[WIP 07/15] connect: convert get_remote_heads to use struct packet_reader

[Brandon Williams <bmwill@google.com>]

In order to allow for better control flow when protocol_v2 is introduced
convert 'get_remote_heads()' to use 'struct packet_reader' to read
packet lines.  This enables a client to be able to peek the first line
of a server's response (without consuming it) in order to determine the
protocol version its speaking and then passing control to the
appropriate handler.

This is needed because the initial response from a server speaking
protocol_v0 includes the first ref, while subsequent protocol versions
respond with a version line.  We want to be able to read this first line
without consuming the first ref sent in the protocol_v0 case so that the
protocol version the server is speaking can be determined outside of
'get_remote_heads()' in a future patch.

Signed-off-by: Brandon Williams <bmwill@google.com>
---
 connect.c | 125 +++++++++++++++++++++++++++++++++++---------------------------
 1 file changed, 70 insertions(+), 55 deletions(-)

diff --git a/connect.c b/connect.c
index 7fbd396b3..f79ea9179 100644
--- a/connect.c
+++ b/connect.c
@@ -48,6 +48,12 @@ int check_ref_type(const struct ref *ref, int flags)
 
 static void die_initial_contact(int unexpected)
 {
+	/*
+	 * A hang-up after seeing some response from the other end
+	 * means that it is unexpected, as we know the other end is
+	 * willing to talk to us.  A hang-up before seeing any
+	 * response does not necessarily mean an ACL problem, though.
+	 */
 	if (unexpected)
 		die(_("The remote end hung up upon initial contact"));
 	else
@@ -56,6 +62,41 @@ static void die_initial_contact(int unexpected)
 		      "and the repository exists."));
 }
 
+static enum protocol_version discover_version(struct packet_reader *reader)
+{
+	enum protocol_version version = protocol_unknown_version;
+
+	/*
+	 * Peek the first line of the server's response to
+	 * determine the protocol version the server is speaking.
+	 */
+	switch (packet_reader_peek(reader)) {
+	case PACKET_READ_ERROR:
+		die_initial_contact(0);
+	case PACKET_READ_FLUSH:
+	case PACKET_READ_DELIM:
+		version = protocol_v0;
+		break;
+	case PACKET_READ_NORMAL:
+		version = determine_protocol_version_client(reader->line);
+		break;
+	}
+
+	/* Maybe process capabilities here, at least for v2 */
+	switch (version) {
+	case protocol_v1:
+		/* Read the peeked version line */
+		packet_reader_read(reader);
+		break;
+	case protocol_v0:
+		break;
+	case protocol_unknown_version:
+		BUG("ERROR");
+	}
+
+	return version;
+}
+
 static void parse_one_symref_info(struct string_list *symref, const char *val, int len)
 {
 	char *sym, *target;
@@ -109,44 +150,10 @@ static void annotate_refs_with_symref_info(struct ref *ref)
 	string_list_clear(&symref, 0);
 }
 
-/*
- * Read one line of a server's ref advertisement into packet_buffer.
- */
-static int read_remote_ref(int in, char **src_buf, size_t *src_len,
-			   int *responded)
-{
-	int len = packet_read(in, src_buf, src_len,
-			      packet_buffer, sizeof(packet_buffer),
-			      PACKET_READ_GENTLE_ON_EOF |
-			      PACKET_READ_CHOMP_NEWLINE);
-	const char *arg;
-	if (len < 0)
-		die_initial_contact(*responded);
-	if (len > 4 && skip_prefix(packet_buffer, "ERR ", &arg))
-		die("remote error: %s", arg);
-
-	*responded = 1;
-
-	return len;
-}
-
-#define EXPECTING_PROTOCOL_VERSION 0
-#define EXPECTING_FIRST_REF 1
-#define EXPECTING_REF 2
-#define EXPECTING_SHALLOW 3
-
-/* Returns 1 if packet_buffer is a protocol version pkt-line, 0 otherwise. */
-static int process_protocol_version(void)
-{
-	switch (determine_protocol_version_client(packet_buffer)) {
-	case protocol_v1:
-		return 1;
-	case protocol_v0:
-		return 0;
-	default:
-		die("server is speaking an unknown protocol");
-	}
-}
+#define EXPECTING_FIRST_REF 0
+#define EXPECTING_REF 1
+#define EXPECTING_SHALLOW 2
+#define EXPECTING_DONE 3
 
 static void process_capabilities(int *len)
 {
@@ -230,28 +237,34 @@ struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 			      struct oid_array *shallow_points)
 {
 	struct ref **orig_list = list;
+	int len = 0;
+	int state = EXPECTING_FIRST_REF;
+	struct packet_reader reader;
+	const char *arg;
 
-	/*
-	 * A hang-up after seeing some response from the other end
-	 * means that it is unexpected, as we know the other end is
-	 * willing to talk to us.  A hang-up before seeing any
-	 * response does not necessarily mean an ACL problem, though.
-	 */
-	int responded = 0;
-	int len;
-	int state = EXPECTING_PROTOCOL_VERSION;
+	packet_reader_init(&reader, in, src_buf, src_len);
+
+	discover_version(&reader);
 
 	*list = NULL;
 
-	while ((len = read_remote_ref(in, &src_buf, &src_len, &responded))) {
+	while (state != EXPECTING_DONE) {
+		switch (packet_reader_read(&reader)) {
+		case PACKET_READ_ERROR:
+			die_initial_contact(1);
+		case PACKET_READ_NORMAL:
+			len = reader.pktlen;
+			if (len > 4 && skip_prefix(packet_buffer, "ERR ", &arg))
+				die("remote error: %s", arg);
+			break;
+		case PACKET_READ_FLUSH:
+			state = EXPECTING_DONE;
+			break;
+		case PACKET_READ_DELIM:
+			die("invalid packet\n");
+		}
+
 		switch (state) {
-		case EXPECTING_PROTOCOL_VERSION:
-			if (process_protocol_version()) {
-				state = EXPECTING_FIRST_REF;
-				break;
-			}
-			state = EXPECTING_FIRST_REF;
-			/* fallthrough */
 		case EXPECTING_FIRST_REF:
 			process_capabilities(&len);
 			if (process_dummy_ref()) {
@@ -269,6 +282,8 @@ struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 			if (process_shallow(len, shallow_points))
 				break;
 			die("protocol error: unexpected '%s'", packet_buffer);
+		case EXPECTING_DONE:
+			break;
 		default:
 			die("unexpected state %d", state);
 		}
-- 
2.15.1.424.g9478a66081-goog