Fix occasional test failures in http tests

't5561-http-backend.sh' is prone to occasional failures; luckily it's not 'git-http-backend's fault, but the test script is a bit racy. I won't go into the details here, patch 4/4's commit message discusses it at length. 4/4 also fixes this issue, but I'm not particularly happy with that fix, the note attached to that patch explains why, along with possible alternatives, hence the RFC.

Even if we settle for a different fix, I think the first two patches are worthy cleanups on their own right.

SZEDER Gábor (4):
t5541: avoid empty line when truncating access log t/lib-httpd: add the strip_access_log() helper function t/lib-httpd: add minor and patch version to $HTTPD_VERSION t/lib-httpd: sort log based on timestamp to avoid occasional failure

 t/lib-httpd.sh              | 24 ++++++++++++++++++++++--
 t/lib-httpd/apache.conf     |  5 +++++
 t/t5541-http-push-smart.sh  | 17 +++--------------
 t/t5551-http-fetch-smart.sh |  7 +------
 t/t5561-http-backend.sh     |  7 +------
 5 files changed, 32 insertions(+), 28 deletions(-)

2.18.0.rc0.207.ga6211da864

The second test of 't5541-http-push-smart.sh', 'no empty path components' truncates Apache's access log by running

echo >.../access.log

which doesn't leave an empty file behind, like a proper truncation would, but a file with a lone newline in it. Consequently, a later test checking the log's contents must consider this improper truncation and include an empty line in the expected content.

There is no need for that newline at all, so drop the 'echo' from the truncation and adjust the expected content accordingly.

---

 t/t5541-http-push-smart.sh | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index 21340e89c9..c961db814d 100755
@@ -54,7 +54,7 @@ test_expect_success 'no empty path components' '
 	# service" test which reads the log too.
 	#
 	# We do this before the actual comparison to ensure the log is cleared.
-	echo > "$HTTPD_ROOT_PATH"/access.log &&
+	>"$HTTPD_ROOT_PATH"/access.log &&
 
 	test_cmp exp act
 '

@@ -124,7 +124,6 @@ test_expect_success 'rejected update prints status' '
 rm -f "$HTTPD_DOCUMENT_ROOT_PATH/test_repo.git/hooks/update"
 
 cat >exp <<EOF
-
 GET  /smart/test_repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-upload-pack HTTP/1.1 200
 GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200

2.18.0.rc0.207.ga6211da864

Four tests in three httpd-related test scripts check the contents of Apache's 'access.log', and they all do so by running 'sed' with the exact same script consisting of four s/// commands to strip uninteresting log fields and to vertically align the requested URLs.

Extract this into a common helper function 'strip_access_log' in 'lib-httpd.sh', and use it in all of those tests.

---
 t/lib-httpd.sh              |  9 +++++++++
 t/t5541-http-push-smart.sh  | 14 ++------------
 t/t5551-http-fetch-smart.sh |  7 +------
 t/t5561-http-backend.sh     |  7 +------
 4 files changed, 13 insertions(+), 24 deletions(-)

index 435a37465a..b6788fea57 100644
@@ -287,3 +287,12 @@ expect_askpass() {
 	test_cmp "$TRASH_DIRECTORY/askpass-expect" \
 		 "$TRASH_DIRECTORY/askpass-query"
 }
+
+strip_access_log() {
+	sed -e "
+		s/^.* \"//
+		s/\"//
+		s/ [1-9][0-9]*\$//
+		s/^GET /GET  /
+	" "$HTTPD_ROOT_PATH"/access.log
+}

index c961db814d..a1385800fa 100755
@@ -43,12 +43,7 @@ test_expect_success 'no empty path components' '
 	cd "$ROOT_PATH" &&
 	git clone $HTTPD_URL/smart/test_repo.git/ test_repo_clone &&
 
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 
 	# Clear the log, so that it does not affect the "used receive-pack
 	# service" test which reads the log too.

@@ -137,12 +132,7 @@ GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-receive-pack HTTP/1.1 200
 EOF
 test_expect_success 'used receive-pack service' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

index f5721b4a59..82a4fb5e16 100755
@@ -98,12 +98,7 @@ GET  /smart/repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/repo.git/git-upload-pack HTTP/1.1 200
 EOF
 test_expect_success 'used upload-pack service' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

index 90e0d6f0fe..6a0f240e6d 100755
@@ -123,12 +123,7 @@ GET  /smart/repo.git/info/refs?service=git-receive-pack HTTP/1.1 403 -
 POST /smart/repo.git/git-receive-pack HTTP/1.1 403 -
 EOF
 test_expect_success 'server request log matches test results' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

2.18.0.rc0.207.ga6211da864

To fix an occasional test failure, the next patch will rely on an Apache log format specifier that was introduced in version 2.2.30. Consequently, 't/lib-httpd.sh' must be able to decide whether the Apache version used for testing is equal or newer than that version to be able to act accordingly when processing the webserver's access log.

$HTTPD_VERSION currently only contains Apache's major version, so change how Apache's version string is parsed to store the minor and patch versions as well. Furthermore, update the only condition looking at $HTTPD_VERSION to deal with the full version.

---
 t/lib-httpd.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index b6788fea57..5915625631 100644
@@ -99,13 +99,13 @@ then
 fi
 
 HTTPD_VERSION=$($LIB_HTTPD_PATH -v | \
-	sed -n 's/^Server version: Apache\/\([0-9]*\)\..*$/\1/p; q')
+	sed -n 's/^Server version: Apache\/\([0-9]*\.[0-9]*\.[0-9]*\).*$/\1/p; q')
 
 if test -n "$HTTPD_VERSION"
 then
 	if test -z "$LIB_HTTPD_MODULE_PATH"
 	then
-		if ! test $HTTPD_VERSION -ge 2
+		if ! test ${HTTPD_VERSION%%.*} -ge 2
 		then
 			test_skip_or_die $GIT_TEST_HTTPD \
 				"at least Apache version 2 is required"

2.18.0.rc0.207.ga6211da864

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, 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.

Now, by default the timestamp of a log entry marks the beginning of the request processing, not when the log entry gets written. Since our requests are sent sequentially, sorting the log entries based on their timestamps would ensure that their order corresponds to the order of our requests.

Therefore, change Apache's log format to use microseconds since epoch as timestamps, the finest resolution Apache has to offer. This will:

- give enough resolution to make reasonably sure that no two requests have the same timestamp; the default timestamp format has only one second resolution, which is not nearly enough.

- make the timestamps sortable; the default timestamp format looks like "[31/Jan/2018:16:33:52 +0000]", which would trip sorting if the test script happens to run over month boundaries.

Modify the strip_access_log() helper function, used by all potentially affected tests, to sort the access log entries based on their timestamp before stripping the uninteresting bits from the entries.

Unfortunately, this "microseconds since epoch" is an extended time format specifier which was only introduced in Apache 2.2.30 [1], and standard strftime() doesn't have any format specifiers with microsecond or finer precision, so we can't use the above solution with Apache versions older than 2.2.30. However, since Apache 2.2 is no longer maintained anyway, it's not that big a deal to leave test runs with those old versions prone to this occasional failure. So use microsecond timestamps and sort access log entries only if the test is run with an Apache version supporting it; otherwise stick to the default timestamp format and leave the order of access log entries unchanged.

[1] http://httpd.apache.org/docs/2.2/mod/mod_log_config.html

---

Notes:
The log of the latest of this test failures:

https://travis-ci.org/szeder/git-cooking-topics-for-travis-ci/jobs/391499735#L3130

I don't really like the fix in this patch. I think an unfortunate clock skew during the test run could mess up the sorting added in this patch and cause test failure. Or if DST or even a leap second happen while the test is running. Do we care? Anyway, this occasional test failure apparently happens more often than DST and leap seconds combined.
Furthermore, in the future we might have computers fast enough to fulfill more than one test requests in a single microsecond; but I guess by that time we'll have higher resolution time formats as well.

An alternative I considered was that we could decide that the order of requests in the access log is not important as long as all the right requests are there. This would inherently eliminate the raciness issue, but if something were to go wrong, then it would become rather hard to find out e.g. which request from which test has gone missing, especially considering that several requests are sent in more than one test. We could address this by truncating the access log at the beginning and checking its contents at the end of each test sending requests. Unfortunately, this would raise additional difficulties, because all requests in t5561 come from tests included from 't556x-common', i.e. from tests shared with 't5560-http-backend-noserver.sh', which as its name suggests doesn't run Apache and doesn't have an access log at all.

Another alternative is to simply accept that these tests are racy, and that the resulting test failures are rare enough that it isn't worth the complexity of the workaround, but adding a comment to the affected tests warning about the raciness is sufficient. (But I wrote this when I first saw and tracked down this failure; since then I observed it four more times... :)

--

Apache doesn't maintain 2.2 anymore; the final maintenance release 2.2.34 was released in July 2017, almost a year ago. OTOH, our 't/lib-httpd/apache.conf' contains a couple of IfVersion directives dealing with versions <2.4, and one even with <2.1. How much do we actually care about these unmaintained Apache versions, and how much of this is just bitrotting?

 t/lib-httpd.sh          | 13 ++++++++++++-
 t/lib-httpd/apache.conf |  5 +++++
 2 files changed, 17 insertions(+), 1 deletion(-)

index 5915625631..1a7fe67141 100644
@@ -289,10 +289,21 @@ expect_askpass() {
 }
 
 strip_access_log() {
+	case "$HTTPD_VERSION" in
+	2.[01].*|2.2.[0-9]|2.2.[12][0-9])
+		# Apache versions < 2.2.30 didn't have any log timestamp
+		# format with microseconds precision, so don't attempt to
+		# sort the access log when testing with such old versions.
+		cat "$HTTPD_ROOT_PATH"/access.log
+		;;
+	*)
+		sort -n -k 4,4 "$HTTPD_ROOT_PATH"/access.log
+		;;
+	esac | \
 	sed -e "
 		s/^.* \"//
 		s/\"//
 		s/ [1-9][0-9]*\$//
 		s/^GET /GET  /
-	" "$HTTPD_ROOT_PATH"/access.log
+	"
 }

index 724d9ae462..8d6da6bdb2 100644
@@ -1,7 +1,12 @@
 ServerName dummy
 PidFile httpd.pid
 DocumentRoot www
+<IfVersion >= 2.2.30>
+LogFormat "%h %l %u %{usec}t \"%r\" %>s %b" common
+</IfVersion>
+<IfVersion < 2.2.30>
 LogFormat "%h %l %u %t \"%r\" %>s %b" common
+</IfVersion>
 CustomLog access.log common
 ErrorLog error.log
 <IfModule !mod_log_config.c>

2.18.0.rc0.207.ga6211da864

A bit of addendum here:

There are basically two "kinds" of requests:

1. requests asking for regular files, e.g.:

     GET  /smart/repo.git/HEAD HTTP/1.1 200
     GET  /smart/repo.git/info/refs HTTP/1.1 200
     GET  /smart/repo.git/objects/info/packs HTTP/1.1 200

2. requests asking for 'git-receive-pack' or 'git-upload-pack',

     e.g.:

     GET  /smart/repo.git/info/refs?service=git-receive-pack HTTP/1.1 200
     POST /smart/repo.git/git-receive-pack HTTP/1.1 200 -

This artificial variable delay shuffles only the access log entries written for the first kind of requests, but doesn't appear to have any effect on the order of log entries written for the second kind of requests.

t5561 sends both kinds of requests, so this variable delay is sufficient to make the test fail most of the time.

t5541 and t5551, however, send only requests of the second kind, so that's why this variable delay can't trigger a similar failure in these test scripts.

I haven't investigated any further what else would be necessary to trigger a failure in t5541 and t5551.

I've occasionally run into these failures on my local box, too. I'm happy somebody is looking into it (I have before, but eventually threw up my hands in disgust).

That's a reasonably clever solution. One thing I wonder, though: are we always guaranteed that the log entries are written _at all_ before we look at them?

I.e., could we have a situation where we make a request, the client finishes, and then we look at the logs, but nothing has been written by apache yet?

We could probably eliminate DST issues by consistently using UTC for the timestamps. Leap seconds are probably infrequent enough not to worry about. More likely is something like clock adjustment due to ntp. Those adjustments are usually small enough not to matter, but if we're talking microseconds, it could trigger.

What if the test script provides the in-order expectation, but we check only the unordered version (by sorting both actual and expected output on the fly)? That would give us a more relaxed check most of the time, but somebody digging in to a failure could run the ordered diff (or we could even show it automatically on failure instead of just using test_cmp).

It's definitely bugged me. I'd be happy to see some solution. I've been close to suggesting that reading apache logs is simply not robust, and we should focus our tests on the git-visible state changes (e.g., seeing successful requests, updated refs, etc).

A side effect of that is that it would become a lot easier to support other webservers in our test scripts (though that may still be a fool's errand just due to the amount of custom config we seem to carry).

I strongly suspect bitrotting. It looks like most of the "< 2.4" directives are from 5 years ago (when Debian switched to 2.4 by default), so even long-term stable systems would generally have it by now (though I'm sure there's probably some RHEL version without it).

-Peff

Hmph, that certainly is "checking only the things that matter", which is desirable.

The existing tests that look at the apache logs are very white-box, and come from Shawn's original smart-http series. I suspect it was as much about sanity-checking the implementation then as it was about protecting against regressions.

So it's possible that these tests have simply outlived their usefulness.

OTOH, they might catch non-functional problems, like if we started sending redundant requests. But then, they are not very comprehensive, as they only cover a few basic situations (so for example, for a while we were sending extra auth-related requests, but I don't think these tests detected that).

-Peff

Heh, well, I found it to be one of the more entertaining debugging sessions... ;)
I'm glad that somebody else saw it, too, and outside of Travis CI.

That's possible, I suppose. Highly improbable, but possible. Or the log entries can be written only partially by the time we look at them.

FWIW, Apache has the 'BufferedLogs' directive, which "causes mod_log_config to store several log entries in memory and write them together to disk, rather than writing them after each request." It very sensibly defaults to off, and our test config doesn't set it.

I would think that stopping Apache should flush the logs, but I don't know whether 'stop_httpd' blocks until the web server is indeed stopped, or it's all async. Perhaps a SIGHUP would suffice to flush the logs, but that's definitely async. Also note that while t5561 checks the access log at the end of the script, t5541 and t5551 do check the logs in the middle of the script, so we would have to stop Apache and then start again.

Anyway, I would prefer not to do anything about it just yet, because I'm curious :) and I'd like to see whether this actually happens in practice.

Ok, so admittedly I haven't quite thought through all these potential clock-related issues that I mentioned.

Setting back the clock at the end of DST is not an issue, because Apache uses the timezone it finds set in the environment, and our test environment does set UTC, so as far as the tests and Apache's log timestamps are concerned there is no such thing as DST. Leap seconds don't go back in time, they are only added to compensate for Earth's slowing rotation, so that's not an issue, either. And I think NTP is supposed to be clever enough to not set back the clock recklessly, but only advance it slower.

Of course, someone can still set back the clock manually while these tests run, but I would put that in the "If it hurts, don't do it" box.

However, having said all this, ...

this is the fix I'll go for in v2.

Digging into earlier cases I had seen on my local box, my impression was that this was the problem I was seeing (not reordering). But it's entirely possible that I was confused by reordering. I've been trying to reproduce, but after 5 minutes of t5561 running in a tight loop under high load, I haven't gotten a single failure.

I can't reproduce for now, so I'm happy either way. :)

Great. That seems much simpler to reason about. I'll take a look.

-Peff

Interesting.

Yeah, the first two are unquestionally good.

The sed expression of the second one could use sq around (instead of dq) to lose hard-to-read backslash quoting from there, but it probably is OK to keep the second one a pure "refactoring" patch, and leave it to a separate "further cleanup" patch to make such a change.

Yes, the first two look like improvements on their own (and I left more detailed comments on the final one). Thanks for tackling this!

-Peff

't5561-http-backend.sh' is prone to occasional failures; luckily it's not 'git-http-backend's fault, but the test script is a bit racy. I won't go into the details here, patch 3/3's commit message discusses it at length.

v1 is here; it haven't been picked up by Junio:

https://public-inbox.org/git/[email protected]/T/#u

The first two patches are identical to those in v1, and the last patch implements a different and simpler fix than in v1, following Peff's suggestion. The third patch was dropped, because it's not necessary for this simpler fix.

SZEDER Gábor (3):
t5541: avoid empty line when truncating access log t/lib-httpd: add the strip_access_log() helper function t/lib-httpd: avoid occasional failures when checking access.log

 t/lib-httpd.sh              | 21 +++++++++++++++++++++
 t/t5541-http-push-smart.sh  | 24 +++++-------------------
 t/t5551-http-fetch-smart.sh |  8 +-------
 t/t5561-http-backend.sh     |  8 +-------
 4 files changed, 28 insertions(+), 33 deletions(-)

2.18.0.273.g57f1ecce9c

The second test of 't5541-http-push-smart.sh', 'no empty path components' truncates Apache's access log by running

echo >.../access.log

which doesn't leave an empty file behind, like a proper truncation would, but a file with a lone newline in it. Consequently, a later test checking the log's contents must consider this improper truncation and include an empty line in the expected content.

There is no need for that newline at all, so drop the 'echo' from the truncation and adjust the expected content accordingly.

---
 t/t5541-http-push-smart.sh | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index a2af693068..d623cbad97 100755
@@ -54,7 +54,7 @@ test_expect_success 'no empty path components' '
 	# service" test which reads the log too.
 	#
 	# We do this before the actual comparison to ensure the log is cleared.
-	echo > "$HTTPD_ROOT_PATH"/access.log &&
+	>"$HTTPD_ROOT_PATH"/access.log &&
 
 	test_cmp exp act
 '

@@ -124,7 +124,6 @@ test_expect_success 'rejected update prints status' '
 rm -f "$HTTPD_DOCUMENT_ROOT_PATH/test_repo.git/hooks/update"
 
 cat >exp <<EOF
-
 GET  /smart/test_repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-upload-pack HTTP/1.1 200
 GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200

2.18.0.273.g57f1ecce9c

Four tests in three httpd-related test scripts check the contents of Apache's 'access.log', and they all do so by running 'sed' with the exact same script consisting of four s/// commands to strip uninteresting log fields and to vertically align the requested URLs.

Extract this into a common helper function 'strip_access_log' in 'lib-httpd.sh', and use it in all of those tests.

---
 t/lib-httpd.sh              |  9 +++++++++
 t/t5541-http-push-smart.sh  | 14 ++------------
 t/t5551-http-fetch-smart.sh |  7 +------
 t/t5561-http-backend.sh     |  7 +------
 4 files changed, 13 insertions(+), 24 deletions(-)

index 435a37465a..b6788fea57 100644
@@ -287,3 +287,12 @@ expect_askpass() {
 	test_cmp "$TRASH_DIRECTORY/askpass-expect" \
 		 "$TRASH_DIRECTORY/askpass-query"
 }
+
+strip_access_log() {
+	sed -e "
+		s/^.* \"//
+		s/\"//
+		s/ [1-9][0-9]*\$//
+		s/^GET /GET  /
+	" "$HTTPD_ROOT_PATH"/access.log
+}

index d623cbad97..6cd986797d 100755
@@ -43,12 +43,7 @@ test_expect_success 'no empty path components' '
 	cd "$ROOT_PATH" &&
 	git clone $HTTPD_URL/smart/test_repo.git/ test_repo_clone &&
 
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 
 	# Clear the log, so that it does not affect the "used receive-pack
 	# service" test which reads the log too.

@@ -137,12 +132,7 @@ GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-receive-pack HTTP/1.1 200
 EOF
 test_expect_success 'used receive-pack service' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

index 913089b144..c8b6ec493b 100755
@@ -103,12 +103,7 @@ GET  /smart/repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/repo.git/git-upload-pack HTTP/1.1 200
 EOF
 test_expect_success 'used upload-pack service' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

index 84a955770a..4248b5a06d 100755
@@ -129,12 +129,7 @@ GET  /smart/repo.git/info/refs?service=git-receive-pack HTTP/1.1 403 -
 POST /smart/repo.git/git-receive-pack HTTP/1.1 403 -
 EOF
 test_expect_success 'server request log matches test results' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

2.18.0.273.g57f1ecce9c

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'.

---
 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(-)

index b6788fea57..7f060aebd0 100644
@@ -296,3 +296,15 @@ strip_access_log() {
 		s/^GET /GET  /
 	" "$HTTPD_ROOT_PATH"/access.log
 }
+
+# Requires one argument: the name of a file containing the expected stripped
+# access log entries.
+check_access_log() {
+	sort "$1" >"$1".sorted &&
+	strip_access_log >access.log.stripped &&
+	sort access.log.stripped >access.log.sorted &&
+	if ! test_cmp "$1".sorted access.log.sorted
+	then
+		test_cmp "$1" access.log.stripped
+	fi
+}

index 6cd986797d..a481e3989a 100755
@@ -43,15 +43,13 @@ test_expect_success 'no empty path components' '
 	cd "$ROOT_PATH" &&
 	git clone $HTTPD_URL/smart/test_repo.git/ test_repo_clone &&
 
-	strip_access_log >act &&
+	check_access_log exp
+'
 
+test_expect_success 'clear access log' '
 	# Clear the log, so that it does not affect the "used receive-pack
 	# service" test which reads the log too.
-	#
-	# We do this before the actual comparison to ensure the log is cleared.
-	>"$HTTPD_ROOT_PATH"/access.log &&
-
-	test_cmp exp act
+	>"$HTTPD_ROOT_PATH"/access.log
 '
 
 test_expect_success 'clone remote repository' '

@@ -132,8 +130,7 @@ GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-receive-pack HTTP/1.1 200
 EOF
 test_expect_success 'used receive-pack service' '
-	strip_access_log >act &&
-	test_cmp exp act
+	check_access_log exp
 '
 
 test_http_push_nonff "$HTTPD_DOCUMENT_ROOT_PATH"/test_repo.git \

index c8b6ec493b..3aab44bdcb 100755
@@ -103,8 +103,7 @@ GET  /smart/repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/repo.git/git-upload-pack HTTP/1.1 200
 EOF
 test_expect_success 'used upload-pack service' '
-	strip_access_log >act &&
-	test_cmp exp act
+	check_access_log exp
 '
 
 test_expect_success 'follow redirects (301)' '

index 4248b5a06d..1c49054595 100755
@@ -129,8 +129,7 @@ GET  /smart/repo.git/info/refs?service=git-receive-pack HTTP/1.1 403 -
 POST /smart/repo.git/git-receive-pack HTTP/1.1 403 -
 EOF
 test_expect_success 'server request log matches test results' '
-	strip_access_log >act &&
-	test_cmp exp act
+	check_access_log exp
 '
 
 stop_httpd

2.18.0.273.g57f1ecce9c

I have no opinion about this, at all. I tried it both ways back then, and didn't find one any better than the other, so I just chose the simpler-shorter one, i.e. no 2>/dev/null redirection in the condition.

Heh, I had to do some head-scratching now as well... That's what I get for updating the patch, and then waiting almost a month to finish it up and update the commit message.

After taking a step back, I think this would fit better into the first patch, wouldn't it?
I will send v3 shortly.

Yeah, I agree it would be even nicer to clean it up there.

-Peff

The first two look obviously correct as before. I picked a few minor nits in the third one. But if your response is "no, I prefer it the way I wrote it", then I am perfectly happy seeing it applied as-is.

Thanks for finishing this up.

-Peff

To recap: 't5561-http-backend.sh' is prone to occasional failures; luckily it's not 'git-http-backend's fault, but the test script is a bit racy; patch 3/3's commit message discusses the details at length.

Changes since v2:

- Use 'test_when_finished' to clear the access log in the first patch. (and necessary adjustments to the subsequent patches because of conflicts)

Interdiff included below. (I wanted to include range-diff, but it didn't make any sense)

SZEDER Gábor (3):
t5541: clean up truncating access log
t/lib-httpd: add the strip_access_log() helper function t/lib-httpd: avoid occasional failures when checking access.log

 t/lib-httpd.sh              | 21 +++++++++++++++++++++
 t/t5541-http-push-smart.sh  | 28 ++++++----------------------
 t/t5551-http-fetch-smart.sh |  8 +-------
 t/t5561-http-backend.sh     |  8 +-------
 4 files changed, 29 insertions(+), 36 deletions(-)

2.18.0.305.g66e9e0a543

index a481e3989a..a9836e4af0 100755
@@ -38,6 +38,10 @@ GET  /smart/test_repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-upload-pack HTTP/1.1 200
 EOF
 test_expect_success 'no empty path components' '
+	# Clear the log, so that it does not affect the "used receive-pack
+	# service" test which reads the log too.
+	test_when_finished ">\"$HTTPD_ROOT_PATH\"/access.log" &&
+
 	# In the URL, add a trailing slash, and see if git appends yet another
 	# slash.
 	cd "$ROOT_PATH" &&

@@ -46,12 +50,6 @@ test_expect_success 'no empty path components' '
 	check_access_log exp
 '
 
-test_expect_success 'clear access log' '
-	# Clear the log, so that it does not affect the "used receive-pack
-	# service" test which reads the log too.
-	>"$HTTPD_ROOT_PATH"/access.log
-'
-
 test_expect_success 'clone remote repository' '
 	rm -rf test_repo_clone &&
 	git clone $HTTPD_URL/smart/test_repo.git test_repo_clone &&

In the second test of 't5541-http-push-smart.sh', 'no empty path components' we truncate Apache's access log by running:

echo >.../access.log

There are two issues with this approach:

- This doesn't leave an empty file behind, like a proper truncation would, but a file with a lone newline in it. Consequently, a later test checking the log's contents must consider this improper truncation and include an empty line in the expected content.

- This truncation is done in the middle of the test, because, quoting the in-code comment, "we do this [truncation] before the actual comparison to ensure the log is cleared" even when subsequent 'test_cmp' fails. Alas, this is not quite robust enough, as it is conceivable that 'git clone' fails after already having sent a request, in which case the access log would not be truncated and would leave stray log entries behind.

Since there is no need for that newline at all, drop the 'echo' from the truncation and adjust the expected content accordingly. Furthermore, make sure that the truncation is performed no matter whether and how 'git clone' fails unexpectedly by specifying it as a 'test_when_finished' command.

---
 t/t5541-http-push-smart.sh | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

index a2af693068..5c9ca19401 100755
@@ -38,6 +38,10 @@ GET  /smart/test_repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-upload-pack HTTP/1.1 200
 EOF
 test_expect_success 'no empty path components' '
+	# Clear the log, so that it does not affect the "used receive-pack
+	# service" test which reads the log too.
+	test_when_finished ">\"$HTTPD_ROOT_PATH\"/access.log" &&
+
 	# In the URL, add a trailing slash, and see if git appends yet another
 	# slash.
 	cd "$ROOT_PATH" &&

@@ -49,13 +53,6 @@ test_expect_success 'no empty path components' '
 		s/ [1-9][0-9]*\$//
 		s/^GET /GET  /
 	" >act <"$HTTPD_ROOT_PATH"/access.log &&
-
-	# Clear the log, so that it does not affect the "used receive-pack
-	# service" test which reads the log too.
-	#
-	# We do this before the actual comparison to ensure the log is cleared.
-	echo > "$HTTPD_ROOT_PATH"/access.log &&
-
 	test_cmp exp act
 '
 

@@ -124,7 +121,6 @@ test_expect_success 'rejected update prints status' '
 rm -f "$HTTPD_DOCUMENT_ROOT_PATH/test_repo.git/hooks/update"
 
 cat >exp <<EOF
-
 GET  /smart/test_repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-upload-pack HTTP/1.1 200
 GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200

2.18.0.305.g66e9e0a543

Four tests in three httpd-related test scripts check the contents of Apache's 'access.log', and they all do so by running 'sed' with the exact same script consisting of four s/// commands to strip uninteresting log fields and to vertically align the requested URLs.

Extract this into a common helper function 'strip_access_log' in 'lib-httpd.sh', and use it in all of those tests.

---
 t/lib-httpd.sh              |  9 +++++++++
 t/t5541-http-push-smart.sh  | 14 ++------------
 t/t5551-http-fetch-smart.sh |  7 +------
 t/t5561-http-backend.sh     |  7 +------
 4 files changed, 13 insertions(+), 24 deletions(-)

index 435a37465a..b6788fea57 100644
@@ -287,3 +287,12 @@ expect_askpass() {
 	test_cmp "$TRASH_DIRECTORY/askpass-expect" \
 		 "$TRASH_DIRECTORY/askpass-query"
 }
+
+strip_access_log() {
+	sed -e "
+		s/^.* \"//
+		s/\"//
+		s/ [1-9][0-9]*\$//
+		s/^GET /GET  /
+	" "$HTTPD_ROOT_PATH"/access.log
+}

index 5c9ca19401..83390f80cc 100755
@@ -47,12 +47,7 @@ test_expect_success 'no empty path components' '
 	cd "$ROOT_PATH" &&
 	git clone $HTTPD_URL/smart/test_repo.git/ test_repo_clone &&
 
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

@@ -134,12 +129,7 @@ GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-receive-pack HTTP/1.1 200
 EOF
 test_expect_success 'used receive-pack service' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

index 913089b144..c8b6ec493b 100755
@@ -103,12 +103,7 @@ GET  /smart/repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/repo.git/git-upload-pack HTTP/1.1 200
 EOF
 test_expect_success 'used upload-pack service' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

index 84a955770a..4248b5a06d 100755
@@ -129,12 +129,7 @@ GET  /smart/repo.git/info/refs?service=git-receive-pack HTTP/1.1 403 -
 POST /smart/repo.git/git-receive-pack HTTP/1.1 403 -
 EOF
 test_expect_success 'server request log matches test results' '
-	sed -e "
-		s/^.* \"//
-		s/\"//
-		s/ [1-9][0-9]*\$//
-		s/^GET /GET  /
-	" >act <"$HTTPD_ROOT_PATH"/access.log &&
+	strip_access_log >act &&
 	test_cmp exp act
 '
 

2.18.0.305.g66e9e0a543

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'.

---
 t/lib-httpd.sh              | 12 ++++++++++++
 t/t5541-http-push-smart.sh  |  6 ++----
 t/t5551-http-fetch-smart.sh |  3 +--
 t/t5561-http-backend.sh     |  3 +--
 4 files changed, 16 insertions(+), 8 deletions(-)

index b6788fea57..7f060aebd0 100644
@@ -296,3 +296,15 @@ strip_access_log() {
 		s/^GET /GET  /
 	" "$HTTPD_ROOT_PATH"/access.log
 }
+
+# Requires one argument: the name of a file containing the expected stripped
+# access log entries.
+check_access_log() {
+	sort "$1" >"$1".sorted &&
+	strip_access_log >access.log.stripped &&
+	sort access.log.stripped >access.log.sorted &&
+	if ! test_cmp "$1".sorted access.log.sorted
+	then
+		test_cmp "$1" access.log.stripped
+	fi
+}

index 83390f80cc..a9836e4af0 100755
@@ -47,8 +47,7 @@ test_expect_success 'no empty path components' '
 	cd "$ROOT_PATH" &&
 	git clone $HTTPD_URL/smart/test_repo.git/ test_repo_clone &&
 
-	strip_access_log >act &&
-	test_cmp exp act
+	check_access_log exp
 '
 
 test_expect_success 'clone remote repository' '

@@ -129,8 +128,7 @@ GET  /smart/test_repo.git/info/refs?service=git-receive-pack HTTP/1.1 200
 POST /smart/test_repo.git/git-receive-pack HTTP/1.1 200
 EOF
 test_expect_success 'used receive-pack service' '
-	strip_access_log >act &&
-	test_cmp exp act
+	check_access_log exp
 '
 
 test_http_push_nonff "$HTTPD_DOCUMENT_ROOT_PATH"/test_repo.git \

index c8b6ec493b..3aab44bdcb 100755
@@ -103,8 +103,7 @@ GET  /smart/repo.git/info/refs?service=git-upload-pack HTTP/1.1 200
 POST /smart/repo.git/git-upload-pack HTTP/1.1 200
 EOF
 test_expect_success 'used upload-pack service' '
-	strip_access_log >act &&
-	test_cmp exp act
+	check_access_log exp
 '
 
 test_expect_success 'follow redirects (301)' '

index 4248b5a06d..1c49054595 100755
@@ -129,8 +129,7 @@ GET  /smart/repo.git/info/refs?service=git-receive-pack HTTP/1.1 403 -
 POST /smart/repo.git/git-receive-pack HTTP/1.1 403 -
 EOF
 test_expect_success 'server request log matches test results' '
-	strip_access_log >act &&
-	test_cmp exp act
+	check_access_log exp
 '
 
 stop_httpd

2.18.0.305.g66e9e0a543