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

From: SZEDER Gábor <[email-masked]>
Date: Sat, 30 Nov 2019 11:46:44 +0100

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 <[email-masked]>
---
 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