path: root/builtin/receive-pack.c
diff options
authorJeff King <>2016-07-15 10:43:47 (GMT)
committerJunio C Hamano <>2016-07-20 19:11:11 (GMT)
commit83558686ceaeb634b630d5177818d571cafafbf4 (patch)
treeafae7f9221fe3b09104b6e5791bed05e49f1c357 /builtin/receive-pack.c
parent6b4cd2f82791b0c9726616832382e0af8d895d99 (diff)
receive-pack: send keepalives during quiet periods
After a client has sent us the complete pack, we may spend some time processing the data and running hooks. If the client asked us to be quiet, receive-pack won't send any progress data during the index-pack or connectivity-check steps. And hooks may or may not produce their own progress output. In these cases, the network connection is totally silent from both ends. Git itself doesn't care about this (it will wait forever), but other parts of the system (e.g., firewalls, load-balancers, etc) might hang up the connection. So we'd like to send some sort of keepalive to let the network and the client side know that we're still alive and processing. We can use the same trick we did in 05e9515 (upload-pack: send keepalive packets during pack computation, 2013-09-08). Namely, we will send an empty sideband data packet every `N` seconds that we do not relay any stderr data over the sideband channel. As with 05e9515, this means that we won't bother sending keepalives when there's actual progress data, but will kick in when it has been disabled (or if there is a lull in the progress data). The concept is simple, but the details are subtle enough that they need discussing here. Before the client sends us the pack, we don't want to do any keepalives. We'll have sent our ref advertisement, and we're waiting for them to send us the pack (and tell us that they support sidebands at all). While we're receiving the pack from the client (or waiting for it to start), there's no need for keepalives; it's up to them to keep the connection active by sending data. Moreover, it would be wrong for us to do so. When we are the server in the smart-http protocol, we must treat our connection as half-duplex. So any keepalives we send while receiving the pack would potentially be buffered by the webserver. Not only does this make them useless (since they would not be delivered in a timely manner), but it could actually cause a deadlock if we fill up the buffer with keepalives. (It wouldn't be wrong to send keepalives in this phase for a full-duplex connection like ssh; it's simply pointless, as it is the client's responsibility to speak). As soon as we've gotten all of the pack data, then the client is waiting for us to speak, and we should start keepalives immediately. From here until the end of the connection, we send one any time we are not otherwise sending data. But there's a catch. Receive-pack doesn't know the moment we've gotten all the data. It passes the descriptor to index-pack, who reads all of the data, and then starts resolving the deltas. We have to communicate that back. To make this work, we instruct the sideband muxer to enable keepalives in three phases: 1. In the beginning, not at all. 2. While reading from index-pack, wait for a signal indicating end-of-input, and then start them. 3. Afterwards, always. The signal from index-pack in phase 2 has to come over the stderr channel which the muxer is reading. We can't use an extra pipe because the portable run-command interface only gives us stderr and stdout. Stdout is already used to pass the .keep filename back to receive-pack. We could also send a signal there, but then we would find out about it in the main thread. And the keepalive needs to be done by the async muxer thread (since it's the one writing sideband data back to the client). And we can't reliably signal the async thread from the main thread, because the async code sometimes uses threads and sometimes uses forked processes. Therefore the signal must come over the stderr channel, where it may be interspersed with other random human-readable messages from index-pack. This patch makes the signal a single NUL byte. This is easy to parse, should not appear in any normal stderr output, and we don't have to worry about any timing issues (like seeing half the signal bytes in one read(), and half in a subsequent one). This is a bit ugly, but it's simple to code and should work reliably. Another option would be to stop using an async thread for muxing entirely, and just poll() both stderr and stdout of index-pack from the main thread. This would work for index-pack (because we aren't doing anything useful in the main thread while it runs anyway). But it would make the connectivity check and the hook muxers much more complicated, as they need to simultaneously feed the sub-programs while reading their stderr. The index-pack phase is the only one that needs this signaling, so it could simply behave differently than the other two. That would mean having two separate implementations of copy_to_sideband (and the keepalive code), though. And it still doesn't get rid of the signaling; it just means we can write a nicer message like "END_OF_INPUT" or something on stdout, since we don't have to worry about separating it from the stderr cruft. One final note: this signaling trick is only done with index-pack, not with unpack-objects. There's no point in doing it for the latter, because by definition it only kicks in for a small number of objects, where keepalives are not as useful (and this conveniently lets us avoid duplicating the implementation). Signed-off-by: Jeff King <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 'builtin/receive-pack.c')
1 files changed, 67 insertions, 1 deletions
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 7db1639..e41f55f 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -76,6 +76,13 @@ static long nonce_stamp_slop;
static unsigned long nonce_stamp_slop_limit;
static struct ref_transaction *transaction;
+static enum {
+} use_keepalive;
+static int keepalive_in_sec = 5;
static enum deny_action parse_deny_action(const char *var, const char *value)
if (value) {
@@ -193,6 +200,11 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
return 0;
+ if (strcmp(var, "receive.keepalive") == 0) {
+ keepalive_in_sec = git_config_int(var, value);
+ return 0;
+ }
return git_default_config(var, value, cb);
@@ -319,10 +331,60 @@ static void rp_error(const char *err, ...)
static int copy_to_sideband(int in, int out, void *arg)
char data[128];
+ int keepalive_active = 0;
+ if (keepalive_in_sec <= 0)
+ use_keepalive = KEEPALIVE_NEVER;
+ if (use_keepalive == KEEPALIVE_ALWAYS)
+ keepalive_active = 1;
while (1) {
- ssize_t sz = xread(in, data, sizeof(data));
+ ssize_t sz;
+ if (keepalive_active) {
+ struct pollfd pfd;
+ int ret;
+ pfd.fd = in;
+ ret = poll(&pfd, 1, 1000 * keepalive_in_sec);
+ if (ret < 0) {
+ if (errno == EINTR)
+ continue;
+ else
+ break;
+ } else if (ret == 0) {
+ /* no data; send a keepalive packet */
+ static const char buf[] = "0005\1";
+ write_or_die(1, buf, sizeof(buf) - 1);
+ continue;
+ } /* else there is actual data to read */
+ }
+ sz = xread(in, data, sizeof(data));
if (sz <= 0)
+ if (use_keepalive == KEEPALIVE_AFTER_NUL && !keepalive_active) {
+ const char *p = memchr(data, '\0', sz);
+ if (p) {
+ /*
+ * The NUL tells us to start sending keepalives. Make
+ * sure we send any other data we read along
+ * with it.
+ */
+ keepalive_active = 1;
+ send_sideband(1, 2, data, p - data, use_sideband);
+ send_sideband(1, 2, p + 1, sz - (p - data + 1), use_sideband);
+ continue;
+ }
+ }
+ /*
+ * Either we're not looking for a NUL signal, or we didn't see
+ * it yet; just pass along the data.
+ */
send_sideband(1, 2, data, sz, use_sideband);
@@ -1566,6 +1628,8 @@ static const char *unpack(int err_fd, struct shallow_info *si)
if (!quiet && err_fd)
argv_array_push(&child.args, "--show-resolving-progress");
+ if (use_sideband)
+ argv_array_push(&child.args, "--report-end-of-input");
if (fsck_objects)
argv_array_pushf(&child.args, "--strict%s",
@@ -1595,6 +1659,7 @@ static const char *unpack_with_sideband(struct shallow_info *si)
if (!use_sideband)
return unpack(0, si);
+ use_keepalive = KEEPALIVE_AFTER_NUL;
memset(&muxer, 0, sizeof(muxer));
muxer.proc = copy_to_sideband; = -1;
@@ -1782,6 +1847,7 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
unpack_status = unpack_with_sideband(&si);
update_shallow_info(commands, &si, &ref);
+ use_keepalive = KEEPALIVE_ALWAYS;
execute_commands(commands, unpack_status, &si);
if (pack_lockfile)