fast-import: checkpoint: dump branches/tags/marks even if object_count==0

The checkpoint command cycles packfiles if object_count != 0, a sensible
test or there would be no pack files to write. Since 820b931012, the
command also dumps branches, tags and marks, but still conditionally.
However, it is possible for a command stream to modify refs or create
marks without creating any new objects.

For example, reset a branch (and keep fast-import running):

	$ git fast-import
	reset refs/heads/master
	from refs/heads/master^

	checkpoint

but refs/heads/master remains unchanged.

Other example: a commit command that re-creates an object that already
exists in the object database.

The man page also states that checkpoint "updates the refs" and that
"placing a progress command immediately after a checkpoint will inform
the reader when the checkpoint has been completed and it can safely
access the refs that fast-import updated". This wasn't always true
without this patch.

This fix unconditionally calls dump_{branches,tags,marks}() for all
checkpoint commands. dump_branches() and dump_tags() are cheap to call
in the case of a no-op.

Add tests to t9300 that observe the (non-packfiles) effects of
checkpoint.

Signed-off-by: Eric Rannaud <e@nanocritical.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -3188,10 +3188,10 @@ static void checkpoint(void)
 	checkpoint_requested = 0;
 	if (object_count) {
 		cycle_packfile();
-		dump_branches();
-		dump_tags();
-		dump_marks();
 	}
+	dump_branches();
+	dump_tags();
+	dump_marks();
 }
 
 static void parse_checkpoint(void)

t/t9300-fast-import.sh

@@ -3120,4 +3120,146 @@ test_expect_success 'U: validate root delete result' '
 	compare_diff_raw expect actual
 '
 
+###
+### series V (checkpoint)
+###
+
+# 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
+	# We don't mind if fast-import has already died by the time the test
+	# ends.
+	test_when_finished "exec 8>&-; exec 9>&-; kill $(cat V.pid) || true"
+
+	# Start in the background to ensure we adhere strictly to (blocking)
+	# pipes writing sequence. We want to assume that the write below could
+	# block, e.g. if fast-import blocks writing its own output to &9
+	# because there is no reader on &9 yet.
+	(
+		cat "$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
+		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)"
+	then
+		echo >&2 "background fast-import terminated too early"
+		false
+	fi
+}
+
+test_expect_success PIPE 'V: checkpoint helper does not get stuck with extra output' '
+	cat >input <<-INPUT_END &&
+	progress foo
+	progress bar
+
+	INPUT_END
+
+	background_import_then_checkpoint "" input &&
+	background_import_still_running
+'
+
+test_expect_success PIPE 'V: checkpoint updates refs after reset' '
+	cat >input <<-\INPUT_END &&
+	reset refs/heads/V
+	from refs/heads/U
+
+	INPUT_END
+
+	background_import_then_checkpoint "" input &&
+	test "$(git rev-parse --verify V)" = "$(git rev-parse --verify U)" &&
+	background_import_still_running
+'
+
+test_expect_success PIPE 'V: checkpoint updates refs and marks after commit' '
+	cat >input <<-INPUT_END &&
+	commit refs/heads/V
+	mark :1
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data 0
+	from refs/heads/U
+
+	INPUT_END
+
+	background_import_then_checkpoint "--export-marks=marks.actual" input &&
+
+	echo ":1 $(git rev-parse --verify V)" >marks.expected &&
+
+	test "$(git rev-parse --verify V^)" = "$(git rev-parse --verify U)" &&
+	test_cmp marks.expected marks.actual &&
+	background_import_still_running
+'
+
+# Re-create the exact same commit, but on a different branch: no new object is
+# created in the database, but the refs and marks still need to be updated.
+test_expect_success PIPE 'V: checkpoint updates refs and marks after commit (no new objects)' '
+	cat >input <<-INPUT_END &&
+	commit refs/heads/V2
+	mark :2
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data 0
+	from refs/heads/U
+
+	INPUT_END
+
+	background_import_then_checkpoint "--export-marks=marks.actual" input &&
+
+	echo ":2 $(git rev-parse --verify V2)" >marks.expected &&
+
+	test "$(git rev-parse --verify V2)" = "$(git rev-parse --verify V)" &&
+	test_cmp marks.expected marks.actual &&
+	background_import_still_running
+'
+
+test_expect_success PIPE 'V: checkpoint updates tags after tag' '
+	cat >input <<-INPUT_END &&
+	tag Vtag
+	from refs/heads/V
+	tagger $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data 0
+
+	INPUT_END
+
+	background_import_then_checkpoint "" input &&
+	git show-ref -d Vtag &&
+	background_import_still_running
+'
+
 test_done