perf(protocol): pack metadata into one frame (PROTOCOL 2.20.0)

src/shared/config.h

@@ -642,8 +642,24 @@ typedef struct Config {
  * anything else) is what keeps a 2.19 client and a 2.18 server from ever
  * reaching that state.  SECURITY: a 2.19 store holds a salted PBKDF2 verifier
  * and cannot verify (and refuses to load) a legacy unsalted-SHA-256 store line,
- * so an old bearer digest can never be replayed against a 2.19 daemon. */
-#define PROTOCOL_VERSION "2.19.0"
+ * so an old bearer digest can never be replayed against a 2.19 daemon.
+ *
+ * Packed Metadata Wave: 2.19.0 -> 2.20.0.
+ *
+ * WHY the bump, grounded in the wire: metadata_send()/metadata_receive() no
+ * longer emit/consume the metadata as up to 12 separate per-field framed
+ * writes.  A file's metadata now crosses the wire as ONE packed frame: a
+ * single int32 present flag (0 = absent, 1 = present) followed, when present,
+ * by the fixed FILE_METADATA_WIRE_SIZE-byte (68-byte) field record produced by
+ * metadata_to_buf().  A 2.19 peer would desynchronize on the removed frames
+ * (it would read the packed record's bytes as a stream of separate field
+ * frames), so the strict same-version handshake (config_receive rejects a
+ * mismatched version before parsing anything else) is what keeps a 2.20 client
+ * and a 2.19 server from ever reaching that state.  The encoded field layout
+ * itself is unchanged (only its framing collapses), so the chunk codec, which
+ * already used the packed metadata_to_buf()/metadata_from_buf() codec, is
+ * byte-identical to before. */
+#define PROTOCOL_VERSION "2.20.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 /* Upper bound on total basis-dir entries (rsync caps --link-dest at 20). */
 #define MAX_BASIS_DIRS 64