Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

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

1 files changed, 751 insertions, 0 deletions

diff --git a/image/StreamingLexer.h b/image/StreamingLexer.h
new file mode 100644
index 0000000000..06dbabde1c
--- /dev/null
+++ b/image/StreamingLexer.h
@@ -0,0 +1,751 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * StreamingLexer is a lexing framework designed to make it simple to write
+ * image decoders without worrying about the details of how the data is arriving
+ * from the network.
+ */
+
+#ifndef mozilla_image_StreamingLexer_h
+#define mozilla_image_StreamingLexer_h
+
+#include <algorithm>
+#include <cstdint>
+#include <utility>
+
+#include "SourceBuffer.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Variant.h"
+#include "mozilla/Vector.h"
+
+namespace mozilla {
+namespace image {
+
+/// Buffering behaviors for StreamingLexer transitions.
+enum class BufferingStrategy {
+  BUFFERED,   // Data will be buffered and processed in one chunk.
+  UNBUFFERED  // Data will be processed as it arrives, in multiple chunks.
+};
+
+/// Control flow behaviors for StreamingLexer transitions.
+enum class ControlFlowStrategy {
+  CONTINUE,  // If there's enough data, proceed to the next state immediately.
+  YIELD      // Yield to the caller before proceeding to the next state.
+};
+
+/// Possible terminal states for the lexer.
+enum class TerminalState { SUCCESS, FAILURE };
+
+/// Possible yield reasons for the lexer.
+enum class Yield {
+  NEED_MORE_DATA,   // The lexer cannot continue without more data.
+  OUTPUT_AVAILABLE  // There is output available for the caller to consume.
+};
+
+/// The result of a call to StreamingLexer::Lex().
+typedef Variant<TerminalState, Yield> LexerResult;
+
+/**
+ * LexerTransition is a type used to give commands to the lexing framework.
+ * Code that uses StreamingLexer can create LexerTransition values using the
+ * static methods on Transition, and then return them to the lexing framework
+ * for execution.
+ */
+template <typename State>
+class LexerTransition {
+ public:
+  // This is implicit so that Terminate{Success,Failure}() can return a
+  // TerminalState and have it implicitly converted to a
+  // LexerTransition<State>, which avoids the need for a "<State>"
+  // qualification to the Terminate{Success,Failure}() callsite.
+  MOZ_IMPLICIT LexerTransition(TerminalState aFinalState)
+      : mNextState(aFinalState) {}
+
+  bool NextStateIsTerminal() const {
+    return mNextState.template is<TerminalState>();
+  }
+
+  TerminalState NextStateAsTerminal() const {
+    return mNextState.template as<TerminalState>();
+  }
+
+  State NextState() const {
+    return mNextState.template as<NonTerminalState>().mState;
+  }
+
+  State UnbufferedState() const {
+    return *mNextState.template as<NonTerminalState>().mUnbufferedState;
+  }
+
+  size_t Size() const {
+    return mNextState.template as<NonTerminalState>().mSize;
+  }
+
+  BufferingStrategy Buffering() const {
+    return mNextState.template as<NonTerminalState>().mBufferingStrategy;
+  }
+
+  ControlFlowStrategy ControlFlow() const {
+    return mNextState.template as<NonTerminalState>().mControlFlowStrategy;
+  }
+
+ private:
+  friend struct Transition;
+
+  LexerTransition(State aNextState, const Maybe<State>& aUnbufferedState,
+                  size_t aSize, BufferingStrategy aBufferingStrategy,
+                  ControlFlowStrategy aControlFlowStrategy)
+      : mNextState(NonTerminalState(aNextState, aUnbufferedState, aSize,
+                                    aBufferingStrategy, aControlFlowStrategy)) {
+  }
+
+  struct NonTerminalState {
+    State mState;
+    Maybe<State> mUnbufferedState;
+    size_t mSize;
+    BufferingStrategy mBufferingStrategy;
+    ControlFlowStrategy mControlFlowStrategy;
+
+    NonTerminalState(State aState, const Maybe<State>& aUnbufferedState,
+                     size_t aSize, BufferingStrategy aBufferingStrategy,
+                     ControlFlowStrategy aControlFlowStrategy)
+        : mState(aState),
+          mUnbufferedState(aUnbufferedState),
+          mSize(aSize),
+          mBufferingStrategy(aBufferingStrategy),
+          mControlFlowStrategy(aControlFlowStrategy) {
+      MOZ_ASSERT_IF(mBufferingStrategy == BufferingStrategy::UNBUFFERED,
+                    mUnbufferedState);
+      MOZ_ASSERT_IF(mUnbufferedState,
+                    mBufferingStrategy == BufferingStrategy::UNBUFFERED);
+    }
+  };
+
+  Variant<NonTerminalState, TerminalState> mNextState;
+};
+
+struct Transition {
+  /// Transition to @aNextState, buffering @aSize bytes of data.
+  template <typename State>
+  static LexerTransition<State> To(const State& aNextState, size_t aSize) {
+    return LexerTransition<State>(aNextState, Nothing(), aSize,
+                                  BufferingStrategy::BUFFERED,
+                                  ControlFlowStrategy::CONTINUE);
+  }
+
+  /// Yield to the caller, transitioning to @aNextState when Lex() is next
+  /// invoked. The same data that was delivered for the current state will be
+  /// delivered again.
+  template <typename State>
+  static LexerTransition<State> ToAfterYield(const State& aNextState) {
+    return LexerTransition<State>(aNextState, Nothing(), 0,
+                                  BufferingStrategy::BUFFERED,
+                                  ControlFlowStrategy::YIELD);
+  }
+
+  /**
+   * Transition to @aNextState via @aUnbufferedState, reading @aSize bytes of
+   * data unbuffered.
+   *
+   * The unbuffered data will be delivered in state @aUnbufferedState, which may
+   * be invoked repeatedly until all @aSize bytes have been delivered. Then,
+   * @aNextState will be invoked with no data. No state transitions are allowed
+   * from @aUnbufferedState except for transitions to a terminal state, so
+   * @aNextState will always be reached unless lexing terminates early.
+   */
+  template <typename State>
+  static LexerTransition<State> ToUnbuffered(const State& aNextState,
+                                             const State& aUnbufferedState,
+                                             size_t aSize) {
+    return LexerTransition<State>(aNextState, Some(aUnbufferedState), aSize,
+                                  BufferingStrategy::UNBUFFERED,
+                                  ControlFlowStrategy::CONTINUE);
+  }
+
+  /**
+   * Continue receiving unbuffered data. @aUnbufferedState should be the same
+   * state as the @aUnbufferedState specified in the preceding call to
+   * ToUnbuffered().
+   *
+   * This should be used during an unbuffered read initiated by ToUnbuffered().
+   */
+  template <typename State>
+  static LexerTransition<State> ContinueUnbuffered(
+      const State& aUnbufferedState) {
+    return LexerTransition<State>(aUnbufferedState, Nothing(), 0,
+                                  BufferingStrategy::BUFFERED,
+                                  ControlFlowStrategy::CONTINUE);
+  }
+
+  /**
+   * Continue receiving unbuffered data. @aUnbufferedState should be the same
+   * state as the @aUnbufferedState specified in the preceding call to
+   * ToUnbuffered(). @aSize indicates the amount of data that has already been
+   * consumed; the next state will receive the same data that was delivered to
+   * the current state, without the first @aSize bytes.
+   *
+   * This should be used during an unbuffered read initiated by ToUnbuffered().
+   */
+  template <typename State>
+  static LexerTransition<State> ContinueUnbufferedAfterYield(
+      const State& aUnbufferedState, size_t aSize) {
+    return LexerTransition<State>(aUnbufferedState, Nothing(), aSize,
+                                  BufferingStrategy::BUFFERED,
+                                  ControlFlowStrategy::YIELD);
+  }
+
+  /**
+   * Terminate lexing, ending up in terminal state SUCCESS. (The implicit
+   * LexerTransition constructor will convert the result to a LexerTransition
+   * as needed.)
+   *
+   * No more data will be delivered after this function is used.
+   */
+  static TerminalState TerminateSuccess() { return TerminalState::SUCCESS; }
+
+  /**
+   * Terminate lexing, ending up in terminal state FAILURE. (The implicit
+   * LexerTransition constructor will convert the result to a LexerTransition
+   * as needed.)
+   *
+   * No more data will be delivered after this function is used.
+   */
+  static TerminalState TerminateFailure() { return TerminalState::FAILURE; }
+
+ private:
+  Transition();
+};
+
+/**
+ * StreamingLexer is a lexing framework designed to make it simple to write
+ * image decoders without worrying about the details of how the data is arriving
+ * from the network.
+ *
+ * To use StreamingLexer:
+ *
+ *  - Create a State type. This should be an |enum class| listing all of the
+ *    states that you can be in while lexing the image format you're trying to
+ *    read.
+ *
+ *  - Add an instance of StreamingLexer<State> to your decoder class. Initialize
+ *    it with a Transition::To() the state that you want to start lexing in, and
+ *    a Transition::To() the state you'd like to use to handle truncated data.
+ *
+ *  - In your decoder's DoDecode() method, call Lex(), passing in the input
+ *    data and length that are passed to DoDecode(). You also need to pass
+ *    a lambda which dispatches to lexing code for each state based on the State
+ *    value that's passed in. The lambda generally should just continue a
+ *    |switch| statement that calls different methods for each State value. Each
+ *    method should return a LexerTransition<State>, which the lambda should
+ *    return in turn.
+ *
+ *  - Write the methods that actually implement lexing for your image format.
+ *    These methods should return either Transition::To(), to move on to another
+ *    state, or Transition::Terminate{Success,Failure}(), if lexing has
+ *    terminated in either success or failure. (There are also additional
+ *    transitions for unbuffered reads; see below.)
+ *
+ * That's the basics. The StreamingLexer will track your position in the input
+ * and buffer enough data so that your lexing methods can process everything in
+ * one pass. Lex() returns Yield::NEED_MORE_DATA if more data is needed, in
+ * which case you should just return from DoDecode(). If lexing reaches a
+ * terminal state, Lex() returns TerminalState::SUCCESS or
+ * TerminalState::FAILURE, and you can check which one to determine if lexing
+ * succeeded or failed and do any necessary cleanup.
+ *
+ * Sometimes, the input data is truncated. StreamingLexer will notify you when
+ * this happens by invoking the truncated data state you passed to the
+ * constructor. At this point you can attempt to recover and return
+ * TerminalState::SUCCESS or TerminalState::FAILURE, depending on whether you
+ * were successful. Note that you can't return anything other than a terminal
+ * state in this situation, since there's no more data to read. For the same
+ * reason, your truncated data state shouldn't require any data. (That is, the
+ * @aSize argument you pass to Transition::To() must be zero.) Violating these
+ * requirements will trigger assertions and an immediate transition to
+ * TerminalState::FAILURE.
+ *
+ * Some lexers may want to *avoid* buffering in some cases, and just process the
+ * data as it comes in. This is useful if, for example, you just want to skip
+ * over a large section of data; there's no point in buffering data you're just
+ * going to ignore.
+ *
+ * You can begin an unbuffered read with Transition::ToUnbuffered(). This works
+ * a little differently than Transition::To() in that you specify *two* states.
+ * The @aUnbufferedState argument specifies a state that will be called
+ * repeatedly with unbuffered data, as soon as it arrives. The implementation
+ * for that state should return either a transition to a terminal state, or a
+ * Transition::ContinueUnbuffered() to the same @aUnbufferedState. (From a
+ * technical perspective, it's not necessary to specify the state again, but
+ * it's helpful to human readers.) Once the amount of data requested in the
+ * original call to Transition::ToUnbuffered() has been delivered, Lex() will
+ * transition to the @aNextState state specified via Transition::ToUnbuffered().
+ * That state will be invoked with *no* data; it's just called to signal that
+ * the unbuffered read is over.
+ *
+ * It's sometimes useful for a lexer to provide incremental results, rather
+ * than simply running to completion and presenting all its output at once. For
+ * example, when decoding animated images, it may be useful to produce each
+ * frame incrementally. StreamingLexer supports this by allowing a lexer to
+ * yield.
+ *
+ * To yield back to the caller, a state implementation can simply return
+ * Transition::ToAfterYield(). ToAfterYield()'s @aNextState argument specifies
+ * the next state that the lexer should transition to, just like when using
+ * Transition::To(), but there are two differences. One is that Lex() will
+ * return to the caller before processing any more data when it encounters a
+ * yield transition. This provides an opportunity for the caller to interact
+ * with the lexer's intermediate results. The second difference is that
+ * @aNextState will be called with *the same data as the state that you returned
+ * Transition::ToAfterYield() from*. This allows a lexer to partially consume
+ * the data, return intermediate results, and then finish consuming the data
+ * when @aNextState is called.
+ *
+ * It's also possible to yield during an unbuffered read. Just return a
+ * Transition::ContinueUnbufferedAfterYield(). Just like with
+ * Transition::ContinueUnbuffered(), the @aUnbufferedState must be the same as
+ * the one originally passed to Transition::ToUnbuffered(). The second argument,
+ * @aSize, specifies the amount of data that the lexer has already consumed.
+ * When @aUnbufferedState is next invoked, it will get the same data that it
+ * received previously, except that the first @aSize bytes will be excluded.
+ * This makes it easy to consume unbuffered data incrementally.
+ *
+ * XXX(seth): We should be able to get of the |State| stuff totally once bug
+ * 1198451 lands, since we can then just return a function representing the next
+ * state directly.
+ */
+template <typename State, size_t InlineBufferSize = 16>
+class StreamingLexer {
+ public:
+  StreamingLexer(const LexerTransition<State>& aStartState,
+                 const LexerTransition<State>& aTruncatedState)
+      : mTransition(TerminalState::FAILURE),
+        mTruncatedTransition(aTruncatedState) {
+    if (!aStartState.NextStateIsTerminal() &&
+        aStartState.ControlFlow() == ControlFlowStrategy::YIELD) {
+      // Allowing a StreamingLexer to start in a yield state doesn't make sense
+      // semantically (since yield states are supposed to deliver the same data
+      // as previous states, and there's no previous state here), but more
+      // importantly, it's necessary to advance a SourceBufferIterator at least
+      // once before you can read from it, and adding the necessary checks to
+      // Lex() to avoid that issue has the potential to mask real bugs. So
+      // instead, it's better to forbid starting in a yield state.
+      MOZ_ASSERT_UNREACHABLE("Starting in a yield state");
+      return;
+    }
+
+    if (!aTruncatedState.NextStateIsTerminal() &&
+        (aTruncatedState.ControlFlow() == ControlFlowStrategy::YIELD ||
+         aTruncatedState.Buffering() == BufferingStrategy::UNBUFFERED ||
+         aTruncatedState.Size() != 0)) {
+      // The truncated state can't receive any data because, by definition,
+      // there is no more data to receive. That means that yielding or an
+      // unbuffered read would not make sense, and that the state must require
+      // zero bytes.
+      MOZ_ASSERT_UNREACHABLE("Truncated state makes no sense");
+      return;
+    }
+
+    SetTransition(aStartState);
+  }
+
+  /**
+   * From the given SourceBufferIterator, aIterator, create a new iterator at
+   * the same position, with the given read limit, aReadLimit. The read limit
+   * applies after adjusting for the position. If the given iterator has been
+   * advanced, but required buffering inside StreamingLexer, the position
+   * of the cloned iterator will be at the beginning of buffered data; this
+   * should match the perspective of the caller.
+   */
+  Maybe<SourceBufferIterator> Clone(SourceBufferIterator& aIterator,
+                                    size_t aReadLimit) const {
+    // In order to advance to the current position of the iterator from the
+    // perspective of the caller, we need to take into account if we are
+    // buffering data.
+    size_t pos = aIterator.Position();
+    if (!mBuffer.empty()) {
+      pos += aIterator.Length();
+      MOZ_ASSERT(pos > mBuffer.length());
+      pos -= mBuffer.length();
+    }
+
+    size_t readLimit = aReadLimit;
+    if (aReadLimit != SIZE_MAX) {
+      readLimit += pos;
+    }
+
+    SourceBufferIterator other = aIterator.Owner()->Iterator(readLimit);
+
+    // Since the current iterator has already advanced to this point, we
+    // know that the state can only be READY or COMPLETE. That does not mean
+    // everything is stored in a single chunk, and may require multiple Advance
+    // calls to get where we want to be.
+    SourceBufferIterator::State state;
+    do {
+      state = other.Advance(pos);
+      if (state != SourceBufferIterator::READY) {
+        // The only way we should fail to advance over data we already seen is
+        // if we hit an error while inserting data into the buffer. This will
+        // cause an early exit.
+        MOZ_ASSERT(NS_FAILED(other.CompletionStatus()));
+        return Nothing();
+      }
+      MOZ_ASSERT(pos >= other.Length());
+      pos -= other.Length();
+    } while (pos > 0);
+
+    // Force the data pointer to be where we expect it to be.
+    state = other.Advance(0);
+    if (state != SourceBufferIterator::READY) {
+      // The current position could be the end of the buffer, in which case
+      // there is no point cloning with no more data to read.
+      MOZ_ASSERT(state == SourceBufferIterator::COMPLETE);
+      return Nothing();
+    }
+    return Some(std::move(other));
+  }
+
+  template <typename Func>
+  LexerResult Lex(SourceBufferIterator& aIterator, IResumable* aOnResume,
+                  Func aFunc) {
+    if (mTransition.NextStateIsTerminal()) {
+      // We've already reached a terminal state. We never deliver any more data
+      // in this case; just return the terminal state again immediately.
+      return LexerResult(mTransition.NextStateAsTerminal());
+    }
+
+    Maybe<LexerResult> result;
+
+    // If the lexer requested a yield last time, we deliver the same data again
+    // before we read anything else from |aIterator|. Note that although to the
+    // callers of Lex(), Yield::NEED_MORE_DATA is just another type of yield,
+    // internally they're different in that we don't redeliver the same data in
+    // the Yield::NEED_MORE_DATA case, and |mYieldingToState| is not set. This
+    // means that for Yield::NEED_MORE_DATA, we go directly to the loop below.
+    if (mYieldingToState) {
+      result = mTransition.Buffering() == BufferingStrategy::UNBUFFERED
+                   ? UnbufferedReadAfterYield(aIterator, aFunc)
+                   : BufferedReadAfterYield(aIterator, aFunc);
+    }
+
+    while (!result) {
+      MOZ_ASSERT_IF(mTransition.Buffering() == BufferingStrategy::UNBUFFERED,
+                    mUnbufferedState);
+
+      // Figure out how much we need to read.
+      const size_t toRead =
+          mTransition.Buffering() == BufferingStrategy::UNBUFFERED
+              ? mUnbufferedState->mBytesRemaining
+              : mTransition.Size() - mBuffer.length();
+
+      // Attempt to advance the iterator by |toRead| bytes.
+      switch (aIterator.AdvanceOrScheduleResume(toRead, aOnResume)) {
+        case SourceBufferIterator::WAITING:
+          // We can't continue because the rest of the data hasn't arrived from
+          // the network yet. We don't have to do anything special; the
+          // SourceBufferIterator will ensure that |aOnResume| gets called when
+          // more data is available.
+          result = Some(LexerResult(Yield::NEED_MORE_DATA));
+          break;
+
+        case SourceBufferIterator::COMPLETE:
+          // The data is truncated; if not, the lexer would've reached a
+          // terminal state by now. We only get to
+          // SourceBufferIterator::COMPLETE after every byte of data has been
+          // delivered to the lexer.
+          result = Truncated(aIterator, aFunc);
+          break;
+
+        case SourceBufferIterator::READY:
+          // Process the new data that became available.
+          MOZ_ASSERT(aIterator.Data());
+
+          result = mTransition.Buffering() == BufferingStrategy::UNBUFFERED
+                       ? UnbufferedRead(aIterator, aFunc)
+                       : BufferedRead(aIterator, aFunc);
+          break;
+
+        default:
+          MOZ_ASSERT_UNREACHABLE("Unknown SourceBufferIterator state");
+          result = SetTransition(Transition::TerminateFailure());
+      }
+    };
+
+    return *result;
+  }
+
+ private:
+  template <typename Func>
+  Maybe<LexerResult> UnbufferedRead(SourceBufferIterator& aIterator,
+                                    Func aFunc) {
+    MOZ_ASSERT(mTransition.Buffering() == BufferingStrategy::UNBUFFERED);
+    MOZ_ASSERT(mUnbufferedState);
+    MOZ_ASSERT(!mYieldingToState);
+    MOZ_ASSERT(mBuffer.empty(),
+               "Buffered read at the same time as unbuffered read?");
+    MOZ_ASSERT(aIterator.Length() <= mUnbufferedState->mBytesRemaining,
+               "Read too much data during unbuffered read?");
+    MOZ_ASSERT(mUnbufferedState->mBytesConsumedInCurrentChunk == 0,
+               "Already consumed data in the current chunk, but not yielding?");
+
+    if (mUnbufferedState->mBytesRemaining == 0) {
+      // We're done with the unbuffered read, so transition to the next state.
+      return SetTransition(aFunc(mTransition.NextState(), nullptr, 0));
+    }
+
+    return ContinueUnbufferedRead(aIterator.Data(), aIterator.Length(),
+                                  aIterator.Length(), aFunc);
+  }
+
+  template <typename Func>
+  Maybe<LexerResult> UnbufferedReadAfterYield(SourceBufferIterator& aIterator,
+                                              Func aFunc) {
+    MOZ_ASSERT(mTransition.Buffering() == BufferingStrategy::UNBUFFERED);
+    MOZ_ASSERT(mUnbufferedState);
+    MOZ_ASSERT(mYieldingToState);
+    MOZ_ASSERT(mBuffer.empty(),
+               "Buffered read at the same time as unbuffered read?");
+    MOZ_ASSERT(aIterator.Length() <= mUnbufferedState->mBytesRemaining,
+               "Read too much data during unbuffered read?");
+    MOZ_ASSERT(
+        mUnbufferedState->mBytesConsumedInCurrentChunk <= aIterator.Length(),
+        "Consumed more data than the current chunk holds?");
+    MOZ_ASSERT(mTransition.UnbufferedState() == *mYieldingToState);
+
+    mYieldingToState = Nothing();
+
+    if (mUnbufferedState->mBytesRemaining == 0) {
+      // We're done with the unbuffered read, so transition to the next state.
+      return SetTransition(aFunc(mTransition.NextState(), nullptr, 0));
+    }
+
+    // Since we've yielded, we may have already consumed some data in this
+    // chunk. Make the necessary adjustments. (Note that the std::min call is
+    // just belt-and-suspenders to keep this code memory safe even if there's
+    // a bug somewhere.)
+    const size_t toSkip = std::min(
+        mUnbufferedState->mBytesConsumedInCurrentChunk, aIterator.Length());
+    const char* data = aIterator.Data() + toSkip;
+    const size_t length = aIterator.Length() - toSkip;
+
+    // If |length| is zero, we've hit the end of the current chunk. This only
+    // happens if we yield right at the end of a chunk. Rather than call |aFunc|
+    // with a |length| of zero bytes (which seems potentially surprising to
+    // decoder authors), we go ahead and read more data.
+    if (length == 0) {
+      return FinishCurrentChunkOfUnbufferedRead(aIterator.Length());
+    }
+
+    return ContinueUnbufferedRead(data, length, aIterator.Length(), aFunc);
+  }
+
+  template <typename Func>
+  Maybe<LexerResult> ContinueUnbufferedRead(const char* aData, size_t aLength,
+                                            size_t aChunkLength, Func aFunc) {
+    // Call aFunc with the unbuffered state to indicate that we're in the
+    // middle of an unbuffered read. We enforce that any state transition
+    // passed back to us is either a terminal state or takes us back to the
+    // unbuffered state.
+    LexerTransition<State> unbufferedTransition =
+        aFunc(mTransition.UnbufferedState(), aData, aLength);
+
+    // If we reached a terminal state, we're done.
+    if (unbufferedTransition.NextStateIsTerminal()) {
+      return SetTransition(unbufferedTransition);
+    }
+
+    MOZ_ASSERT(mTransition.UnbufferedState() ==
+               unbufferedTransition.NextState());
+
+    // Perform bookkeeping.
+    if (unbufferedTransition.ControlFlow() == ControlFlowStrategy::YIELD) {
+      mUnbufferedState->mBytesConsumedInCurrentChunk +=
+          unbufferedTransition.Size();
+      return SetTransition(unbufferedTransition);
+    }
+
+    MOZ_ASSERT(unbufferedTransition.Size() == 0);
+    return FinishCurrentChunkOfUnbufferedRead(aChunkLength);
+  }
+
+  Maybe<LexerResult> FinishCurrentChunkOfUnbufferedRead(size_t aChunkLength) {
+    // We've finished an unbuffered read of a chunk of length |aChunkLength|, so
+    // update |myBytesRemaining| to reflect that we're |aChunkLength| closer to
+    // the end of the unbuffered read. (The std::min call is just
+    // belt-and-suspenders to keep this code memory safe even if there's a bug
+    // somewhere.)
+    mUnbufferedState->mBytesRemaining -=
+        std::min(mUnbufferedState->mBytesRemaining, aChunkLength);
+
+    // Since we're moving on to a new chunk, we can forget about the count of
+    // bytes consumed by yielding in the current chunk.
+    mUnbufferedState->mBytesConsumedInCurrentChunk = 0;
+
+    return Nothing();  // Keep processing.
+  }
+
+  template <typename Func>
+  Maybe<LexerResult> BufferedRead(SourceBufferIterator& aIterator, Func aFunc) {
+    MOZ_ASSERT(mTransition.Buffering() == BufferingStrategy::BUFFERED);
+    MOZ_ASSERT(!mYieldingToState);
+    MOZ_ASSERT(!mUnbufferedState,
+               "Buffered read at the same time as unbuffered read?");
+    MOZ_ASSERT(mBuffer.length() < mTransition.Size() ||
+                   (mBuffer.length() == 0 && mTransition.Size() == 0),
+               "Buffered more than we needed?");
+
+    // If we have all the data, we don't actually need to buffer anything.
+    if (mBuffer.empty() && aIterator.Length() == mTransition.Size()) {
+      return SetTransition(
+          aFunc(mTransition.NextState(), aIterator.Data(), aIterator.Length()));
+    }
+
+    // We do need to buffer, so make sure the buffer has enough capacity. We
+    // deliberately wait until we know for sure we need to buffer to call
+    // reserve() since it could require memory allocation.
+    if (!mBuffer.reserve(mTransition.Size())) {
+      return SetTransition(Transition::TerminateFailure());
+    }
+
+    // Append the new data we just got to the buffer.
+    if (!mBuffer.append(aIterator.Data(), aIterator.Length())) {
+      return SetTransition(Transition::TerminateFailure());
+    }
+
+    if (mBuffer.length() != mTransition.Size()) {
+      return Nothing();  // Keep processing.
+    }
+
+    // We've buffered everything, so transition to the next state.
+    return SetTransition(
+        aFunc(mTransition.NextState(), mBuffer.begin(), mBuffer.length()));
+  }
+
+  template <typename Func>
+  Maybe<LexerResult> BufferedReadAfterYield(SourceBufferIterator& aIterator,
+                                            Func aFunc) {
+    MOZ_ASSERT(mTransition.Buffering() == BufferingStrategy::BUFFERED);
+    MOZ_ASSERT(mYieldingToState);
+    MOZ_ASSERT(!mUnbufferedState,
+               "Buffered read at the same time as unbuffered read?");
+    MOZ_ASSERT(mBuffer.length() <= mTransition.Size(),
+               "Buffered more than we needed?");
+
+    State nextState = std::move(*mYieldingToState);
+
+    // After a yield, we need to take the same data that we delivered to the
+    // last state, and deliver it again to the new state. We know that this is
+    // happening right at a state transition, and that the last state was a
+    // buffered read, so there are two cases:
+
+    // 1. We got the data from the SourceBufferIterator directly.
+    if (mBuffer.empty() && aIterator.Length() == mTransition.Size()) {
+      return SetTransition(
+          aFunc(nextState, aIterator.Data(), aIterator.Length()));
+    }
+
+    // 2. We got the data from the buffer.
+    if (mBuffer.length() == mTransition.Size()) {
+      return SetTransition(aFunc(nextState, mBuffer.begin(), mBuffer.length()));
+    }
+
+    // Anything else indicates a bug.
+    MOZ_ASSERT_UNREACHABLE("Unexpected state encountered during yield");
+    return SetTransition(Transition::TerminateFailure());
+  }
+
+  template <typename Func>
+  Maybe<LexerResult> Truncated(SourceBufferIterator& aIterator, Func aFunc) {
+    // The data is truncated. Let the lexer clean up and decide which terminal
+    // state we should end up in.
+    LexerTransition<State> transition =
+        mTruncatedTransition.NextStateIsTerminal()
+            ? mTruncatedTransition
+            : aFunc(mTruncatedTransition.NextState(), nullptr, 0);
+
+    if (!transition.NextStateIsTerminal()) {
+      MOZ_ASSERT_UNREACHABLE("Truncated state didn't lead to terminal state?");
+      return SetTransition(Transition::TerminateFailure());
+    }
+
+    // If the SourceBuffer was completed with a failing state, we end in
+    // TerminalState::FAILURE no matter what. This only happens in exceptional
+    // situations like SourceBuffer itself encountering a failure due to OOM.
+    if (NS_FAILED(aIterator.CompletionStatus())) {
+      return SetTransition(Transition::TerminateFailure());
+    }
+
+    return SetTransition(transition);
+  }
+
+  Maybe<LexerResult> SetTransition(const LexerTransition<State>& aTransition) {
+    // There should be no transitions while we're buffering for a buffered read
+    // unless they're to terminal states. (The terminal state transitions would
+    // generally be triggered by error handling code.)
+    MOZ_ASSERT_IF(!mBuffer.empty(), aTransition.NextStateIsTerminal() ||
+                                        mBuffer.length() == mTransition.Size());
+
+    // Similarly, the only transitions allowed in the middle of an unbuffered
+    // read are to a terminal state, or a yield to the same state. Otherwise, we
+    // should remain in the same state until the unbuffered read completes.
+    MOZ_ASSERT_IF(
+        mUnbufferedState,
+        aTransition.NextStateIsTerminal() ||
+            (aTransition.ControlFlow() == ControlFlowStrategy::YIELD &&
+             aTransition.NextState() == mTransition.UnbufferedState()) ||
+            mUnbufferedState->mBytesRemaining == 0);
+
+    // If this transition is a yield, save the next state and return. We'll
+    // handle the rest when Lex() gets called again.
+    if (!aTransition.NextStateIsTerminal() &&
+        aTransition.ControlFlow() == ControlFlowStrategy::YIELD) {
+      mYieldingToState = Some(aTransition.NextState());
+      return Some(LexerResult(Yield::OUTPUT_AVAILABLE));
+    }
+
+    // Update our transition.
+    mTransition = aTransition;
+
+    // Get rid of anything left over from the previous state.
+    mBuffer.clear();
+    mYieldingToState = Nothing();
+    mUnbufferedState = Nothing();
+
+    // If we reached a terminal state, let the caller know.
+    if (mTransition.NextStateIsTerminal()) {
+      return Some(LexerResult(mTransition.NextStateAsTerminal()));
+    }
+
+    // If we're entering an unbuffered state, record how long we'll stay in it.
+    if (mTransition.Buffering() == BufferingStrategy::UNBUFFERED) {
+      mUnbufferedState.emplace(mTransition.Size());
+    }
+
+    return Nothing();  // Keep processing.
+  }
+
+  // State that tracks our position within an unbuffered read.
+  struct UnbufferedState {
+    explicit UnbufferedState(size_t aBytesRemaining)
+        : mBytesRemaining(aBytesRemaining), mBytesConsumedInCurrentChunk(0) {}
+
+    size_t mBytesRemaining;
+    size_t mBytesConsumedInCurrentChunk;
+  };
+
+  Vector<char, InlineBufferSize> mBuffer;
+  LexerTransition<State> mTransition;
+  const LexerTransition<State> mTruncatedTransition;
+  Maybe<State> mYieldingToState;
+  Maybe<UnbufferedState> mUnbufferedState;
+};
+
+}  // namespace image
+}  // namespace mozilla
+
+#endif  // mozilla_image_StreamingLexer_h