adb sync cleanup.

We can double the speed of "adb sync" (on N9) if we increase SYNC_DATA_MAX
from 64KiB to 256KiB. This change doesn't do that, because I still haven't
managed to plumb through the information about whether we're a new adb/adbd
to file_sync_client.cpp and file_sync_service.cpp. But this is already a big
change with a lot of cleanup, so let's do the cleanup and worry about the
intended change another day...

This change does improve performance somewhat by halving the number of
lstat(2) calls made on the client side, and ensuring that most packets are
sent with a single write. This has the pleasing result of making the null
sync on an AOSP N9 go from just over 300ms to around 100ms, which means it
now seems instantaneous (https://en.wikipedia.org/wiki/Mental_chronometry).

Change-Id: If9f6d4c1f93ec752b95f71211bbbb1c513045166
diff --git a/adb_io.h b/adb_io.h
index 8d50a6d..9c3b2a5 100644
--- a/adb_io.h
+++ b/adb_io.h
@@ -30,23 +30,22 @@
 // Writes a protocol-format string; a four hex digit length followed by the string data.
 bool SendProtocolString(int fd, const std::string& s);
 
-/*
- * Reads exactly len bytes from fd into buf.
- *
- * Returns false if there is an error or if EOF was reached before len bytes
- * were read. If EOF was found, errno will be set to 0.
- *
- * If this function fails, the contents of buf are undefined.
- */
-bool ReadFdExactly(int fd, void *buf, size_t len);
+// Reads a protocol-format string; a four hex digit length followed by the string data.
+bool ReadProtocolString(int fd, std::string* s, std::string* error);
 
-/*
- * Writes exactly len bytes from buf to fd.
- *
- * Returns false if there is an error or if the fd was closed before the write
- * completed. If the other end of the fd (such as in a socket, pipe, or fifo),
- * is closed, errno will be set to 0.
- */
+// Reads exactly len bytes from fd into buf.
+//
+// Returns false if there is an error or if EOF was reached before len bytes
+// were read. If EOF was found, errno will be set to 0.
+//
+// If this function fails, the contents of buf are undefined.
+bool ReadFdExactly(int fd, void* buf, size_t len);
+
+// Writes exactly len bytes from buf to fd.
+//
+// Returns false if there is an error or if the fd was closed before the write
+// completed. If the other end of the fd (such as in a socket, pipe, or fifo),
+// is closed, errno will be set to 0.
 bool WriteFdExactly(int fd, const void* buf, size_t len);
 
 // Same as above, but for strings.