+      }
+      GoToNextChunk();
+    }
+  }
+
+  // Check if the current position is likely to point at a valid block.
+  // (Size should be reasonable, and block should fully fit inside buffer.)
+  // MOZ_ASSERTs on failure, to catch incorrect uses of block indices (which
+  // should only point at valid blocks if still in range). Non-asserting build
+  // fallback should still be handled.
+  [[nodiscard]] bool ShouldPointAtValidBlock() const {
+    if (IsNull()) {
+      // Pointer is null, no blocks here.
+      MOZ_ASSERT(false, "ShouldPointAtValidBlock - null pointer");
+      return false;
+    }
+    // Use a copy, so we don't modify `*this`.
+    InChunkPointer pointer = *this;
+    // Try to read the entry size.
+    Length entrySize = pointer.ReadEntrySize();
+    if (entrySize == 0) {
+      // Entry size of zero means we read 0 or a way-too-big value.
+      MOZ_ASSERT(false, "ShouldPointAtValidBlock - invalid size");
+      return false;
+    }
+    // See if the last byte of the entry is still inside the buffer.
+    pointer += entrySize - 1;
+    MOZ_ASSERT(!IsNull(), "ShouldPointAtValidBlock - past end of buffer");
+    return !IsNull();
+  }
+
+  const ProfileBufferChunk* mChunk;
+  const ProfileBufferChunk* mNextChunkGroup;
+  Length mOffsetInChunk;
+};
+
+}  // namespace detail
+
+// Thread-safe buffer that can store blocks of different sizes during defined
+// sessions, using Chunks (from a ChunkManager) as storage.
+//
+// Each *block* contains an *entry* and the entry size:
+// [ entry_size | entry ] [ entry_size | entry ] ...
+//
+// *In-session* is a period of time during which `ProfileChunkedBuffer` allows
+// reading and writing.
+// *Out-of-session*, the `ProfileChunkedBuffer` object is still valid, but
+// contains no data, and gracefully denies accesses.
+//
+// To write an entry, the buffer reserves a block of sufficient size (to contain
+// user data of predetermined size), writes the entry size, and lets the caller
+// fill the entry contents using a ProfileBufferEntryWriter. E.g.:
+// ```
+// ProfileChunkedBuffer cb(...);
+// cb.ReserveAndPut([]() { return sizeof(123); },
+//                  [&](Maybe<ProfileBufferEntryWriter>& aEW) {
+//                    if (aEW) { aEW->WriteObject(123); }
+//                  });
+// ```
+// Other `Put...` functions may be used as shortcuts for simple entries.
+// The objects given to the caller's callbacks should only be used inside the
+// callbacks and not stored elsewhere, because they keep their own references to
+// chunk memory and therefore should not live longer.
+// Different type of objects may be serialized into an entry, see
+// `ProfileBufferEntryWriter::Serializer` for more information.
+//
+// When reading data, the buffer iterates over blocks (it knows how to read the
+// entry size, and therefore move to the next block), and lets the caller read
+// the entry inside of each block. E.g.:
+// ```
+// cb.ReadEach([](ProfileBufferEntryReader& aER) {
+//   /* Use ProfileBufferEntryReader functions to read serialized objects. */
+//   int n = aER.ReadObject<int>();
+// });
+// ```
+// Different type of objects may be deserialized from an entry, see
+// `ProfileBufferEntryReader::Deserializer` for more information.
+//
+// Writers may retrieve the block index corresponding to an entry
+// (`ProfileBufferBlockIndex` is an opaque type preventing the user from easily
+// modifying it). That index may later be used with `ReadAt` to get back to the
+// entry in that particular block -- if it still exists.
+class ProfileChunkedBuffer {
+ public:
+  using Byte = ProfileBufferChunk::Byte;
+  using Length = ProfileBufferChunk::Length;
+
+  enum class ThreadSafety { WithoutMutex, WithMutex };
+
+  // Default constructor starts out-of-session (nothing to read or write).
+  explicit ProfileChunkedBuffer(ThreadSafety aThreadSafety)
+      : mMutex(aThreadSafety != ThreadSafety::WithoutMutex) {}
+
+  // Start in-session with external chunk manager.
+  ProfileChunkedBuffer(ThreadSafety aThreadSafety,
+                       ProfileBufferChunkManager& aChunkManager)
+      : mMutex(aThreadSafety != ThreadSafety::WithoutMutex) {
+    SetChunkManager(aChunkManager);
+  }
+
+  // Start in-session with owned chunk manager.
+  ProfileChunkedBuffer(ThreadSafety aThreadSafety,
+                       UniquePtr<ProfileBufferChunkManager>&& aChunkManager)
+      : mMutex(aThreadSafety != ThreadSafety::WithoutMutex) {
+    SetChunkManager(std::move(aChunkManager));
+  }
+
+  ~ProfileChunkedBuffer() {
+    // Do proper clean-up by resetting the chunk manager.
+    ResetChunkManager();
+  }
+
+  // This cannot change during the lifetime of this buffer, so there's no need
+  // to lock.
+  [[nodiscard]] bool IsThreadSafe() const { return mMutex.IsActivated(); }
+
+  [[nodiscard]] bool IsInSession() const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return !!mChunkManager;
+  }
+
+  // Stop using the current chunk manager.
+  // If we own the current chunk manager, it will be destroyed.
+  // This will always clear currently-held chunks, if any.
+  void ResetChunkManager() {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    Unused << ResetChunkManager(lock);
+  }
+
+  // Set the current chunk manager.
+  // The caller is responsible for keeping the chunk manager alive as along as
+  // it's used here (until the next (Re)SetChunkManager, or
+  // ~ProfileChunkedBuffer).
+  void SetChunkManager(ProfileBufferChunkManager& aChunkManager) {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    Unused << ResetChunkManager(lock);
+    SetChunkManager(aChunkManager, lock);
+  }
+
+  // Set the current chunk manager, and keep ownership of it.
+  void SetChunkManager(UniquePtr<ProfileBufferChunkManager>&& aChunkManager) {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    Unused << ResetChunkManager(lock);
+    mOwnedChunkManager = std::move(aChunkManager);
+    if (mOwnedChunkManager) {
+      SetChunkManager(*mOwnedChunkManager, lock);
+    }
+  }
+
+  // Stop using the current chunk manager, and return it if owned here.
+  [[nodiscard]] UniquePtr<ProfileBufferChunkManager> ExtractChunkManager() {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return ResetChunkManager(lock);
+  }
+
+  // Clear the contents of this buffer, ready to receive new chunks.
+  // Note that memory is not freed: No chunks are destroyed, they are all
+  // receycled.
+  // Also the range doesn't reset, instead it continues at some point after the
+  // previous range. This may be useful if the caller may be keeping indexes
+  // into old chunks that have now been cleared, using these indexes will fail
+  // gracefully (instead of potentially pointing into new data).
+  void Clear() {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      // Out-of-session.
+      return;
+    }
+
+    mRangeStart = mRangeEnd = mNextChunkRangeStart;
+    mPushedBlockCount = 0;
+    mClearedBlockCount = 0;
+    mFailedPutBytes = 0;
+
+    // Recycle all released chunks as "next" chunks. This will reduce the number
+    // of future allocations. Also, when using ProfileBufferChunkManagerSingle,
+    // this retrieves the one chunk if it was released.
+    UniquePtr<ProfileBufferChunk> releasedChunks =
+        mChunkManager->GetExtantReleasedChunks();
+    if (releasedChunks) {
+      // Released chunks should be in the "Done" state, they need to be marked
+      // "recycled" before they can be reused.
+      for (ProfileBufferChunk* chunk = releasedChunks.get(); chunk;
+           chunk = chunk->GetNext()) {
+        chunk->MarkRecycled();
+      }
+      mNextChunks = ProfileBufferChunk::Join(std::move(mNextChunks),
+                                             std::move(releasedChunks));
+    }
+
+    if (mCurrentChunk) {
+      // We already have a current chunk (empty or in-use), mark it "done" and
+      // then "recycled", ready to be reused.
+      mCurrentChunk->MarkDone();
+      mCurrentChunk->MarkRecycled();
+    } else {
+      if (!mNextChunks) {
+        // No current chunk, and no next chunks to recycle, nothing more to do.
+        // The next "Put" operation will try to allocate a chunk as needed.
+        return;
+      }
+
+      // No current chunk, take a next chunk.
+      mCurrentChunk = std::exchange(mNextChunks, mNextChunks->ReleaseNext());
+    }
+
+    // Here, there was already a current chunk, or one has just been taken.
+    // Make sure it's ready to receive new entries.
+    InitializeCurrentChunk(lock);
+  }
+
+  // Buffer maximum length in bytes.
+  Maybe<size_t> BufferLength() const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    if (!mChunkManager) {
+      return Nothing{};
+    }
+    return Some(mChunkManager->MaxTotalSize());
+  }
+
+  [[nodiscard]] size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return SizeOfExcludingThis(aMallocSizeOf, lock);
+  }
+
+  [[nodiscard]] size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf, lock);
+  }
+
+  // Snapshot of the buffer state.
+  struct State {
+    // Index to/before the first block.
+    ProfileBufferIndex mRangeStart = 1;
+
+    // Index past the last block. Equals mRangeStart if empty.
+    ProfileBufferIndex mRangeEnd = 1;
+
+    // Number of blocks that have been pushed into this buffer.
+    uint64_t mPushedBlockCount = 0;
+
+    // Number of blocks that have been removed from this buffer.
+    // Note: Live entries = pushed - cleared.
+    uint64_t mClearedBlockCount = 0;
+
+    // Number of bytes that could not be put into this buffer.
+    uint64_t mFailedPutBytes = 0;
+  };
+
+  // Get a snapshot of the current state.
+  // When out-of-session, mFirstReadIndex==mNextWriteIndex, and
+  // mPushedBlockCount==mClearedBlockCount==0.
+  // Note that these may change right after this thread-safe call, so they
+  // should only be used for statistical purposes.
+  [[nodiscard]] State GetState() const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return {mRangeStart, mRangeEnd, mPushedBlockCount, mClearedBlockCount,
+            mFailedPutBytes};
+  }
+
+  [[nodiscard]] bool IsEmpty() const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return mRangeStart == mRangeEnd;
+  }
+
+  // True if this buffer is already locked on this thread.
+  // This should be used if some functions may call an already-locked buffer,
+  // e.g.: Put -> memory hook -> profiler_add_native_allocation_marker -> Put.
+  [[nodiscard]] bool IsThreadSafeAndLockedOnCurrentThread() const {
+    return mMutex.IsActivatedAndLockedOnCurrentThread();
+  }
+
+  // Lock the buffer mutex and run the provided callback.
+  // This can be useful when the caller needs to explicitly lock down this
+  // buffer, but not do anything else with it.
+  template <typename Callback>
+  auto LockAndRun(Callback&& aCallback) const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return std::forward<Callback>(aCallback)();
+  }
+
+  // Reserve a block that can hold an entry of the given `aCallbackEntryBytes()`
+  // size, write the entry size (ULEB128-encoded), and invoke and return
+  // `aCallback(Maybe<ProfileBufferEntryWriter>&)`.
+  // Note: `aCallbackEntryBytes` is a callback instead of a simple value, to
+  // delay this potentially-expensive computation until after we're checked that
+  // we're in-session; use `Put(Length, Callback)` below if you know the size
+  // already.
+  template <typename CallbackEntryBytes, typename Callback>
+  auto ReserveAndPut(CallbackEntryBytes&& aCallbackEntryBytes,
+                     Callback&& aCallback)
+      -> decltype(std::forward<Callback>(aCallback)(
+          std::declval<Maybe<ProfileBufferEntryWriter>&>())) {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+
+    // This can only be read in the 2nd lambda below after it has been written
+    // by the first lambda.
+    Length entryBytes;
+
+    return ReserveAndPutRaw(
+        [&]() {
+          entryBytes = std::forward<CallbackEntryBytes>(aCallbackEntryBytes)();
+          MOZ_ASSERT(entryBytes != 0, "Empty entries are not allowed");
+          return ULEB128Size(entryBytes) + entryBytes;
+        },
+        [&](Maybe<ProfileBufferEntryWriter>& aMaybeEntryWriter) {
+          if (aMaybeEntryWriter.isSome()) {
+            aMaybeEntryWriter->WriteULEB128(entryBytes);
+            MOZ_ASSERT(aMaybeEntryWriter->RemainingBytes() == entryBytes);
+          }
+          return std::forward<Callback>(aCallback)(aMaybeEntryWriter);
+        },
+        lock);
+  }
+
+  template <typename Callback>
+  auto Put(Length aEntryBytes, Callback&& aCallback) {
+    return ReserveAndPut([aEntryBytes]() { return aEntryBytes; },
+                         std::forward<Callback>(aCallback));
+  }
+
+  // Add a new entry copied from the given buffer, return block index.
+  ProfileBufferBlockIndex PutFrom(const void* aSrc, Length aBytes) {
+    return ReserveAndPut(
+        [aBytes]() { return aBytes; },
+        [aSrc, aBytes](Maybe<ProfileBufferEntryWriter>& aMaybeEntryWriter) {
+          if (aMaybeEntryWriter.isNothing()) {
+            return ProfileBufferBlockIndex{};
+          }
+          aMaybeEntryWriter->WriteBytes(aSrc, aBytes);
+          return aMaybeEntryWriter->CurrentBlockIndex();
+        });
+  }
+
+  // Add a new single entry with *all* given object (using a Serializer for
+  // each), return block index.
+  template <typename... Ts>
+  ProfileBufferBlockIndex PutObjects(const Ts&... aTs) {
+    static_assert(sizeof...(Ts) > 0,
+                  "PutObjects must be given at least one object.");
+    return ReserveAndPut(
+        [&]() { return ProfileBufferEntryWriter::SumBytes(aTs...); },
+        [&](Maybe<ProfileBufferEntryWriter>& aMaybeEntryWriter) {
+          if (aMaybeEntryWriter.isNothing()) {
+            return ProfileBufferBlockIndex{};
+          }
+          aMaybeEntryWriter->WriteObjects(aTs...);
+          return aMaybeEntryWriter->CurrentBlockIndex();
+        });
+  }
+
+  // Add a new entry copied from the given object, return block index.
+  template <typename T>
+  ProfileBufferBlockIndex PutObject(const T& aOb) {
+    return PutObjects(aOb);
+  }
+
+  // Get *all* chunks related to this buffer, including extant chunks in its
+  // ChunkManager, and yet-unused new/recycled chunks.
+  // We don't expect this buffer to be used again, though it's still possible
+  // and will allocate the first buffer when needed.
+  [[nodiscard]] UniquePtr<ProfileBufferChunk> GetAllChunks() {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      // Out-of-session.
+      return nullptr;
+    }
+    UniquePtr<ProfileBufferChunk> chunks =
+        mChunkManager->GetExtantReleasedChunks();
+    Unused << HandleRequestedChunk_IsPending(lock);
+    if (MOZ_LIKELY(!!mCurrentChunk)) {
+      mCurrentChunk->MarkDone();
+      chunks =
+          ProfileBufferChunk::Join(std::move(chunks), std::move(mCurrentChunk));
+    }
+    chunks =
+        ProfileBufferChunk::Join(std::move(chunks), std::move(mNextChunks));
+    mChunkManager->ForgetUnreleasedChunks();
+    mRangeStart = mRangeEnd = mNextChunkRangeStart;
+    return chunks;
+  }
+
+  class Reader;
+
+  // Class that can iterate through blocks and provide
+  // `ProfileBufferEntryReader`s.
+  // Created through `Reader`, lives within a lock guard lifetime.
+  class BlockIterator {
+   public:
+#ifdef DEBUG
+    ~BlockIterator() {
+      // No BlockIterator should live outside of a mutexed call.
+      mBuffer->mMutex.AssertCurrentThreadOwns();
+    }
+#endif  // DEBUG
+
+    // Comparison with other iterator, mostly used in range-for loops.
+    [[nodiscard]] bool operator==(const BlockIterator& aRhs) const {
+      MOZ_ASSERT(mBuffer == aRhs.mBuffer);
+      return mCurrentBlockIndex == aRhs.mCurrentBlockIndex;
+    }
+    [[nodiscard]] bool operator!=(const BlockIterator& aRhs) const {
+      MOZ_ASSERT(mBuffer == aRhs.mBuffer);
+      return mCurrentBlockIndex != aRhs.mCurrentBlockIndex;
+    }
+
+    // Advance to next BlockIterator.
+    BlockIterator& operator++() {
+      mBuffer->mMutex.AssertCurrentThreadOwns();
+      mCurrentBlockIndex =
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+              mNextBlockPointer.GlobalRangePosition());
+      mCurrentEntry =
+          mNextBlockPointer.EntryReader(mNextBlockPointer.ReadEntrySize());
+      return *this;
+    }
+
+    // Dereferencing creates a `ProfileBufferEntryReader` object for the entry
+    // inside this block.
+    // (Note: It would be possible to return a `const
+    // ProfileBufferEntryReader&`, but not useful in practice, because in most
+    // case the user will want to read, which is non-const.)
+    [[nodiscard]] ProfileBufferEntryReader operator*() const {
+      return mCurrentEntry;
+    }
+
+    // True if this iterator is just past the last entry.
+    [[nodiscard]] bool IsAtEnd() const {
+      return mCurrentEntry.RemainingBytes() == 0;
+    }
+
+    // Can be used as reference to come back to this entry with `GetEntryAt()`.
+    [[nodiscard]] ProfileBufferBlockIndex CurrentBlockIndex() const {
+      return mCurrentBlockIndex;
+    }
+
+    // Index past the end of this block, which is the start of the next block.
+    [[nodiscard]] ProfileBufferBlockIndex NextBlockIndex() const {
+      MOZ_ASSERT(!IsAtEnd());
+      return ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+          mNextBlockPointer.GlobalRangePosition());
+    }
+
+    // Index of the first block in the whole buffer.
+    [[nodiscard]] ProfileBufferBlockIndex BufferRangeStart() const {
+      mBuffer->mMutex.AssertCurrentThreadOwns();
+      return ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+          mBuffer->mRangeStart);
+    }
+
+    // Index past the last block in the whole buffer.
+    [[nodiscard]] ProfileBufferBlockIndex BufferRangeEnd() const {
+      mBuffer->mMutex.AssertCurrentThreadOwns();
+      return ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+          mBuffer->mRangeEnd);
+    }
+
+   private:
+    // Only a Reader can instantiate a BlockIterator.
+    friend class Reader;
+
+    BlockIterator(const ProfileChunkedBuffer& aBuffer,
+                  const ProfileBufferChunk* aChunks0,
+                  const ProfileBufferChunk* aChunks1,
+                  ProfileBufferBlockIndex aBlockIndex)
+        : mNextBlockPointer(aChunks0, aChunks1, aBlockIndex),
+          mCurrentBlockIndex(
+              ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+                  mNextBlockPointer.GlobalRangePosition())),
+          mCurrentEntry(
+              mNextBlockPointer.EntryReader(mNextBlockPointer.ReadEntrySize())),
+          mBuffer(WrapNotNull(&aBuffer)) {
+      // No BlockIterator should live outside of a mutexed call.
+      mBuffer->mMutex.AssertCurrentThreadOwns();
+    }
+
+    detail::InChunkPointer mNextBlockPointer;
+
+    ProfileBufferBlockIndex mCurrentBlockIndex;
+
+    ProfileBufferEntryReader mCurrentEntry;
+
+    // Using a non-null pointer instead of a reference, to allow copying.
+    // This BlockIterator should only live inside one of the thread-safe
+    // ProfileChunkedBuffer functions, for this reference to stay valid.
+    NotNull<const ProfileChunkedBuffer*> mBuffer;
+  };
+
+  // Class that can create `BlockIterator`s (e.g., for range-for), or just
+  // iterate through entries; lives within a lock guard lifetime.
+  class MOZ_RAII Reader {
+   public:
+    Reader(const Reader&) = delete;
+    Reader& operator=(const Reader&) = delete;
+    Reader(Reader&&) = delete;
+    Reader& operator=(Reader&&) = delete;
+
+#ifdef DEBUG
+    ~Reader() {
+      // No Reader should live outside of a mutexed call.
+      mBuffer.mMutex.AssertCurrentThreadOwns();
+    }
+#endif  // DEBUG
+
+    // Index of the first block in the whole buffer.
+    [[nodiscard]] ProfileBufferBlockIndex BufferRangeStart() const {
+      mBuffer.mMutex.AssertCurrentThreadOwns();
+      return ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+          mBuffer.mRangeStart);
+    }
+
+    // Index past the last block in the whole buffer.
+    [[nodiscard]] ProfileBufferBlockIndex BufferRangeEnd() const {
+      mBuffer.mMutex.AssertCurrentThreadOwns();
+      return ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+          mBuffer.mRangeEnd);
+    }
+
+    // Iterators to the first and past-the-last blocks.
+    // Compatible with range-for (see `ForEach` below as example).
+    [[nodiscard]] BlockIterator begin() const {
+      return BlockIterator(mBuffer, mChunks0, mChunks1, nullptr);
+    }
+    // Note that a `BlockIterator` at the `end()` should not be dereferenced, as
+    // there is no actual block there!
+    [[nodiscard]] BlockIterator end() const {
+      return BlockIterator(mBuffer, nullptr, nullptr, nullptr);
+    }
+
+    // Get a `BlockIterator` at the given `ProfileBufferBlockIndex`, clamped to
+    // the stored range. Note that a `BlockIterator` at the `end()` should not
+    // be dereferenced, as there is no actual block there!
+    [[nodiscard]] BlockIterator At(ProfileBufferBlockIndex aBlockIndex) const {
+      if (aBlockIndex < BufferRangeStart()) {
+        // Anything before the range (including null ProfileBufferBlockIndex) is
+        // clamped at the beginning.
+        return begin();
+      }
+      // Otherwise we at least expect the index to be valid (pointing exactly at
+      // a live block, or just past the end.)
+      return BlockIterator(mBuffer, mChunks0, mChunks1, aBlockIndex);
+    }
+
+    // Run `aCallback(ProfileBufferEntryReader&)` on each entry from first to
+    // last. Callback should not store `ProfileBufferEntryReader`, as it may
+    // become invalid after this thread-safe call.
+    template <typename Callback>
+    void ForEach(Callback&& aCallback) const {
+      for (ProfileBufferEntryReader reader : *this) {
+        aCallback(reader);
+      }
+    }
+
+    // If this reader only points at one chunk with some data, this data will be
+    // exposed as a single entry.
+    [[nodiscard]] ProfileBufferEntryReader SingleChunkDataAsEntry() {
+      const ProfileBufferChunk* onlyNonEmptyChunk = nullptr;
+      for (const ProfileBufferChunk* chunkList : {mChunks0, mChunks1}) {
+        for (const ProfileBufferChunk* chunk = chunkList; chunk;
+             chunk = chunk->GetNext()) {
+          if (chunk->OffsetFirstBlock() != chunk->OffsetPastLastBlock()) {
+            if (onlyNonEmptyChunk) {
+              // More than one non-empty chunk.
+              return ProfileBufferEntryReader();
+            }
+            onlyNonEmptyChunk = chunk;
+          }
+        }
+      }
+      if (!onlyNonEmptyChunk) {
+        // No non-empty chunks.
+        return ProfileBufferEntryReader();
+      }
+      // Here, we have found one chunk that had some data.
+      return ProfileBufferEntryReader(
+          onlyNonEmptyChunk->BufferSpan().FromTo(
+              onlyNonEmptyChunk->OffsetFirstBlock(),
+              onlyNonEmptyChunk->OffsetPastLastBlock()),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+              onlyNonEmptyChunk->RangeStart()),
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+              onlyNonEmptyChunk->RangeStart() +
+              (onlyNonEmptyChunk->OffsetPastLastBlock() -
+               onlyNonEmptyChunk->OffsetFirstBlock())));
+    }
+
+   private:
+    friend class ProfileChunkedBuffer;
+
+    explicit Reader(const ProfileChunkedBuffer& aBuffer,
+                    const ProfileBufferChunk* aChunks0,
+                    const ProfileBufferChunk* aChunks1)
+        : mBuffer(aBuffer), mChunks0(aChunks0), mChunks1(aChunks1) {
+      // No Reader should live outside of a mutexed call.
+      mBuffer.mMutex.AssertCurrentThreadOwns();
+    }
+
+    // This Reader should only live inside one of the thread-safe
+    // ProfileChunkedBuffer functions, for this reference to stay valid.
+    const ProfileChunkedBuffer& mBuffer;
+    const ProfileBufferChunk* mChunks0;
+    const ProfileBufferChunk* mChunks1;
+  };
+
+  // In in-session, call `aCallback(ProfileChunkedBuffer::Reader&)` and return
+  // true. Callback should not store `Reader`, because it may become invalid
+  // after this call.
+  // If out-of-session, return false (callback is not invoked).
+  template <typename Callback>
+  [[nodiscard]] auto Read(Callback&& aCallback) const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      // Out-of-session.
+      return std::forward<Callback>(aCallback)(static_cast<Reader*>(nullptr));
+    }
+    return mChunkManager->PeekExtantReleasedChunks(
+        [&](const ProfileBufferChunk* aOldestChunk) {
+          Reader reader(*this, aOldestChunk, mCurrentChunk.get());
+          return std::forward<Callback>(aCallback)(&reader);
+        });
+  }
+
+  // Invoke `aCallback(ProfileBufferEntryReader& [, ProfileBufferBlockIndex])`
+  // on each entry, it must read or at least skip everything. Either/both chunk
+  // pointers may be null.
+  template <typename Callback>
+  static void ReadEach(const ProfileBufferChunk* aChunks0,
+                       const ProfileBufferChunk* aChunks1,
+                       Callback&& aCallback) {
+    static_assert(std::is_invocable_v<Callback, ProfileBufferEntryReader&> ||
+                      std::is_invocable_v<Callback, ProfileBufferEntryReader&,
+                                          ProfileBufferBlockIndex>,
+                  "ReadEach callback must take ProfileBufferEntryReader& and "
+                  "optionally a ProfileBufferBlockIndex");
+    detail::InChunkPointer p{aChunks0, aChunks1};
+    while (!p.IsNull()) {
+      // The position right before an entry size *is* a block index.
+      const ProfileBufferBlockIndex blockIndex =
+          ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+              p.GlobalRangePosition());
+      Length entrySize = p.ReadEntrySize();
+      if (entrySize == 0) {
+        return;
+      }
+      ProfileBufferEntryReader entryReader = p.EntryReader(entrySize);
+      if (entryReader.RemainingBytes() == 0) {
+        return;
+      }
+      MOZ_ASSERT(entryReader.RemainingBytes() == entrySize);
+      if constexpr (std::is_invocable_v<Callback, ProfileBufferEntryReader&,
+                                        ProfileBufferBlockIndex>) {
+        aCallback(entryReader, blockIndex);
+      } else {
+        Unused << blockIndex;
+        aCallback(entryReader);
+      }
+      MOZ_ASSERT(entryReader.RemainingBytes() == 0);
+    }
+  }
+
+  // Invoke `aCallback(ProfileBufferEntryReader& [, ProfileBufferBlockIndex])`
+  // on each entry, it must read or at least skip everything.
+  template <typename Callback>
+  void ReadEach(Callback&& aCallback) const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      // Out-of-session.
+      return;
+    }
+    mChunkManager->PeekExtantReleasedChunks(
+        [&](const ProfileBufferChunk* aOldestChunk) {
+          ReadEach(aOldestChunk, mCurrentChunk.get(),
+                   std::forward<Callback>(aCallback));
+        });
+  }
+
+  // Call `aCallback(Maybe<ProfileBufferEntryReader>&&)` on the entry at
+  // the given ProfileBufferBlockIndex; The `Maybe` will be `Nothing` if
+  // out-of-session, or if that entry doesn't exist anymore, or if we've reached
+  // just past the last entry. Return whatever `aCallback` returns. Callback
+  // should not store `ProfileBufferEntryReader`, because it may become invalid
+  // after this call.
+  // Either/both chunk pointers may be null.
+  template <typename Callback>
+  [[nodiscard]] static auto ReadAt(ProfileBufferBlockIndex aMinimumBlockIndex,
+                                   const ProfileBufferChunk* aChunks0,
+                                   const ProfileBufferChunk* aChunks1,
+                                   Callback&& aCallback) {
+    static_assert(
+        std::is_invocable_v<Callback, Maybe<ProfileBufferEntryReader>&&>,
+        "ReadAt callback must take a Maybe<ProfileBufferEntryReader>&&");
+    Maybe<ProfileBufferEntryReader> maybeEntryReader;
+    if (detail::InChunkPointer p{aChunks0, aChunks1}; !p.IsNull()) {
+      // If the pointer position is before the given position, try to advance.
+      if (p.GlobalRangePosition() >=
+              aMinimumBlockIndex.ConvertToProfileBufferIndex() ||
+          p.AdvanceToGlobalRangePosition(
+              aMinimumBlockIndex.ConvertToProfileBufferIndex())) {
+        MOZ_ASSERT(p.GlobalRangePosition() >=
+                   aMinimumBlockIndex.ConvertToProfileBufferIndex());
+        // Here we're pointing at the start of a block, try to read the entry
+        // size. (Entries cannot be empty, so 0 means failure.)
+        if (Length entrySize = p.ReadEntrySize(); entrySize != 0) {
+          maybeEntryReader.emplace(p.EntryReader(entrySize));
+          if (maybeEntryReader->RemainingBytes() == 0) {
+            // An empty entry reader means there was no complete block at the
+            // given index.
+            maybeEntryReader.reset();
+          } else {
+            MOZ_ASSERT(maybeEntryReader->RemainingBytes() == entrySize);
+          }
+        }
+      }
+    }
+#ifdef DEBUG
+    auto assertAllRead = MakeScopeExit([&]() {
+      MOZ_ASSERT(!maybeEntryReader || maybeEntryReader->RemainingBytes() == 0);
+    });
+#endif  // DEBUG
+    return std::forward<Callback>(aCallback)(std::move(maybeEntryReader));
+  }
+
+  // Call `aCallback(Maybe<ProfileBufferEntryReader>&&)` on the entry at
+  // the given ProfileBufferBlockIndex; The `Maybe` will be `Nothing` if
+  // out-of-session, or if that entry doesn't exist anymore, or if we've reached
+  // just past the last entry. Return whatever `aCallback` returns. Callback
+  // should not store `ProfileBufferEntryReader`, because it may become invalid
+  // after this call.
+  template <typename Callback>
+  [[nodiscard]] auto ReadAt(ProfileBufferBlockIndex aBlockIndex,
+                            Callback&& aCallback) const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      // Out-of-session.
+      return std::forward<Callback>(aCallback)(Nothing{});
+    }
+    return mChunkManager->PeekExtantReleasedChunks(
+        [&](const ProfileBufferChunk* aOldestChunk) {
+          return ReadAt(aBlockIndex, aOldestChunk, mCurrentChunk.get(),
+                        std::forward<Callback>(aCallback));
+        });
+  }
+
+  // Append the contents of another ProfileChunkedBuffer to this one.
+  ProfileBufferBlockIndex AppendContents(const ProfileChunkedBuffer& aSrc) {
+    ProfileBufferBlockIndex firstBlockIndex;
+    // If we start failing, we'll stop writing.
+    bool failed = false;
+    aSrc.ReadEach([&](ProfileBufferEntryReader& aER) {
+      if (failed) {
+        return;
+      }
+      failed =
+          !Put(aER.RemainingBytes(), [&](Maybe<ProfileBufferEntryWriter>& aEW) {
+            if (aEW.isNothing()) {
+              return false;
+            }
+            if (!firstBlockIndex) {
+              firstBlockIndex = aEW->CurrentBlockIndex();
+            }
+            aEW->WriteFromReader(aER, aER.RemainingBytes());
+            return true;
+          });
+    });
+    return failed ? nullptr : firstBlockIndex;
+  }
+
+#ifdef DEBUG
+  void Dump(std::FILE* aFile = stdout) const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    fprintf(aFile,
+            "ProfileChunkedBuffer[%p] State: range %u-%u pushed=%u cleared=%u "
+            "(live=%u) failed-puts=%u bytes",
+            this, unsigned(mRangeStart), unsigned(mRangeEnd),
+            unsigned(mPushedBlockCount), unsigned(mClearedBlockCount),
+            unsigned(mPushedBlockCount) - unsigned(mClearedBlockCount),
+            unsigned(mFailedPutBytes));
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      fprintf(aFile, " - Out-of-session\n");
+      return;
+    }
+    fprintf(aFile, " - chunks:\n");
+    bool hasChunks = false;
+    mChunkManager->PeekExtantReleasedChunks(
+        [&](const ProfileBufferChunk* aOldestChunk) {
+          for (const ProfileBufferChunk* chunk = aOldestChunk; chunk;
+               chunk = chunk->GetNext()) {
+            fprintf(aFile, "R ");
+            chunk->Dump(aFile);
+            hasChunks = true;
+          }
+        });
+    if (mCurrentChunk) {
+      fprintf(aFile, "C ");
+      mCurrentChunk->Dump(aFile);
+      hasChunks = true;
+    }
+    for (const ProfileBufferChunk* chunk = mNextChunks.get(); chunk;
+         chunk = chunk->GetNext()) {
+      fprintf(aFile, "N ");
+      chunk->Dump(aFile);
+      hasChunks = true;
+    }
+    switch (mRequestedChunkHolder->GetState()) {
+      case RequestedChunkRefCountedHolder::State::Unused:
+        fprintf(aFile, " - No request pending.\n");
+        break;
+      case RequestedChunkRefCountedHolder::State::Requested:
+        fprintf(aFile, " - Request pending.\n");
+        break;
+      case RequestedChunkRefCountedHolder::State::Fulfilled:
+        fprintf(aFile, " - Request fulfilled.\n");
+        break;
+    }
+    if (!hasChunks) {
+      fprintf(aFile, " No chunks.\n");
+    }
+  }
+#endif  // DEBUG
+
+ private:
+  // Used to de/serialize a ProfileChunkedBuffer (e.g., containing a backtrace).
+  friend ProfileBufferEntryWriter::Serializer<ProfileChunkedBuffer>;
+  friend ProfileBufferEntryReader::Deserializer<ProfileChunkedBuffer>;
+  friend ProfileBufferEntryWriter::Serializer<UniquePtr<ProfileChunkedBuffer>>;
+  friend ProfileBufferEntryReader::Deserializer<
+      UniquePtr<ProfileChunkedBuffer>>;
+
+  [[nodiscard]] UniquePtr<ProfileBufferChunkManager> ResetChunkManager(
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock&) {
+    UniquePtr<ProfileBufferChunkManager> chunkManager;
+    if (mChunkManager) {
+      mRequestedChunkHolder = nullptr;
+      mChunkManager->ForgetUnreleasedChunks();
+#ifdef DEBUG
+      mChunkManager->DeregisteredFrom(this);
+#endif
+      mChunkManager = nullptr;
+      chunkManager = std::move(mOwnedChunkManager);
+      if (mCurrentChunk) {
+        mCurrentChunk->MarkDone();
+        mCurrentChunk = nullptr;
+      }
+      mNextChunks = nullptr;
+      mNextChunkRangeStart = mRangeEnd;
+      mRangeStart = mRangeEnd;
+      mPushedBlockCount = 0;
+      mClearedBlockCount = 0;
+      mFailedPutBytes = 0;
+    }
+    return chunkManager;
+  }
+
+  void SetChunkManager(
+      ProfileBufferChunkManager& aChunkManager,
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock& aLock) {
+    MOZ_ASSERT(!mChunkManager);
+    mChunkManager = &aChunkManager;
+#ifdef DEBUG
+    mChunkManager->RegisteredWith(this);
+#endif
+
+    mChunkManager->SetChunkDestroyedCallback(
+        [this](const ProfileBufferChunk& aChunk) {
+          for (;;) {
+            ProfileBufferIndex rangeStart = mRangeStart;
+            if (MOZ_LIKELY(rangeStart <= aChunk.RangeStart())) {
+              if (MOZ_LIKELY(mRangeStart.compareExchange(
+                      rangeStart,
+                      aChunk.RangeStart() + aChunk.BufferBytes()))) {
+                break;
+              }
+            }
+          }
+          mClearedBlockCount += aChunk.BlockCount();
+        });
+
+    // We start with one chunk right away, and request a following one now
+    // so it should be available before the current chunk is full.
+    SetAndInitializeCurrentChunk(mChunkManager->GetChunk(), aLock);
+    mRequestedChunkHolder = MakeRefPtr<RequestedChunkRefCountedHolder>();
+    RequestChunk(aLock);
+  }
+
+  [[nodiscard]] size_t SizeOfExcludingThis(
+      MallocSizeOf aMallocSizeOf,
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock&) const {
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      // Out-of-session.
+      return 0;
+    }
+    size_t size = mChunkManager->SizeOfIncludingThis(aMallocSizeOf);
+    if (mCurrentChunk) {
+      size += mCurrentChunk->SizeOfIncludingThis(aMallocSizeOf);
+    }
+    if (mNextChunks) {
+      size += mNextChunks->SizeOfIncludingThis(aMallocSizeOf);
+    }
+    return size;
+  }
+
+  void InitializeCurrentChunk(
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock&) {
+    MOZ_ASSERT(!!mCurrentChunk);
+    mCurrentChunk->SetRangeStart(mNextChunkRangeStart);
+    mNextChunkRangeStart += mCurrentChunk->BufferBytes();
+    Unused << mCurrentChunk->ReserveInitialBlockAsTail(0);
+  }
+
+  void SetAndInitializeCurrentChunk(
+      UniquePtr<ProfileBufferChunk>&& aChunk,
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock& aLock) {
+    mCurrentChunk = std::move(aChunk);
+    if (mCurrentChunk) {
+      InitializeCurrentChunk(aLock);
+    }
+  }
+
+  void RequestChunk(
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock& aLock) {
+    if (HandleRequestedChunk_IsPending(aLock)) {
+      // There is already a pending request, don't start a new one.
+      return;
+    }
+
+    // Ensure the `RequestedChunkHolder` knows we're starting a request.
+    mRequestedChunkHolder->StartRequest();
+
+    // Request a chunk, the callback carries a `RefPtr` of the
+    // `RequestedChunkHolder`, so it's guaranteed to live until it's invoked,
+    // even if this `ProfileChunkedBuffer` changes its `ChunkManager` or is
+    // destroyed.
+    mChunkManager->RequestChunk(
+        [requestedChunkHolder = RefPtr<RequestedChunkRefCountedHolder>(
+             mRequestedChunkHolder)](UniquePtr<ProfileBufferChunk> aChunk) {
+          requestedChunkHolder->AddRequestedChunk(std::move(aChunk));
+        });
+  }
+
+  [[nodiscard]] bool HandleRequestedChunk_IsPending(
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock& aLock) {
+    MOZ_ASSERT(!!mChunkManager);
+    MOZ_ASSERT(!!mRequestedChunkHolder);
+
+    if (mRequestedChunkHolder->GetState() ==
+        RequestedChunkRefCountedHolder::State::Unused) {
+      return false;
+    }
+
+    // A request is either in-flight or fulfilled.
+    Maybe<UniquePtr<ProfileBufferChunk>> maybeChunk =
+        mRequestedChunkHolder->GetChunkIfFulfilled();
+    if (maybeChunk.isNothing()) {
+      // Request is still pending.
+      return true;
+    }
+
+    // Since we extracted the provided chunk, the holder should now be unused.
+    MOZ_ASSERT(mRequestedChunkHolder->GetState() ==
+               RequestedChunkRefCountedHolder::State::Unused);
+
+    // Request has been fulfilled.
+    UniquePtr<ProfileBufferChunk>& chunk = *maybeChunk;
+    if (chunk) {
+      // Try to use as current chunk if needed.
+      if (!mCurrentChunk) {
+        SetAndInitializeCurrentChunk(std::move(chunk), aLock);
+        // We've just received a chunk and made it current, request a next chunk
+        // for later.
+        MOZ_ASSERT(!mNextChunks);
+        RequestChunk(aLock);
+        return true;
+      }
+
+      if (!mNextChunks) {
+        mNextChunks = std::move(chunk);
+      } else {
+        mNextChunks->InsertNext(std::move(chunk));
+      }
+    }
+
+    return false;
+  }
+
+  // Get a pointer to the next chunk available
+  [[nodiscard]] ProfileBufferChunk* GetOrCreateCurrentChunk(
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock& aLock) {
+    ProfileBufferChunk* current = mCurrentChunk.get();
+    if (MOZ_UNLIKELY(!current)) {
+      // No current chunk ready.
+      MOZ_ASSERT(!mNextChunks,
+                 "There shouldn't be next chunks when there is no current one");
+      // See if a request has recently been fulfilled, ignore pending status.
+      Unused << HandleRequestedChunk_IsPending(aLock);
+      current = mCurrentChunk.get();
+      if (MOZ_UNLIKELY(!current)) {
+        // There was no pending chunk, try to get one right now.
+        // This may still fail, but we can't do anything else about it, the
+        // caller must handle the nullptr case.
+        // Attempt a request for later.
+        SetAndInitializeCurrentChunk(mChunkManager->GetChunk(), aLock);
+        current = mCurrentChunk.get();
+      }
+    }
+    return current;
+  }
+
+  // Get a pointer to the next chunk available
+  [[nodiscard]] ProfileBufferChunk* GetOrCreateNextChunk(
+      const baseprofiler::detail::BaseProfilerMaybeAutoLock& aLock) {
+    MOZ_ASSERT(!!mCurrentChunk,
+               "Why ask for a next chunk when there isn't even a current one?");
+    ProfileBufferChunk* next = mNextChunks.get();
+    if (MOZ_UNLIKELY(!next)) {
+      // No next chunk ready, see if a request has recently been fulfilled,
+      // ignore pending status.
+      Unused << HandleRequestedChunk_IsPending(aLock);
+      next = mNextChunks.get();
+      if (MOZ_UNLIKELY(!next)) {
+        // There was no pending chunk, try to get one right now.
+        mNextChunks = mChunkManager->GetChunk();
+        next = mNextChunks.get();
+        // This may still fail, but we can't do anything else about it, the
+        // caller must handle the nullptr case.
+        if (MOZ_UNLIKELY(!next)) {
+          // Attempt a request for later.
+          RequestChunk(aLock);
+        }
+      }
+    }
+    return next;
+  }
+
+  // Reserve a block of `aCallbackBlockBytes()` size, and invoke and return
+  // `aCallback(Maybe<ProfileBufferEntryWriter>&)`. Note that this is the "raw"
+  // version that doesn't write the entry size at the beginning of the block.
+  // Note: `aCallbackBlockBytes` is a callback instead of a simple value, to
+  // delay this potentially-expensive computation until after we're checked that
+  // we're in-session; use `Put(Length, Callback)` below if you know the size
+  // already.
+  template <typename CallbackBlockBytes, typename Callback>
+  auto ReserveAndPutRaw(CallbackBlockBytes&& aCallbackBlockBytes,
+                        Callback&& aCallback,
+                        baseprofiler::detail::BaseProfilerMaybeAutoLock& aLock,
+                        uint64_t aBlockCount = 1) {
+    // The entry writer that will point into one or two chunks to write
+    // into, empty by default (failure).
+    Maybe<ProfileBufferEntryWriter> maybeEntryWriter;
+
+    // The current chunk will be filled if we need to write more than its
+    // remaining space.
+    bool currentChunkFilled = false;
+
+    // If the current chunk gets filled, we may or may not initialize the next
+    // chunk!
+    bool nextChunkInitialized = false;
+
+    if (MOZ_LIKELY(mChunkManager)) {
+      // In-session.
+
+      const Length blockBytes =
+          std::forward<CallbackBlockBytes>(aCallbackBlockBytes)();
+
+      if (ProfileBufferChunk* current = GetOrCreateCurrentChunk(aLock);
+          MOZ_LIKELY(current)) {
+        if (blockBytes <= current->RemainingBytes()) {
+          // Block fits in current chunk with only one span.
+          currentChunkFilled = blockBytes == current->RemainingBytes();
+          const auto [mem0, blockIndex] = current->ReserveBlock(blockBytes);
+          MOZ_ASSERT(mem0.LengthBytes() == blockBytes);
+          maybeEntryWriter.emplace(
+              mem0, blockIndex,
+              ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+                  blockIndex.ConvertToProfileBufferIndex() + blockBytes));
+          MOZ_ASSERT(maybeEntryWriter->RemainingBytes() == blockBytes);
+          mRangeEnd += blockBytes;
+          mPushedBlockCount += aBlockCount;
+        } else {
+          // Block doesn't fit fully in current chunk, it needs to overflow into
+          // the next one.
+          // Whether or not we can write this entry, the current chunk is now
+          // considered full, so it will be released. (Otherwise we could refuse
+          // this entry, but later accept a smaller entry into this chunk, which
+          // would be somewhat inconsistent.)
+          currentChunkFilled = true;
+          // Make sure the next chunk is available (from a previous request),
+          // otherwise create one on the spot.
+          if (ProfileBufferChunk* next = GetOrCreateNextChunk(aLock);
+              MOZ_LIKELY(next)) {
+            // Here, we know we have a current and a next chunk.
+            // Reserve head of block at the end of the current chunk.
+            const auto [mem0, blockIndex] =
+                current->ReserveBlock(current->RemainingBytes());
+            MOZ_ASSERT(mem0.LengthBytes() < blockBytes);
+            MOZ_ASSERT(current->RemainingBytes() == 0);
+            // Set the next chunk range, and reserve the needed space for the
+            // tail of the block.
+            next->SetRangeStart(mNextChunkRangeStart);
+            mNextChunkRangeStart += next->BufferBytes();
+            const auto mem1 = next->ReserveInitialBlockAsTail(
+                blockBytes - mem0.LengthBytes());
+            MOZ_ASSERT(next->RemainingBytes() != 0);
+            nextChunkInitialized = true;
+            // Block is split in two spans.
+            maybeEntryWriter.emplace(
+                mem0, mem1, blockIndex,
+                ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+                    blockIndex.ConvertToProfileBufferIndex() + blockBytes));
+            MOZ_ASSERT(maybeEntryWriter->RemainingBytes() == blockBytes);
+            mRangeEnd += blockBytes;
+            mPushedBlockCount += aBlockCount;
+          } else {
+            // Cannot get a new chunk. Record put failure.
+            mFailedPutBytes += blockBytes;
+          }
+        }
+      } else {
+        // Cannot get a current chunk. Record put failure.
+        mFailedPutBytes += blockBytes;
+      }
+    }  // end of `if (MOZ_LIKELY(mChunkManager))`
+
+    // Here, we either have a `Nothing` (failure), or a non-empty entry writer
+    // pointing at the start of the block.
+
+    // After we invoke the callback and return, we may need to handle the
+    // current chunk being filled.
+    auto handleFilledChunk = MakeScopeExit([&]() {
+      // If the entry writer was not already empty, the callback *must* have
+      // filled the full entry.
+      MOZ_ASSERT(!maybeEntryWriter || maybeEntryWriter->RemainingBytes() == 0);
+
+      if (currentChunkFilled) {
+        // Extract current (now filled) chunk.
+        UniquePtr<ProfileBufferChunk> filled = std::move(mCurrentChunk);
+
+        if (mNextChunks) {
+          // Cycle to the next chunk.
+          mCurrentChunk =
+              std::exchange(mNextChunks, mNextChunks->ReleaseNext());
+
+          // Make sure it is initialized (it is now the current chunk).
+          if (!nextChunkInitialized) {
+            InitializeCurrentChunk(aLock);
+          }
+        }
+
+        // And finally mark filled chunk done and release it.
+        filled->MarkDone();
+        mChunkManager->ReleaseChunk(std::move(filled));
+
+        // Request another chunk if needed.
+        // In most cases, here we should have one current chunk and no next
+        // chunk, so we want to do a request so there hopefully will be a next
+        // chunk available when the current one gets filled.
+        // But we also for a request if we don't even have a current chunk (if
+        // it's too late, it's ok because the next `ReserveAndPutRaw` wil just
+        // allocate one on the spot.)
+        // And if we already have a next chunk, there's no need for more now.
+        if (!mCurrentChunk || !mNextChunks) {
+          RequestChunk(aLock);
+        }
+      }
+    });
+
+    return std::forward<Callback>(aCallback)(maybeEntryWriter);
+  }
+
+  // Reserve a block of `aBlockBytes` size, and invoke and return
+  // `aCallback(Maybe<ProfileBufferEntryWriter>&)`. Note that this is the "raw"
+  // version that doesn't write the entry size at the beginning of the block.
+  template <typename Callback>
+  auto ReserveAndPutRaw(Length aBlockBytes, Callback&& aCallback,
+                        uint64_t aBlockCount) {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    return ReserveAndPutRaw([aBlockBytes]() { return aBlockBytes; },
+                            std::forward<Callback>(aCallback), lock,
+                            aBlockCount);
+  }
+
+  // Mutex guarding the following members.
+  mutable baseprofiler::detail::BaseProfilerMaybeMutex mMutex;
+
+  // Pointer to the current Chunk Manager (or null when out-of-session.)
+  // It may be owned locally (see below) or externally.
+  ProfileBufferChunkManager* mChunkManager = nullptr;
+
+  // Only non-null when we own the current Chunk Manager.
+  UniquePtr<ProfileBufferChunkManager> mOwnedChunkManager;
+
+  UniquePtr<ProfileBufferChunk> mCurrentChunk;
+
+  UniquePtr<ProfileBufferChunk> mNextChunks;
+
+  // Class used to transfer requested chunks from a `ChunkManager` to a
+  // `ProfileChunkedBuffer`.
+  // It needs to be ref-counted because the request may be fulfilled
+  // asynchronously, and either side may be destroyed during the request.
+  // It cannot use the `ProfileChunkedBuffer` mutex, because that buffer and its
+  // mutex could be destroyed during the request.
+  class RequestedChunkRefCountedHolder
+      : public external::AtomicRefCounted<RequestedChunkRefCountedHolder> {
+   public:
+    MOZ_DECLARE_REFCOUNTED_TYPENAME(RequestedChunkRefCountedHolder)
+
+    enum class State { Unused, Requested, Fulfilled };
+
+    // Get the current state. Note that it may change after the function
+    // returns, so it should be used carefully, e.g., `ProfileChunkedBuffer` can
+    // see if a request is pending or fulfilled, to avoid starting another
+    // request.
+    [[nodiscard]] State GetState() const {
+      baseprofiler::detail::BaseProfilerAutoLock lock(mRequestMutex);
+      return mState;
+    }
+
+    // Must be called by `ProfileChunkedBuffer` when it requests a chunk.
+    // There cannot be more than one request in-flight.
+    void StartRequest() {
+      baseprofiler::detail::BaseProfilerAutoLock lock(mRequestMutex);
+      MOZ_ASSERT(mState == State::Unused, "Already requested or fulfilled");
+      mState = State::Requested;
+    }
+
+    // Must be called by the `ChunkManager` with a chunk.
+    // If the `ChunkManager` cannot provide a chunk (because of memory limits,
+    // or it gets destroyed), it must call this anyway with a nullptr.
+    void AddRequestedChunk(UniquePtr<ProfileBufferChunk>&& aChunk) {
+      baseprofiler::detail::BaseProfilerAutoLock lock(mRequestMutex);
+      MOZ_ASSERT(mState == State::Requested);
+      mState = State::Fulfilled;
+      mRequestedChunk = std::move(aChunk);
+    }
+
+    // The `ProfileChunkedBuffer` can try to extract the provided chunk after a
+    // request:
+    // - Nothing -> Request is not fulfilled yet.
+    // - Some(nullptr) -> The `ChunkManager` was not able to provide a chunk.
+    // - Some(chunk) -> Requested chunk.
+    [[nodiscard]] Maybe<UniquePtr<ProfileBufferChunk>> GetChunkIfFulfilled() {
+      Maybe<UniquePtr<ProfileBufferChunk>> maybeChunk;
+      baseprofiler::detail::BaseProfilerAutoLock lock(mRequestMutex);
+      MOZ_ASSERT(mState == State::Requested || mState == State::Fulfilled);
+      if (mState == State::Fulfilled) {
+        mState = State::Unused;
+        maybeChunk.emplace(std::move(mRequestedChunk));
+      }
+      return maybeChunk;
+    }
+
+   private:
+    // Mutex guarding the following members.
+    mutable baseprofiler::detail::BaseProfilerMutex mRequestMutex;
+    State mState = State::Unused;
+    UniquePtr<ProfileBufferChunk> mRequestedChunk;
+  };
+
+  // Requested-chunk holder, kept alive when in-session, but may also live
+  // longer if a request is in-flight.
+  RefPtr<RequestedChunkRefCountedHolder> mRequestedChunkHolder;
+
+  // Range start of the next chunk to become current. Starting at 1 because
+  // 0 is a reserved index similar to nullptr.
+  ProfileBufferIndex mNextChunkRangeStart = 1;
+
+  // Index to the first block.
+  // Atomic because it may be increased when a Chunk is destroyed, and the
+  // callback may be invoked from anywhere, including from inside one of our
+  // locked section, so we cannot protect it with a mutex.
+  Atomic<ProfileBufferIndex, MemoryOrdering::ReleaseAcquire> mRangeStart{1};
+
+  // Index past the last block. Equals mRangeStart if empty.
+  ProfileBufferIndex mRangeEnd = 1;
+
+  // Number of blocks that have been pushed into this buffer.
+  uint64_t mPushedBlockCount = 0;
+
+  // Number of blocks that have been removed from this buffer.
+  // Note: Live entries = pushed - cleared.
+  // Atomic because it may be updated when a Chunk is destroyed, and the
+  // callback may be invoked from anywhere, including from inside one of our
+  // locked section, so we cannot protect it with a mutex.
+  Atomic<uint64_t, MemoryOrdering::ReleaseAcquire> mClearedBlockCount{0};
+
+  // Number of bytes that could not be put into this buffer.
+  uint64_t mFailedPutBytes = 0;
+};
+
+// ----------------------------------------------------------------------------
+// ProfileChunkedBuffer serialization
+
+// A ProfileChunkedBuffer can hide another one!
+// This will be used to store marker backtraces; They can be read back into a
+// UniquePtr<ProfileChunkedBuffer>.
+// Format: len (ULEB128) | start | end | buffer (len bytes) | pushed | cleared
+// len==0 marks an out-of-session buffer, or empty buffer.
+template <>
+struct ProfileBufferEntryWriter::Serializer<ProfileChunkedBuffer> {
+  static Length Bytes(const ProfileChunkedBuffer& aBuffer) {
+    return aBuffer.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+      if (!aReader) {
+        // Out-of-session, we only need 1 byte to store a length of 0.
+        return ULEB128Size<Length>(0);
+      }
+      ProfileBufferEntryReader reader = aReader->SingleChunkDataAsEntry();
+      const ProfileBufferIndex start =
+          reader.CurrentBlockIndex().ConvertToProfileBufferIndex();
+      const ProfileBufferIndex end =
+          reader.NextBlockIndex().ConvertToProfileBufferIndex();
+      MOZ_ASSERT(end - start <= std::numeric_limits<Length>::max());
+      const Length len = static_cast<Length>(end - start);
+      if (len == 0) {
+        // In-session but empty, also store a length of 0.
+        return ULEB128Size<Length>(0);
+      }
+      // In-session.
+      return static_cast<Length>(ULEB128Size(len) + sizeof(start) + len +
+                                 sizeof(aBuffer.mPushedBlockCount) +
+                                 sizeof(aBuffer.mClearedBlockCount));
+    });
+  }
+
+  static void Write(ProfileBufferEntryWriter& aEW,
+                    const ProfileChunkedBuffer& aBuffer) {
+    aBuffer.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+      if (!aReader) {
+        // Out-of-session, only store a length of 0.
+        aEW.WriteULEB128<Length>(0);
+        return;
+      }
+      ProfileBufferEntryReader reader = aReader->SingleChunkDataAsEntry();
+      const ProfileBufferIndex start =
+          reader.CurrentBlockIndex().ConvertToProfileBufferIndex();
+      const ProfileBufferIndex end =
+          reader.NextBlockIndex().ConvertToProfileBufferIndex();
+      MOZ_ASSERT(end - start <= std::numeric_limits<Length>::max());
+      const Length len = static_cast<Length>(end - start);
+      MOZ_ASSERT(len <= aEW.RemainingBytes());
+      if (len == 0) {
+        // In-session but empty, only store a length of 0.
+        aEW.WriteULEB128<Length>(0);
+        return;
+      }
+      // In-session.
+      // Store buffer length, and start index.
+      aEW.WriteULEB128(len);
+      aEW.WriteObject(start);
+      // Write all the bytes.
+      aEW.WriteFromReader(reader, reader.RemainingBytes());
+      // And write stats.
+      aEW.WriteObject(static_cast<uint64_t>(aBuffer.mPushedBlockCount));
+      aEW.WriteObject(static_cast<uint64_t>(aBuffer.mClearedBlockCount));
+      // Note: Failed pushes are not important to serialize.
+    });
+  }
+};
+
+// A serialized ProfileChunkedBuffer can be read into an empty buffer (either
+// out-of-session, or in-session with enough room).
+template <>
+struct ProfileBufferEntryReader::Deserializer<ProfileChunkedBuffer> {
+  static void ReadInto(ProfileBufferEntryReader& aER,
+                       ProfileChunkedBuffer& aBuffer) {
+    // Expect an empty buffer, as we're going to overwrite it.
+    MOZ_ASSERT(aBuffer.GetState().mRangeStart == aBuffer.GetState().mRangeEnd);
+    // Read the stored buffer length.
+    const auto len = aER.ReadULEB128<ProfileChunkedBuffer::Length>();
+    if (len == 0) {
+      // 0-length means an "uninteresting" buffer, just return now.
+      return;
+    }
+    // We have a non-empty buffer to read.
+
+    // Read start and end indices.
+    const auto start = aER.ReadObject<ProfileBufferIndex>();
+    aBuffer.mRangeStart = start;
+    // For now, set the end to be the start (the buffer is still empty). It will
+    // be updated in `ReserveAndPutRaw()` below.
+    aBuffer.mRangeEnd = start;
+
+    if (aBuffer.IsInSession()) {
+      // Output buffer is in-session (i.e., it already has a memory buffer
+      // attached). Make sure the caller allocated enough space.
+      MOZ_RELEASE_ASSERT(aBuffer.BufferLength().value() >= len);
+    } else {
+      // Output buffer is out-of-session, set a new chunk manager that will
+      // provide a single chunk of just the right size.
+      aBuffer.SetChunkManager(MakeUnique<ProfileBufferChunkManagerSingle>(len));
+      MOZ_ASSERT(aBuffer.BufferLength().value() >= len);
+    }
+
+    // Copy bytes into the buffer.
+    aBuffer.ReserveAndPutRaw(
+        len,
+        [&](Maybe<ProfileBufferEntryWriter>& aEW) {
+          MOZ_RELEASE_ASSERT(aEW.isSome());
+          aEW->WriteFromReader(aER, len);
+        },
+        0);
+    // Finally copy stats.
+    aBuffer.mPushedBlockCount = aER.ReadObject<uint64_t>();
+    aBuffer.mClearedBlockCount = aER.ReadObject<uint64_t>();
+    // Failed puts are not important to keep.
+    aBuffer.mFailedPutBytes = 0;
+  }
+
+  // We cannot output a ProfileChunkedBuffer object (not copyable), use
+  // `ReadInto()` or `aER.ReadObject<UniquePtr<BlocksRinbBuffer>>()` instead.
+  static ProfileChunkedBuffer Read(ProfileBufferEntryReader& aER) = delete;
+};
+
+// A ProfileChunkedBuffer is usually refererenced through a UniquePtr, for
+// convenience we support (de)serializing that UniquePtr directly.
+// This is compatible with the non-UniquePtr serialization above, with a null
+// pointer being treated like an out-of-session or empty buffer; and any of
+// these would be deserialized into a null pointer.
+template <>
+struct ProfileBufferEntryWriter::Serializer<UniquePtr<ProfileChunkedBuffer>> {
+  static Length Bytes(const UniquePtr<ProfileChunkedBuffer>& aBufferUPtr) {
+    if (!aBufferUPtr) {
+      // Null pointer, treat it like an empty buffer, i.e., write length of 0.
+      return ULEB128Size<Length>(0);
+    }
+    // Otherwise write the pointed-at ProfileChunkedBuffer (which could be
+    // out-of-session or empty.)
+    return SumBytes(*aBufferUPtr);
+  }
+
+  static void Write(ProfileBufferEntryWriter& aEW,
+                    const UniquePtr<ProfileChunkedBuffer>& aBufferUPtr) {
+    if (!aBufferUPtr) {
+      // Null pointer, treat it like an empty buffer, i.e., write length of 0.
+      aEW.WriteULEB128<Length>(0);
+      return;
+    }
+    // Otherwise write the pointed-at ProfileChunkedBuffer (which could be
+    // out-of-session or empty.)
+    aEW.WriteObject(*aBufferUPtr);
+  }
+};
+
+// Serialization of a raw pointer to ProfileChunkedBuffer.
+// Use Deserializer<UniquePtr<ProfileChunkedBuffer>> to read it back.
+template <>
+struct ProfileBufferEntryWriter::Serializer<ProfileChunkedBuffer*> {
+  static Length Bytes(ProfileChunkedBuffer* aBufferUPtr) {
+    if (!aBufferUPtr) {
+      // Null pointer, treat it like an empty buffer, i.e., write length of 0.
+      return ULEB128Size<Length>(0);
+    }
+    // Otherwise write the pointed-at ProfileChunkedBuffer (which could be
+    // out-of-session or empty.)
+    return SumBytes(*aBufferUPtr);
+  }
+
+  static void Write(ProfileBufferEntryWriter& aEW,
+                    ProfileChunkedBuffer* aBufferUPtr) {
+    if (!aBufferUPtr) {
+      // Null pointer, treat it like an empty buffer, i.e., write length of 0.
+      aEW.WriteULEB128<Length>(0);
+      return;
+    }
+    // Otherwise write the pointed-at ProfileChunkedBuffer (which could be
+    // out-of-session or empty.)
+    aEW.WriteObject(*aBufferUPtr);
+  }
+};
+
+template <>
+struct ProfileBufferEntryReader::Deserializer<UniquePtr<ProfileChunkedBuffer>> {
+  static void ReadInto(ProfileBufferEntryReader& aER,
+                       UniquePtr<ProfileChunkedBuffer>& aBuffer) {
+    aBuffer = Read(aER);
+  }
+
+  static UniquePtr<ProfileChunkedBuffer> Read(ProfileBufferEntryReader& aER) {
+    UniquePtr<ProfileChunkedBuffer> bufferUPtr;
+    // Keep a copy of the reader before reading the length, so we can restart
+    // from here below.
+    ProfileBufferEntryReader readerBeforeLen = aER;
+    // Read the stored buffer length.
+    const auto len = aER.ReadULEB128<ProfileChunkedBuffer::Length>();
+    if (len == 0) {
+      // 0-length means an "uninteresting" buffer, just return nullptr.
+      return bufferUPtr;
+    }
+    // We have a non-empty buffer.
+    // allocate an empty ProfileChunkedBuffer without mutex.
+    bufferUPtr = MakeUnique<ProfileChunkedBuffer>(
+        ProfileChunkedBuffer::ThreadSafety::WithoutMutex);
+    // Rewind the reader before the length and deserialize the contents, using
+    // the non-UniquePtr Deserializer.
+    aER = readerBeforeLen;
+    aER.ReadIntoObject(*bufferUPtr);
+    return bufferUPtr;
+  }
+};
+
+}  // namespace mozilla
+
+#endif  // ProfileChunkedBuffer_h