[WIP 03/15] pkt-line: add delim packet support

[Brandon Williams <bmwill@google.com>]

One of the design goals of protocol-v2 is to improve the semantics of
flush packets.  Currently in protocol-v1, flush packets are used both to
indicate a break in a list of packet lines as well as an indication that
one side has finished speaking.  This makes it particularly difficult
to implement proxies as a proxy would need to completely understand git
protocol instead of simply looking for a flush packet.

To do this, introduce the special deliminator packet '0001'.  A delim
packet can then be used as a deliminator between lists of packet lines
while flush packets can be reserved to indicate the end of a response.

Signed-off-by: Brandon Williams <bmwill@google.com>
---
 pkt-line.c | 19 ++++++++++++++++++-
 pkt-line.h |  3 +++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/pkt-line.c b/pkt-line.c
index 518109bbe..222e1e310 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -91,6 +91,12 @@ void packet_flush(int fd)
 	write_or_die(fd, "0000", 4);
 }
 
+void packet_delim(int fd)
+{
+	packet_trace("0001", 4, 1);
+	write_or_die(fd, "0001", 4);
+}
+
 int packet_flush_gently(int fd)
 {
 	packet_trace("0000", 4, 1);
@@ -105,6 +111,12 @@ void packet_buf_flush(struct strbuf *buf)
 	strbuf_add(buf, "0000", 4);
 }
 
+void packet_buf_delim(struct strbuf *buf)
+{
+	packet_trace("0001", 4, 1);
+	strbuf_add(buf, "0001", 4);
+}
+
 static void set_packet_header(char *buf, const int size)
 {
 	static char hexchar[] = "0123456789abcdef";
@@ -297,7 +309,10 @@ enum packet_read_status packet_read_with_status(int fd, char **src_buffer, size_
 	if (len == 0) {
 		packet_trace("0000", 4, 0);
 		return PACKET_READ_FLUSH;
-	} else if (len >= 1 && len <= 3) {
+	} else if (len == 1) {
+		packet_trace("0001", 4, 0);
+		return PACKET_READ_DELIM;
+	} else if (len >= 2 && len <= 3) {
 		die("protocol error: bad line length character: %.4s", linelen);
 	}
 
@@ -333,6 +348,7 @@ int packet_read(int fd, char **src_buffer, size_t *src_len,
 		break;
 	case PACKET_READ_NORMAL:
 		break;
+	case PACKET_READ_DELIM:
 	case PACKET_READ_FLUSH:
 		pktlen = 0;
 		break;
@@ -447,6 +463,7 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
 	case PACKET_READ_NORMAL:
 		reader->line = reader->buffer;
 		break;
+	case PACKET_READ_DELIM:
 	case PACKET_READ_FLUSH:
 		reader->pktlen = 0;
 		reader->line = NULL;
diff --git a/pkt-line.h b/pkt-line.h
index 2b5c7cf11..49ec80c80 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -20,8 +20,10 @@
  * side can't, we stay with pure read/write interfaces.
  */
 void packet_flush(int fd);
+void packet_delim(int fd);
 void packet_write_fmt(int fd, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 void packet_buf_flush(struct strbuf *buf);
+void packet_buf_delim(struct strbuf *buf);
 void packet_write(int fd_out, const char *buf, size_t size);
 void packet_buf_write(struct strbuf *buf, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 int packet_flush_gently(int fd);
@@ -64,6 +66,7 @@ enum packet_read_status {
 	PACKET_READ_ERROR = -1,
 	PACKET_READ_NORMAL,
 	PACKET_READ_FLUSH,
+	PACKET_READ_DELIM,
 };
 #define PACKET_READ_GENTLE_ON_EOF (1u<<0)
 #define PACKET_READ_CHOMP_NEWLINE (1u<<1)
-- 
2.15.1.424.g9478a66081-goog