[PATCH 3/8] receive-pack.c: use a single transaction when atomic-push is negotiated

[Ronnie Sahlberg <sahlberg@google.com>]

Update receive-pack to use an atomic transaction iff the client negotiated
that it wanted atomic-push.
This leaves the default behavior to be the old non-atomic one ref at a
time update. This is to cause as little disruption as possible to existing
clients. It is unknown if there are client scripts that depend on the old
non-atomic behavior so we make it opt-in for now.

Later patch in this series also adds a configuration variable where you can
override the atomic push behavior on the receiving repo and force it
to use atomic updates always.

If it turns out over time that there are no client scripts that depend on the
old behavior we can change git to default to use atomic pushes and instead
offer an opt-out argument for people that do not want atomic pushes.

Change-Id: Ice9739aa2676f76d2e7fab2d54f37047b2eb277e
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
---
 builtin/receive-pack.c | 73 +++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 58 insertions(+), 15 deletions(-)

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index b8ffd9e..6991d22 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -67,6 +67,8 @@ static const char *NONCE_SLOP = "SLOP";
 static const char *nonce_status;
 static long nonce_stamp_slop;
 static unsigned long nonce_stamp_slop_limit;
+struct strbuf err = STRBUF_INIT;
+struct transaction *transaction;
 
 static enum deny_action parse_deny_action(const char *var, const char *value)
 {
@@ -827,33 +829,55 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 				cmd->did_not_exist = 1;
 			}
 		}
-		if (delete_ref(namespaced_name, old_sha1, 0)) {
-			rp_error("failed to delete %s", name);
-			return "failed to delete";
+		if (!use_atomic_push) {
+			if (delete_ref(namespaced_name, old_sha1, 0)) {
+				rp_error("failed to delete %s", name);
+				return "failed to delete";
+			}
+		} else {
+			if (transaction_delete_ref(transaction,
+						   namespaced_name,
+						   old_sha1,
+						   0, old_sha1 != NULL,
+						   "push", &err)) {
+				rp_error("%s", err.buf);
+				strbuf_release(&err);
+				return "failed to delete";
+			}
 		}
 		return NULL; /* good */
 	}
 	else {
-		struct strbuf err = STRBUF_INIT;
-		struct transaction *transaction;
-
 		if (shallow_update && si->shallow_ref[cmd->index] &&
 		    update_shallow_ref(cmd, si))
 			return "shallow error";
 
-		transaction = transaction_begin(&err);
-		if (!transaction ||
-		    transaction_update_ref(transaction, namespaced_name,
-					   new_sha1, old_sha1, 0, 1, "push",
-					   &err) ||
-		    transaction_commit(transaction, &err)) {
-			transaction_free(transaction);
+		if (!use_atomic_push) {
+			transaction = transaction_begin(&err);
+			if (!transaction) {
+				rp_error("%s", err.buf);
+				strbuf_release(&err);
+				return "failed to start transaction";
+			}
+		}
+		if (transaction_update_ref(transaction,
+					   namespaced_name,
+					   new_sha1, old_sha1,
+					   0, 1, "push",
+					   &err)) {
 			rp_error("%s", err.buf);
 			strbuf_release(&err);
 			return "failed to update ref";
 		}
-
-		transaction_free(transaction);
+		if (!use_atomic_push) {
+			if (transaction_commit(transaction, &err)) {
+				transaction_free(transaction);
+				rp_error("%s", err.buf);
+				strbuf_release(&err);
+				return "failed to update ref";
+			}
+			transaction_free(transaction);
+		}
 		strbuf_release(&err);
 		return NULL; /* good */
 	}
@@ -1053,6 +1077,16 @@ static void execute_commands(struct command *commands,
 		return;
 	}
 
+	if (use_atomic_push) {
+		transaction = transaction_begin(&err);
+		if (!transaction) {
+			error("%s", err.buf);
+			strbuf_release(&err);
+			for (cmd = commands; cmd; cmd = cmd->next)
+				cmd->error_string = "transaction error";
+			return;
+		}
+	}
 	data.cmds = commands;
 	data.si = si;
 	if (check_everything_connected(iterate_receive_command_list, 0, &data))
@@ -1090,6 +1124,14 @@ static void execute_commands(struct command *commands,
 		}
 	}
 
+	if (use_atomic_push) {
+		if (transaction_commit(transaction, &err)) {
+			rp_error("%s", err.buf);
+			for (cmd = commands; cmd; cmd = cmd->next)
+				cmd->error_string = err.buf;
+		}
+		transaction_free(transaction);
+	}
 	if (shallow_update && !checked_connectivity)
 		error("BUG: run 'git fsck' for safety.\n"
 		      "If there are errors, try to remove "
@@ -1537,5 +1579,6 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 	sha1_array_clear(&shallow);
 	sha1_array_clear(&ref);
 	free((void *)push_cert_nonce);
+	strbuf_release(&err);
 	return 0;
 }
-- 
2.1.0.rc2.206.gedb03e5