[PATCH v2 3/3] t/lib-httpd: avoid occasional failures when checking access.log

The last test of 't5561-http-backend.sh', 'server request log matches test results' may fail occasionally, because the order of entries in Apache's access log doesn't match the order of requests sent in the previous tests, although all the right requests are there. I saw it fail on Travis CI five times in the span of about half a year, when the order of two subsequent requests was flipped, and could trigger the failure with a modified Git. However, I was unable to trigger it with stock Git on my machine. Three tests in 't5541-http-push-smart.sh' and 't5551-http-fetch-smart.sh' check requests in the log the same way, so they might be prone to a similar occasional failure as well.

When a test sends a HTTP request, it can continue execution after 'git-http-backend' fulfilled that request, but Apache writes the corresponding access log entry only after 'git-http-backend' exited. Some time inevitably passes between fulfilling the request and writing the log entry, and, under unfavourable circumstances, enough time might pass for the subsequent request to be sent and fulfilled by a different Apache thread or process, and then Apache writes access log entries racily.

This effect can be exacerbated by adding a bit of variable delay after the request is fulfilled but before 'git-http-backend' exits, e.g. like this:

  diff --git a/http-backend.c b/http-backend.c
  index f3dc218b2..bbf4c125b 100644
  --- a/http-backend.c
  +++ b/http-backend.c
  @@ -709,5 +709,7 @@ int cmd_main(int argc, const char **argv)
   					   max_request_buffer);

   	cmd->imp(&hdr, cmd_arg);
  +	if (getpid() % 2)
  +		sleep(1);
   	return 0;
   }

This delay considerably increases the chances of log entries being written out of order, and in turn makes t5561's last test fail almost every time. Alas, it doesn't seem to be enough to trigger a similar failure in t5541 and t5551.

So, since we can't just rely on the order of access log entries always corresponding the order of requests, make checking the access log more deterministic by sorting (simply lexicographically) both the stripped access log entries and the expected entries before the comparison with 'test_cmp'. This way the order of log entries won't matter and occasional out-of-order entries won't trigger a test failure, but the comparison will still notice any unexpected or missing log entries.

OTOH, this sorting will make it harder to identify from which test an unexpected log entry came from or which test's request went missing. Therefore, in case of an error include the comparison of the unsorted log enries in the test output as well.

And since all this should be performed in four tests in three test scripts, put this into a new helper function 'check_access_log' in 't/lib-httpd.sh'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/lib-httpd.sh              | 12 ++++++++++++
 t/t5541-http-push-smart.sh  | 13 +++++--------
 t/t5551-http-fetch-smart.sh |  3 +--
 t/t5561-http-backend.sh     |  3 +--
 4 files changed, 19 insertions(+), 12 deletions(-)