CommandProcessor: start dump implementation

Update CommandProcessor, to support dumping existing
logs. In order to keep the CL size reasonable, this
CL only implements dumping of the timestamps of each
message. Dumping the payload will be added in a follow-on
CL.

Note that the new unit tests use ::testing::Invoke(),
in a way that mixes the mocking and faking test
strategies in a single object. The GMock documentation
discourages this kind of mixing, in favor of splitting
a class into one piece that is to be mocked, and another
that is to be faked [1].

The reason we mix the strategies, despite the recommendation
to the contrary, is that we don't always want to fake
Os::Write(). Some times, we just want to mock that method.

Along the way:
- add a ScopedRewinder to MessageBuffer
- add the kDumpBuffers command to protocol::Opcode.
  This command is numbered 0x20, to leave room for
  addition kWrite<MessageType> commands.

[1]
https://github.com/google/googletest/blob/master/googlemock/docs/v1_6/CookBook.md
Specifically, "Having to mix a mock and a fake..."

Bug: 32098312
Test: ./runtests.sh
Change-Id: If221b47ae5615bbc114db5755ce9eb46b9934b6e

1 files changed, 1 insertions, 0 deletions

diff --git a/protocol.h b/protocol.h
index 61c3627..369a9e5 100644
--- a/protocol.h
+++ b/protocol.h
@@ -39,6 +39,7 @@ constexpr size_t kMaxMessageSize = 4096;
 
 enum class Opcode : uint16_t {
   kWriteAsciiMessage,
+  kDumpBuffers = 0x20,
 };
 
 enum class MessageSeverity : uint8_t {