difftool: add support for --trust-exit-code

Teach difftool to exit when a diff tool returns a non-zero exit
code when either --trust-exit-code is specified or
difftool.trustExitCode is true.

Forward exit codes from invoked diff tools to the caller when
--trust-exit-code is used.

Suggested-by: Adri Farr <14farresa@gmail.com>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-difftool.txt

@@ -91,6 +91,15 @@ instead.  `--no-symlinks` is the default on Windows.
 	the default diff tool will be read from the configured
 	`diff.guitool` variable instead of `diff.tool`.
 
+--[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.
++
+'git-difftool' will forward the exit code of the invoked tool when
+'--trust-exit-code' is used.
+
 See linkgit:git-diff[1] for the full list of supported options.
 
 CONFIG VARIABLES
@@ -116,6 +125,11 @@ See the `--tool=<tool>` option above for more details.
 difftool.prompt::
 	Prompt before each invocation of the diff tool.
 
+difftool.trustExitCode::
+	Exit difftool if the invoked diff tool returns a non-zero exit status.
++
+See the `--trust-exit-code` option above for more details.
+
 SEE ALSO
 --------
 linkgit:git-diff[1]::

git-difftool--helper.sh

@@ -85,6 +85,12 @@ else
 	while test $# -gt 6
 	do
 		launch_merge_tool "$1" "$2" "$5"
+		status=$?
+		if test "$status" != 0 &&
+			test "$GIT_DIFFTOOL_TRUST_EXIT_CODE" = true
+		then
+			exit $status
+		fi
 		shift 7
 	done
 fi

git-difftool.perl

@@ -346,6 +346,7 @@ sub main
 		symlinks => $^O ne 'cygwin' &&
 				$^O ne 'MSWin32' && $^O ne 'msys',
 		tool_help => undef,
+		trust_exit_code => undef,
 	);
 	GetOptions('g|gui!' => \$opts{gui},
 		'd|dir-diff' => \$opts{dirdiff},
@@ -356,6 +357,8 @@ sub main
 		'no-symlinks' => sub { $opts{symlinks} = 0; },
 		't|tool:s' => \$opts{difftool_cmd},
 		'tool-help' => \$opts{tool_help},
+		'trust-exit-code' => \$opts{trust_exit_code},
+		'no-trust-exit-code' => sub { $opts{trust_exit_code} = 0; },
 		'x|extcmd:s' => \$opts{extcmd});
 
 	if (defined($opts{help})) {
@@ -387,6 +390,15 @@ sub main
 		}
 	}
 
+	if (!defined $opts{trust_exit_code}) {
+		$opts{trust_exit_code} = Git::config_bool('difftool.trustExitCode');
+	}
+	if ($opts{trust_exit_code}) {
+		$ENV{GIT_DIFFTOOL_TRUST_EXIT_CODE} = 'true';
+	} else {
+		$ENV{GIT_DIFFTOOL_TRUST_EXIT_CODE} = 'false';
+	}
+
 	# In directory diff mode, 'git-difftool--helper' is called once
 	# to compare the a/b directories.  In file diff mode, 'git diff'
 	# will invoke a separate instance of 'git-difftool--helper' for

t/t7800-difftool.sh

@@ -76,6 +76,49 @@ test_expect_success PERL 'difftool forwards arguments to diff' '
 	rm for-diff
 '
 
+test_expect_success PERL 'difftool ignores exit code' '
+	test_config difftool.error.cmd false &&
+	git difftool -y -t error branch
+'
+
+test_expect_success PERL '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 PERL '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 PERL '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 PERL '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 PERL '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 "$2"
+	exit 1
+	EOF
+	>for-diff &&
+	git add for-diff &&
+	echo file >expect &&
+	test_must_fail git difftool -y --trust-exit-code \
+		--extcmd .git/fail-right-file branch >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success PERL 'difftool honors --gui' '
 	difftool_test_setup &&
 	test_config merge.tool bogus-tool &&