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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