t9902: verify that completion does not print anything

The Bash completion script must not print anything to either stdout or
stderr. Instead, it is only expected to populate certain variables.
Tighten our `test_completion ()` test helper to verify this requirement.

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

Author: pks-t

t/t9902-completion.sh

@@ -5,6 +5,12 @@
 
 test_description='test bash completion'
 
+# The Bash completion scripts must not print anything to either stdout or
+# stderr, which we try to verify. When tracing is enabled without support for
+# BASH_XTRACEFD this assertion will fail, so we have to mark the test as
+# untraceable with such ancient Bash versions.
+test_untraceable=UnfortunatelyYes
+
 . ./lib-bash.sh
 
 complete ()
@@ -87,9 +93,11 @@ test_completion ()
 	else
 		sed -e 's/Z$//' |sort >expected
 	fi &&
-	run_completion "$1" &&
+	run_completion "$1" >"$TRASH_DIRECTORY"/bash-completion-output 2>&1 &&
 	sort out >out_sorted &&
-	test_cmp expected out_sorted
+	test_cmp expected out_sorted &&
+	test_must_be_empty "$TRASH_DIRECTORY"/bash-completion-output &&
+	rm "$TRASH_DIRECTORY"/bash-completion-output
 }
 
 # Test __gitcomp.