[PATCH 2/2] pkt-line: show packets in async processes as "sideband"

From: Jeff King <peff@peff.net>
Date: Sep 1, 2015, 20:24 UTC
Message-ID: <20150901202412.GA8020@sigill.intra.peff.net>
In-Reply-To: <20150901202215.GA17370@sigill.intra.peff.net>

If you run "GIT_TRACE_PACKET=1 git push", you may get confusing output like (line prefixes omitted for clarity):

   packet:      push< \1000eunpack ok0019ok refs/heads/master0000
   packet:      push< unpack ok
   packet:      push< ok refs/heads/master
   packet:      push< 0000
   packet:      push< 0000

Why do we see the data twice, once apparently wrapped inside another pkt-line, and once unwrapped? Why do we get two flush packets?

The answer is that we start an async process to demux the sideband data. The first entry comes from the sideband process reading the data, and the second from push itself. Likewise, the first flush is inside the demuxed packet, and the second is an actual sideband flush.

We can make this a bit more clear by marking the sideband demuxer explicitly as "sideband" rather than "push". The most elegant way to do this would be to simply call packet_trace_identity() inside the sideband demuxer. But we can't do that reliably, because it relies on a global variable, which might be shared if pthreads are in use.

What we really need is thread-local storage for packet_trace_identity. But the async code does not provide an interface for that, and it would be messy to add it here (we'd have to care about pthreads, initializing our pthread_key_t ahead of time, etc).

So instead, let us just assume that any async process is handling sideband data. That's always true now, and is likely to remain so in the future.

The output looks like:

   packet:  sideband< \1000eunpack ok0019ok refs/heads/master0000
   packet:      push< unpack ok
   packet:      push< ok refs/heads/master
   packet:      push< 0000
   packet:  sideband< 0000

Signed-off-by: Jeff King <peff@peff.net>
---
 pkt-line.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/pkt-line.c b/pkt-line.c
index 08a1427..62fdb37 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "pkt-line.h"
+#include "run-command.h"
 
 char packet_buffer[LARGE_PACKET_MAX];
 static const char *packet_trace_prefix = "git";
@@ -11,6 +12,11 @@ void packet_trace_identity(const char *prog)
 	packet_trace_prefix = xstrdup(prog);
 }
 
+static const char *get_trace_prefix(void)
+{
+	return in_async() ? "sideband" : packet_trace_prefix;
+}
+
 static int packet_trace_pack(const char *buf, unsigned int len, int sideband)
 {
 	if (!sideband) {
@@ -57,7 +63,7 @@ static void packet_trace(const char *buf, unsigned int len, int write)
 	strbuf_init(&out, len+32);
 
 	strbuf_addf(&out, "packet: %12s%c ",
-		    packet_trace_prefix, write ? '>' : '<');
+		    get_trace_prefix(), write ? '>' : '<');
 
 	/* XXX we should really handle printable utf8 */
 	for (i = 0; i < len; i++) {

-- 
2.5.1.739.g7891f6b