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diff --git a/mozglue/baseprofiler/public/ProfileChunkedBufferDetail.h b/mozglue/baseprofiler/public/ProfileChunkedBufferDetail.h
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+++ b/mozglue/baseprofiler/public/ProfileChunkedBufferDetail.h
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+/* -*- 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 ProfileChunkedBufferDetail_h
+#define ProfileChunkedBufferDetail_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Likely.h"
+#include "mozilla/ProfileBufferChunk.h"
+#include "mozilla/ProfileBufferEntrySerialization.h"
+
+namespace mozilla::profiler::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(!pointer.IsNull(),
+               "ShouldPointAtValidBlock - past end of buffer");
+    return !pointer.IsNull();
+  }
+
+  const ProfileBufferChunk* mChunk;
+  const ProfileBufferChunk* mNextChunkGroup;
+  Length mOffsetInChunk;
+};
+
+}  // namespace mozilla::profiler::detail
+
+#endif  // ProfileChunkedBufferDetail_h