diff: bypass diff process with --no-ext-diff and in format-patch

Make --no-ext-diff disable diff.<driver>.process in addition to
diff.<driver>.command.  The two mechanisms serve the same purpose
(external diff tools), so a single flag should control both.

Replace the OPT_BOOL for --ext-diff with an OPT_CALLBACK that
sets both allow_external and the new no_diff_process flag, keeping
the option's user-facing behavior unchanged while extending it to
cover the diff process.  Passing --ext-diff explicitly clears
no_diff_process, so a later --ext-diff overrides an earlier
--no-ext-diff.

Disable the diff process unconditionally in format-patch so that
generated patches are always based on the builtin diff algorithm
and can be applied reliably by recipients who do not have the
external tool.

Document that --diff-algorithm also bypasses the diff process,
since it sets ignore_driver_algorithm which diff_process_fill_hunks
already checks.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>

Author: mmontalbo

Documentation/diff-algorithm-option.adoc

@@ -18,3 +18,6 @@
 For instance, if you configured the `diff.algorithm` variable to a
 non-default value and want to use the default one, then you
 have to use `--diff-algorithm=default` option.
++
+If you explicitly choose a diff algorithm, it also bypasses
+`diff.<driver>.process` (see linkgit:gitattributes[5]).

Documentation/diff-options.adoc

@@ -825,7 +825,8 @@ endif::git-format-patch[]
 	to use this option with linkgit:git-log[1] and friends.
 
 `--no-ext-diff`::
-	Disallow external diff drivers.
+	Disallow external diff drivers.  Also disables
+	`diff.<driver>.process` (see linkgit:gitattributes[5]).
 
 `--textconv`::
 `--no-textconv`::

builtin/log.c

@@ -2213,6 +2213,13 @@ int cmd_format_patch(int argc,
 	if (argc > 1)
 		die(_("unrecognized argument: %s"), argv[1]);
 
+	/*
+	 * Disable diff.<driver>.process so that patches generated by
+	 * format-patch are always based on the builtin diff algorithm
+	 * and can be applied reliably.
+	 */
+	rev.diffopt.flags.no_diff_process = 1;
+
 	if (rev.diffopt.output_format & DIFF_FORMAT_NAME)
 		die(_("--name-only does not make sense"));
 	if (rev.diffopt.output_format & DIFF_FORMAT_NAME_STATUS)

diff.c

@@ -5913,6 +5913,17 @@ static int diff_opt_submodule(const struct option *opt,
 	return 0;
 }
 
+static int diff_opt_ext_diff(const struct option *opt,
+			     const char *arg, int unset)
+{
+	struct diff_options *options = opt->value;
+
+	BUG_ON_OPT_ARG(arg);
+	options->flags.allow_external = !unset;
+	options->flags.no_diff_process = unset;
+	return 0;
+}
+
 static int diff_opt_textconv(const struct option *opt,
 			     const char *arg, int unset)
 {
@@ -6241,8 +6252,9 @@ struct option *add_diff_options(const struct option *opts,
 			 N_("exit with 1 if there were differences, 0 otherwise")),
 		OPT_BOOL(0, "quiet", &options->flags.quick,
 			 N_("disable all output of the program")),
-		OPT_BOOL(0, "ext-diff", &options->flags.allow_external,
-			 N_("allow an external diff helper to be executed")),
+		OPT_CALLBACK_F(0, "ext-diff", options, NULL,
+			       N_("allow an external diff helper to be executed"),
+			       PARSE_OPT_NOARG, diff_opt_ext_diff),
 		OPT_CALLBACK_F(0, "textconv", options, NULL,
 			       N_("run external text conversion filters when comparing binary files"),
 			       PARSE_OPT_NOARG, diff_opt_textconv),

t/t4080-diff-process.sh

@@ -333,6 +333,14 @@ test_expect_success PYTHON 'diff process not used by --stat' '
 	test_path_is_missing backend.log
 '
 
+test_expect_success PYTHON 'diff process bypassed by --no-ext-diff' '
+	rm -f backend.log &&
+	git -c diff.cdiff.process="$BACKEND --log=backend.log" \
+		diff --no-ext-diff worddiff.c >actual &&
+	test_grep "return 999" actual &&
+	test_path_is_missing backend.log
+'
+
 test_expect_success PYTHON 'diff process works with git log -p' '
 	cat >logtest.c <<-\EOF &&
 	int logfunc(void) { return 1; }
@@ -354,6 +362,14 @@ test_expect_success PYTHON 'diff process works with git log -p' '
 	test_must_be_empty stderr
 '
 
+test_expect_success PYTHON 'diff process not used by format-patch' '
+	rm -f backend.log &&
+	git -c diff.cdiff.process="$BACKEND --log=backend.log" \
+		format-patch -1 --stdout -- logtest.c >actual &&
+	test_grep "return 2" actual &&
+	test_path_is_missing backend.log
+'
+
 test_expect_success PYTHON 'diff process startup failure only warns once' '
 	git -c diff.cdiff.process="/nonexistent/tool" \
 		diff -- multi1.c multi2.c >actual 2>stderr &&