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

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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: szeder

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"