diff tool unconditionally in that case.

This was changed a month later via c41d3fedd8 (difftool--helper: add
explicit exit statement, 2014-11-20), where an explicit `exit 0` was
added to the end of git-difftool--helper.sh. While the stated intent of
that commit was merely a cleanup, it had the consequence that we now
to ignore the exit code of the diff tool when `--dir-diff` was set. This
change in behaviour is thus very likely an unintended side effect of
this patch.

Now there are two ways to fix this:

  - We can either restore the original behaviour, which unconditionally
    returned the exit code of the diffing tool when `--dir-diff` is
    passed.

  - Or we can make the `--dir-diff` case respect the `--trust-exit-code`
    flag.

The fact that we have been ignoring exit codes for 7 years by now makes
me rather lean towards the latter option. Furthermore, respecting the
flag in one case but not the other would needlessly make the user
interface more complex.

Fix the bug so that we also honor `--trust-exit-code` for dir diffs and
adjust the documentation accordingly.

Reported-by: Jean-Rémy Falleri <jr.falleri@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 Documentation/git-difftool.txt |  1 -
 git-difftool--helper.sh        | 14 +++++
 t/t7800-difftool.sh            | 99 ++++++++++++++++++----------------
 3 files changed, 68 insertions(+), 46 deletions(-)

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index c05f97aca9..a616f8b2e6 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -105,7 +105,6 @@ instead.  `--no-symlinks` is the default on Windows.
 	`merge.tool` until a tool is found.
 
 --[no-]trust-exit-code::
-	'git-difftool' invokes a diff tool individually on each file.
 	Errors reported by the diff tool are ignored by default.
 	Use `--trust-exit-code` to make 'git-difftool' exit when an
 	invoked diff tool returns a non-zero exit code.
diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
index e4e820e680..09d8542917 100755
--- a/git-difftool--helper.sh
+++ b/git-difftool--helper.sh
@@ -91,6 +91,20 @@ then
 	# ignore the error from the above --- run_merge_tool
 	# will diagnose unusable tool by itself
 	run_merge_tool "$merge_tool" false
+
+	status=$?
+	if test $status -ge 126
+	then
+		# Command not found (127), not executable (126) or
+		# exited via a signal (>= 128).
+		exit $status
+	fi
+
+	if test "$status" != 0 &&
+		test "$GIT_DIFFTOOL_TRUST_EXIT_CODE" = true
+	then
+		exit $status
+	fi
 else
 	# Launch the merge tool on each path provided by 'git diff'
 	while test $# -gt 6
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 6a36be1e63..96ae5d5880 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -91,58 +91,67 @@ test_expect_success 'difftool forwards arguments to diff' '
 	rm for-diff
 '
 
-test_expect_success 'difftool ignores exit code' '
-	test_config difftool.error.cmd false &&
-	git difftool -y -t error branch
-'
+for opt in '' '--dir-diff'
+do
+	test_expect_success "difftool ${opt} ignores exit code" "
+		test_config difftool.error.cmd false &&
+		git difftool ${opt} -y -t error branch
+	"
 
-test_expect_success 'difftool forwards exit code with --trust-exit-code' '
-	test_config difftool.error.cmd false &&
-	test_must_fail git difftool -y --trust-exit-code -t error branch
-'
+	test_expect_success "difftool ${opt} forwards exit code with --trust-exit-code" "
+		test_config difftool.error.cmd false &&
+		test_must_fail git difftool ${opt} -y --trust-exit-code -t error branch
+	"
 
-test_expect_success 'difftool forwards exit code with --trust-exit-code for built-ins' '
-	test_config difftool.vimdiff.path false &&
-	test_must_fail git difftool -y --trust-exit-code -t vimdiff branch
-'
+	test_expect_success "difftool ${opt} forwards exit code with --trust-exit-code for built-ins" "
+		test_config difftool.vimdiff.path false &&
+		test_must_fail git difftool ${opt} -y --trust-exit-code -t vimdiff branch
+	"
 
-test_expect_success 'difftool honors difftool.trustExitCode = true' '
-	test_config difftool.error.cmd false &&
-	test_config difftool.trustExitCode true &&
-	test_must_fail git difftool -y -t error branch
-'
+	test_expect_success "difftool ${opt} honors difftool.trustExitCode = true" "
+		test_config difftool.error.cmd false &&
+		test_config difftool.trustExitCode true &&
+		test_must_fail git difftool ${opt} -y -t error branch
+	"
 
-test_expect_success 'difftool honors difftool.trustExitCode = false' '
-	test_config difftool.error.cmd false &&
-	test_config difftool.trustExitCode false &&
-	git difftool -y -t error branch
-'
+	test_expect_success "difftool ${opt} honors difftool.trustExitCode = false" "
+		test_config difftool.error.cmd false &&
+		test_config difftool.trustExitCode false &&
+		git difftool ${opt} -y -t error branch
+	"
 
-test_expect_success 'difftool ignores exit code with --no-trust-exit-code' '
-	test_config difftool.error.cmd false &&
-	test_config difftool.trustExitCode true &&
-	git difftool -y --no-trust-exit-code -t error branch
-'
+	test_expect_success "difftool ${opt} ignores exit code with --no-trust-exit-code" "
+		test_config difftool.error.cmd false &&
+		test_config difftool.trustExitCode true &&
+		git difftool ${opt} -y --no-trust-exit-code -t error branch
+	"
 
-test_expect_success 'difftool stops on error with --trust-exit-code' '
-	test_when_finished "rm -f for-diff .git/fail-right-file" &&
-	test_when_finished "git reset -- for-diff" &&
-	write_script .git/fail-right-file <<-\EOF &&
-	echo failed
-	exit 1
-	EOF
-	>for-diff &&
-	git add for-diff &&
-	test_must_fail git difftool -y --trust-exit-code \
-		--extcmd .git/fail-right-file branch >actual &&
-	test_line_count = 1 actual
-'
+	test_expect_success "difftool ${opt} stops on error with --trust-exit-code" "
+		test_when_finished 'rm -f for-diff .git/fail-right-file' &&
+		test_when_finished 'git reset -- for-diff' &&
+		write_script .git/fail-right-file <<-\EOF &&
+		echo failed
+		exit 1
+		EOF
+		>for-diff &&
+		git add for-diff &&
+		test_must_fail git difftool ${opt} -y --trust-exit-code \
+			--extcmd .git/fail-right-file branch >actual &&
+		test_line_count = 1 actual
+	"
 
-test_expect_success 'difftool honors exit status if command not found' '
-	test_config difftool.nonexistent.cmd i-dont-exist &&
-	test_config difftool.trustExitCode false &&
-	test_must_fail git difftool -y -t nonexistent branch
-'
+	test_expect_success "difftool ${opt} honors exit status if command not found" "
+		test_config difftool.nonexistent.cmd i-dont-exist &&
+		test_config difftool.trustExitCode false &&
+		if test "${opt}" = '--dir-diff'
+		then
+			expected_code=127
+		else
+			expected_code=128
+		fi &&
+		test_expect_code \${expected_code} git difftool ${opt} -y -t nonexistent branch
+	"
+done
 
 test_expect_success 'difftool honors --gui' '
 	difftool_test_setup &&
