MessageBuffer: use CopyFromBufferOrDie()

Update ConsumeNextMessage() to make use of
local_utils::CopyFromBufferOrDie(). This (hopefully)
accomplishes two things:
- makes the code a bit more obvious, and
- makes the code more consistent with CommandProcessor

Along the way: fix a bug in the way that we check
that the MessageBuffer contains enough bytes for
a LengthHeader.

(Previously, we were checking that the LengthHeader
fit within the memory bounds of |data_|. Really,
though, we should check that the LengthHeader fits
within the populated area of |data_|.)

Bug: 31971382
Test: ./runtests.sh (on bullhead)
Change-Id: Ie3a4c38fefb69a3212142a53e79fdc720af6c601

1 files changed, 4 insertions, 0 deletions

diff --git a/message_buffer.h b/message_buffer.h
index 1a1932b..15e394e 100644
--- a/message_buffer.h
+++ b/message_buffer.h
@@ -82,6 +82,10 @@ class MessageBuffer {
   // Writes arbitrary data into the buffer.
   void AppendRawBytes(NONNULL const void* data_start, size_t data_len);
 
+  // Returns the total number of bytes available for reading. This number
+  // includes headers.
+  size_t GetReadableSize() const { return write_pos_ - read_pos_; }
+
   std::unique_ptr<uint8_t[]> data_;
   size_t capacity_;
   size_t read_pos_;