[PATCH] t9300-fast-import: don't hang if background fast-import exits too early

[PATCH] t9300-fast-import: don't hang if background fast-import exits too early

[SZEDER Gábor]

The five tests checking 'git fast-import's checkpoint handling in
't9300-fast-import.sh', all with the prefix "V:" in their test
description, can hang indefinitely if 'git fast-import' unexpectedly
dies early in any of these tests.

These five tests run 'git fast-import' in the background, while
feeding instructions to its standard input through a fifo from a
background subshell, and reading and verifying its standard output
through another fifo in the test script's main shell process.  This
"reading and verifying" is basically a 'while read ...' shell loop
iterating until 'git fast-import' outputs the expected line, ignoring
any other output.  This doesn't work very well when 'git fast-import'
dies before printing that particular line, because the 'read' builtin
doesn't get EOF after the death of 'git fast-import', as their input
and output are not connected directly but through a fifo.
Consequently, that 'read' hangs waiting for the next line from the
already dead 'git fast-import', leaving the test script and in turn
the whole test suite hanging.

Avoid this hang by checking whether the background 'git fast-import'
process exited unexpectedly early, and interrupt the 'while read' loop
if it did.  We have to jump through some hoops to achive that, though:

  - Start the background 'git fast-import' in another background
    subshell, which then waits until that 'git fast-import' process
    exits.  If it does exit, then report its exit code, and write a
    message to the fifo used for 'git fast-import's standard output,
    thus un-block the 'read' builtin in the main shell process.

  - Modify that 'while read' loop to break the loop upon seeing that
    message, and fail the test in the usual way.

  - Once the test is finished kill that background subshell as well,
    and do so before killing the background 'git fast-import'.
    Otherwise the background 'git fast-import' and subshell would die
    racily, and if 'git fast-import' were to die sooner, then we might
    get some undesired and potentially confusing messages in the
    test's output.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t9300-fast-import.sh | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index e707fb861e..dcfaa9cc36 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -3148,63 +3148,74 @@ test_expect_success 'U: validate root delete result' '
 # The commands in input_file should not produce any output on the file
 # descriptor set with --cat-blob-fd (or stdout if unspecified).
 #
 # To make sure you're observing the side effects of checkpoint *before*
 # fast-import terminates (and thus writes out its state), check that the
 # fast-import process is still running using background_import_still_running
 # *after* evaluating the test conditions.
 background_import_then_checkpoint () {
 	options=$1
 	input_file=$2
 
 	mkfifo V.input
 	exec 8<>V.input
 	rm V.input
 
 	mkfifo V.output
 	exec 9<>V.output
 	rm V.output
 
-	git fast-import $options <&8 >&9 &
-	echo $! >V.pid
+	(
+		git fast-import $options <&8 >&9 &
+		echo $! >V.fi.pid
+		wait $!
+		echo >&2 "background fast-import terminated too early with exit code $?"
+		# Un-block the read loop in the main shell process.
+		echo >&9 UNEXPECTED
+	) &
+	echo $! >V.sh.pid
 	# We don't mind if fast-import has already died by the time the test
 	# ends.
-	test_when_finished "
+	test_when_finished '
 		exec 8>&-; exec 9>&-;
-		kill $(cat V.pid) && wait $(cat V.pid)
-		true"
+		kill $(cat V.sh.pid) && wait $(cat V.sh.pid)
+		kill $(cat V.fi.pid) && wait $(cat V.sh.pid)
+		true'
 
 	# Start in the background to ensure we adhere strictly to (blocking)
 	# pipes writing sequence. We want to assume that the write below could
 	# block, e.g. if fast-import blocks writing its own output to &9
 	# because there is no reader on &9 yet.
 	(
 		cat "$input_file"
 		echo "checkpoint"
 		echo "progress checkpoint"
 	) >&8 &
 
 	error=1 ;# assume the worst
 	while read output <&9
 	do
 		if test "$output" = "progress checkpoint"
 		then
 			error=0
 			break
+		elif test "$output" = "UNEXPECTED"
+		then
+			break
 		fi
 		# otherwise ignore cruft
 		echo >&2 "cruft: $output"
 	done
 
 	if test $error -eq 1
 	then
 		false
 	fi
 }
 
 background_import_still_running () {
-	if ! kill -0 "$(cat V.pid)"
+	if ! kill -0 "$(cat V.fi.pid)"
 	then
 		echo >&2 "background fast-import terminated too early"
 		false
 	fi
 }
-- 
2.24.0.643.gac013444ca

Re: [PATCH] t9300-fast-import: don't hang if background fast-import exits too early

[Junio C Hamano]

SZEDER Gábor <szeder.dev@gmail.com> writes:

> +	(
> +		git fast-import $options <&8 >&9 &
> +		echo $! >V.fi.pid
> +		wait $!
> +		echo >&2 "background fast-import terminated too early with exit code $?"
> +		# Un-block the read loop in the main shell process.
> +		echo >&9 UNEXPECTED
> +	) &
> +	echo $! >V.sh.pid
>  	# We don't mind if fast-import has already died by the time the test
>  	# ends.
> -	test_when_finished "
> +	test_when_finished '
>  		exec 8>&-; exec 9>&-;
> -		kill $(cat V.pid) && wait $(cat V.pid)
> -		true"
> +		kill $(cat V.sh.pid) && wait $(cat V.sh.pid)
> +		kill $(cat V.fi.pid) && wait $(cat V.sh.pid)
> +		true'

The original interpolates the PID of the fast-import when
"when-finished" program is registered, so it is OK if somebody else
removed V.pid file; the new one interpolates when "when-finished"
program is run, reading from V.??.pid, so somebody needs to make
sure these pid files will stay around.  I do not think it is an
issue as I suspect we've left it to the global clean-up procedure
that removes the trash directory to remove the pid file.

By the way, does the second "kill && wait" wait for the right
process?

>  background_import_still_running () {
> -	if ! kill -0 "$(cat V.pid)"
> +	if ! kill -0 "$(cat V.fi.pid)"
>  	then
>  		echo >&2 "background fast-import terminated too early"
>  		false
>  	fi
>  }

Re: [PATCH] t9300-fast-import: don't hang if background fast-import exits too early

[SZEDER Gábor]

On Sat, Nov 30, 2019 at 01:16:30PM -0800, Junio C Hamano wrote:
> SZEDER Gábor <szeder.dev@gmail.com> writes:
> 
> > +	(
> > +		git fast-import $options <&8 >&9 &
> > +		echo $! >V.fi.pid
> > +		wait $!
> > +		echo >&2 "background fast-import terminated too early with exit code $?"
> > +		# Un-block the read loop in the main shell process.
> > +		echo >&9 UNEXPECTED
> > +	) &
> > +	echo $! >V.sh.pid
> >  	# We don't mind if fast-import has already died by the time the test
> >  	# ends.
> > -	test_when_finished "
> > +	test_when_finished '
> >  		exec 8>&-; exec 9>&-;
> > -		kill $(cat V.pid) && wait $(cat V.pid)
> > -		true"
> > +		kill $(cat V.sh.pid) && wait $(cat V.sh.pid)
> > +		kill $(cat V.fi.pid) && wait $(cat V.sh.pid)
> > +		true'
> 
> The original interpolates the PID of the fast-import when
> "when-finished" program is registered, so it is OK if somebody else
> removed V.pid file; the new one interpolates when "when-finished"
> program is run, reading from V.??.pid, so somebody needs to make
> sure these pid files will stay around.  I do not think it is an
> issue as I suspect we've left it to the global clean-up procedure
> that removes the trash directory to remove the pid file.

In the original the same shell process starts 'git fast-import',
writes its pidfile, and registers the test_when_finished commands, so
we can be sure that the pid file is already present when the shell
runs the $(cat V.pid) command substitutions.

With this patch that's not the case anymore, because the background
subshell starts 'git fast-import' and writes the pidfile, but the main
shell process registers the test_when_finished commands.  IOW these
two shell processes are racing, and it's possible that the
test_when_finished command is executed before the background subshell
can write the pidfile.  So double quotes around the block of
test_when_finished commands are not good.

> By the way, does the second "kill && wait" wait for the right
> process?

Ouch, it clearly doesn't.  Copy-paste error I suppose.
Thanks for spotting it.

> 
> >  background_import_still_running () {
> > -	if ! kill -0 "$(cat V.pid)"
> > +	if ! kill -0 "$(cat V.fi.pid)"
> >  	then
> >  		echo >&2 "background fast-import terminated too early"
> >  		false
> >  	fi
> >  }

Re: [PATCH] t9300-fast-import: don't hang if background fast-import exits too early

[Junio C Hamano]

SZEDER Gábor <szeder.dev@gmail.com> writes:

>> > -	test_when_finished "
>> > +	test_when_finished '
>> >  		exec 8>&-; exec 9>&-;
>> > -		kill $(cat V.pid) && wait $(cat V.pid)
>> > -		true"
>> > +		kill $(cat V.sh.pid) && wait $(cat V.sh.pid)
>> > +		kill $(cat V.fi.pid) && wait $(cat V.sh.pid)
>> > +		true'
>> 
>> The original interpolates the PID of the fast-import when
>> "when-finished" program is registered, so it is OK if somebody else
>> removed V.pid file; the new one interpolates when "when-finished"
>> program is run, reading from V.??.pid, so somebody needs to make
>> sure these pid files will stay around.  I do not think it is an
>> issue as I suspect we've left it to the global clean-up procedure
>> that removes the trash directory to remove the pid file.
>
> In the original the same shell process starts 'git fast-import',
> writes its pidfile, and registers the test_when_finished commands, so
> we can be sure that the pid file is already present when the shell
> runs the $(cat V.pid) command substitutions.

Yes.  It also means that V.pid file was not very useful in the
when-finished handler in the original.  We could have just used a
shell variable.

> With this patch that's not the case anymore, because the background
> subshell starts 'git fast-import' and writes the pidfile, but the main
> shell process registers the test_when_finished commands.  IOW these
> two shell processes are racing, and it's possible that the
> test_when_finished command is executed before the background subshell
> can write the pidfile.  So double quotes around the block of
> test_when_finished commands are not good.

Oh, I was not questioning that.  I wanted to make sure, and I was
doing so aloud, that these files are

 (1) created way before, and 
 (2) left on the filesystem

when-finished handler actually runs, because the original did not
need any guarantee for (2), but now the sq version does.

[PATCH v2 0/2] t9300-fast-import: don't hang if background fast-import exits too early

[SZEDER Gábor]

The five tests checking 'git fast-import's checkpoint handling in
't9300-fast-import.sh', all with the prefix "V:" in their test
description, can hang indefinitely if 'git fast-import' unexpectedly
dies early in any of these tests.

Changes since v1:

  - The first patch is new, and refactors those tests to store the PID
    of the background 'git fast-import' process in a shell variable
    instead of in a pidfile.

  - The second patch is conceptually the same as the only patch v1,
    but it has been updated to store the PID of the background
    subshell in a variable as well.

SZEDER Gábor (2):
  t9300-fast-import: store the PID in a variable instead of pidfile
  t9300-fast-import: don't hang if background fast-import exits too
    early

 t/t9300-fast-import.sh | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

-- 
2.24.0.801.g241c134b8d

[PATCH v2 1/2] t9300-fast-import: store the PID in a variable instead of pidfile

[SZEDER Gábor]

The five tests running 'git fast-import' in the background in
't9300-fast-import.sh' store the PID of that background process in a
pidfile, to be used to check whether that background process survived
each test and then to kill it in test_when_finished commands.  To
achieve this all these five tests run three $(cat <pidfile>) command
substitutions each.

Store the PID of the background 'git fast-import' in a variable to
avoid those extra processes.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t9300-fast-import.sh | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index e707fb861e..6820ebbb63 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -3165,12 +3165,12 @@ background_import_then_checkpoint () {
 	rm V.output
 
 	git fast-import $options <&8 >&9 &
-	echo $! >V.pid
+	fi_pid=$!
 	# We don't mind if fast-import has already died by the time the test
 	# ends.
 	test_when_finished "
 		exec 8>&-; exec 9>&-;
-		kill $(cat V.pid) && wait $(cat V.pid)
+		kill $fi_pid && wait $fi_pid
 		true"
 
 	# Start in the background to ensure we adhere strictly to (blocking)
@@ -3202,7 +3202,7 @@ background_import_then_checkpoint () {
 }
 
 background_import_still_running () {
-	if ! kill -0 "$(cat V.pid)"
+	if ! kill -0 "$fi_pid"
 	then
 		echo >&2 "background fast-import terminated too early"
 		false
-- 
2.24.0.801.g241c134b8d

[PATCH v2 2/2] t9300-fast-import: don't hang if background fast-import exits too early

[SZEDER Gábor]

The five tests checking 'git fast-import's checkpoint handling in
't9300-fast-import.sh', all with the prefix "V:" in their test
description, can hang indefinitely if 'git fast-import' unexpectedly
dies early in any of these tests.

These five tests run 'git fast-import' in the background, while
feeding instructions to its standard input through a fifo (fd 8) from
a background subshell, and reading and verifying its standard output
through another fifo (fd 9) in the test script's main shell process.
This "reading and verifying" is basically a 'while read ...' shell
loop iterating until 'git fast-import' outputs the expected line,
ignoring any other output.  This doesn't work very well when 'git
fast-import' dies before printing that particular line, because the
'read' builtin doesn't get EOF after the death of 'git fast-import',
as their input and output are not connected directly but through a
fifo.  Consequently, that 'read' hangs waiting for the next line from
the already dead 'git fast-import', leaving the test script and in
turn the whole test suite hanging.

Avoid this hang by checking whether the background 'git fast-import'
process exited unexpectedly early, and interrupt the 'while read' loop
if it did.  We have to jump through some hoops to achive that, though:

  - Start the background 'git fast-import' in another background
    subshell, which then:

      - prints the PID of that 'git fast-import' process to the fifo,
	to be read by the main shell process, so it will know which
	process to kill when the test is finished.

      - waits until that 'git fast-import' process exits.  If it does
	exit, then report its exit code, and write a message to the
	fifo used for 'git fast-import's standard output, thus
	un-block the 'read' builtin in the main shell process.

  - Modify that 'while read' loop to break the loop upon seeing that
    message, and fail the test in the usual way.

  - Once the test is finished kill that background subshell as well,
    and do so before killing the background 'git fast-import'.
    Otherwise the background 'git fast-import' and subshell processes
    would die racily, and if 'git fast-import' were to die sooner,
    then we might get some undesired and potentially confusing
    messages in the test's output.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t9300-fast-import.sh | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 6820ebbb63..8f6f80f021 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -3164,12 +3164,21 @@ background_import_then_checkpoint () {
 	exec 9<>V.output
 	rm V.output
 
-	git fast-import $options <&8 >&9 &
-	fi_pid=$!
+	(
+		git fast-import $options <&8 >&9 &
+		echo $! >&9
+		wait $!
+		echo >&2 "background fast-import terminated too early with exit code $?"
+		# Un-block the read loop in the main shell process.
+		echo >&9 UNEXPECTED
+	) &
+	sh_pid=$!
+	read fi_pid <&9
 	# We don't mind if fast-import has already died by the time the test
 	# ends.
 	test_when_finished "
 		exec 8>&-; exec 9>&-;
+		kill $sh_pid && wait $sh_pid
 		kill $fi_pid && wait $fi_pid
 		true"
 
@@ -3190,6 +3199,9 @@ background_import_then_checkpoint () {
 		then
 			error=0
 			break
+		elif test "$output" = "UNEXPECTED"
+		then
+			break
 		fi
 		# otherwise ignore cruft
 		echo >&2 "cruft: $output"
-- 
2.24.0.801.g241c134b8d