signed push: remove duplicated protocol info

With the interim protocol, we used to send the update commands even
though we already send a signed copy of the same information when
push certificate is in use.  Update the send-pack/receive-pack pair
not to do so.

The notable thing on the receive-pack side is that it makes sure
that there is no command sent over the traditional protocol packet
outside the push certificate.  Otherwise a pusher can claim to be
pushing one set of ref updates in the signed certificate while
issuing commands to update unrelated refs, and such an update will
evade later audits.

Finally, start documenting the protocol.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/pack-protocol.txt

@@ -465,7 +465,7 @@ contain all the objects that the server will need to complete the new
 references.
 
 ----
-  update-request    =  *shallow command-list [pack-file]
+  update-request    =  *shallow ( command-list | push-cert ) [pack-file]
 
   shallow           =  PKT-LINE("shallow" SP obj-id)
 
@@ -481,12 +481,25 @@ references.
   old-id            =  obj-id
   new-id            =  obj-id
 
+  push-cert         = PKT-LINE("push-cert" NUL capability-list LF)
+		      PKT-LINE("certificate version 0.1" LF)
+		      PKT-LINE("pusher" SP ident LF)
+		      PKT-LINE(LF)
+		      *PKT-LINE(command LF)
+		      *PKT-LINE(gpg-signature-lines LF)
+		      PKT-LINE("push-cert-end" LF)
+
   pack-file         = "PACK" 28*(OCTET)
 ----
 
 If the receiving end does not support delete-refs, the sending end MUST
 NOT ask for delete command.
 
+If the receiving end does not support push-cert, the sending end
+MUST NOT send a push-cert command.  When a push-cert command is
+sent, command-list MUST NOT be sent; the commands recorded in the
+push certificate is used instead.
+
 The pack-file MUST NOT be sent if the only command used is 'delete'.
 
 A pack-file MUST be sent if either create or update command is used,
@@ -501,6 +514,24 @@ was being processed (the obj-id is still the same as the old-id), and
 it will run any update hooks to make sure that the update is acceptable.
 If all of that is fine, the server will then update the references.
 
+Push Certificate
+----------------
+
+A push certificate begins with a set of header lines.  After the
+header and an empty line, the protocol commands follow, one per
+line.
+
+Currently, the following header fields are defined:
+
+`pusher` ident::
+	Identify the GPG key in "Human Readable Name <email@address>"
+	format.
+
+The GPG signature lines are a detached signature for the contents
+recorded in the push certificate before the signature block begins.
+The detached signature is used to certify that the commands were
+given by the pusher, who must be the signer.
+
 Report Status
 -------------
 

Documentation/technical/protocol-capabilities.txt

@@ -18,8 +18,8 @@ was sent.  Server MUST NOT ignore capabilities that client requested
 and server advertised.  As a consequence of these rules, server MUST
 NOT advertise capabilities it does not understand.
 
-The 'report-status', 'delete-refs', and 'quiet' capabilities are sent and
-recognized by the receive-pack (push to server) process.
+The 'report-status', 'delete-refs', 'quiet', and 'push-cert' capabilities
+are sent and recognized by the receive-pack (push to server) process.
 
 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
 by both upload-pack and receive-pack protocols.  The 'agent' capability
@@ -250,3 +250,11 @@ allow-tip-sha1-in-want
 If the upload-pack server advertises this capability, fetch-pack may
 send "want" lines with SHA-1s that exist at the server but are not
 advertised by upload-pack.
+
+push-cert
+---------
+
+The receive-pack server that advertises this capability is willing
+to accept a signed push certificate.  A send-pack client MUST NOT
+send a push-cert packet unless the receive-pack server advertises
+this capability.

builtin/receive-pack.c

@@ -926,6 +926,28 @@ static struct command **queue_command(struct command **tail,
 	return &cmd->next;
 }
 
+static void queue_commands_from_cert(struct command **tail,
+				     struct strbuf *push_cert)
+{
+	const char *boc, *eoc;
+
+	if (*tail)
+		die("protocol error: got both push certificate and unsigned commands");
+
+	boc = strstr(push_cert->buf, "\n\n");
+	if (!boc)
+		die("malformed push certificate %.*s", 100, push_cert->buf);
+	else
+		boc += 2;
+	eoc = push_cert->buf + parse_signature(push_cert->buf, push_cert->len);
+
+	while (boc < eoc) {
+		const char *eol = memchr(boc, '\n', eoc - boc);
+		tail = queue_command(tail, boc, eol ? eol - boc : eoc - eol);
+		boc = eol ? eol + 1 : eoc;
+	}
+}
+
 static struct command *read_head_info(struct sha1_array *shallow)
 {
 	struct command *commands = NULL;
@@ -981,6 +1003,10 @@ static struct command *read_head_info(struct sha1_array *shallow)
 
 		p = queue_command(p, line, linelen);
 	}
+
+	if (push_cert.len)
+		queue_commands_from_cert(p, &push_cert);
+
 	return commands;
 }
 

send-pack.c

@@ -363,7 +363,7 @@ int send_pack(struct send_pack_args *args,
 	for (ref = remote_refs; ref; ref = ref->next) {
 		char *old_hex, *new_hex;
 
-		if (args->dry_run)
+		if (args->dry_run || args->push_cert)
 			continue;
 
 		if (!ref_update_to_be_sent(ref, args))