config: values of pathname type can be prefixed with :(optional)

Sometimes people want to specify additional configuration data
as "best effort" basis.  Maybe commit.template configuration file points
at somewhere in ~/template/ but on a particular system, the file may not
exist and the user may be OK without using the template in such a case.

When the value given to a configuration variable whose type is
pathname wants to signal such an optional file, it can be marked by
prepending ":(optional)" in front of it.  Such a setting that is
marked optional would avoid getting the command barf for a missing
file, as an optional configuration setting that names a missing or
an empty file is not even seen.

cf. <xmqq5ywehb69.fsf@gitster.g>

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Documentation/config.txt

@@ -358,7 +358,10 @@ compiled without runtime prefix support, the compiled-in prefix will be
 substituted instead. In the unlikely event that a literal path needs to
 be specified that should _not_ be expanded, it needs to be prefixed by
 `./`, like so: `./%(prefix)/bin`.
-
++
+If prefixed with `:(optional)`, the configuration variable is treated
+as if it does not exist, if the named path does not exist or names an
+empty file.
 
 Variables
 ~~~~~~~~~

config.c

@@ -1364,11 +1364,23 @@ int git_config_string(char **dest, const char *var, const char *value)
 
 int git_config_pathname(char **dest, const char *var, const char *value)
 {
+	int is_optional;
+	char *path;
+
 	if (!value)
 		return config_error_nonbool(var);
-	*dest = interpolate_path(value, 0);
-	if (!*dest)
+
+	is_optional = skip_prefix(value, ":(optional)", &value);
+	path = interpolate_path(value, 0);
+	if (!path)
 		die(_("failed to expand user dir in: '%s'"), value);
+
+	if (is_optional && is_empty_or_missing_file(path)) {
+		free(path);
+		return 0;
+	}
+
+	*dest = path;
 	return 0;
 }
 

t/t7500-commit-template-squash-signoff.sh

@@ -46,6 +46,15 @@ test_expect_success 'nonexistent template file in config should return error' '
 	)
 '
 
+test_expect_success 'nonexistent optional template file in config' '
+	test_config commit.template ":(optional)$PWD"/notexist &&
+	(
+		GIT_EDITOR="echo hello >\"\$1\"" &&
+		export GIT_EDITOR &&
+		git commit --allow-empty
+	)
+'
+
 # From now on we'll use a template file that exists.
 TEMPLATE="$PWD"/template