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
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>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.adoc

diff --git a/Documentation/config.adoc b/Documentation/config.adoc
index cc76925..7301ced 100644
--- a/Documentation/config.adoc
+++ b/Documentation/config.adoc
@@ -358,7 +358,9 @@
 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.
 
 Variables
 ~~~~~~~~~

config.c

diff --git a/config.c b/config.c
index 97ffef4..73fc74c 100644
--- a/config.c
+++ b/config.c
@@ -1279,11 +1279,23 @@ 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_missing_file(path)) {
+		free(path);
+		return 0;
+	}
+
+	*dest = path;
 	return 0;
 }
 

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

diff --git a/t/t7500-commit-template-squash-signoff.sh b/t/t7500-commit-template-squash-signoff.sh
index 4922543..a85229e 100755
--- a/t/t7500-commit-template-squash-signoff.sh
+++ b/t/t7500-commit-template-squash-signoff.sh
@@ -46,6 +46,14 @@
 	)
 '
 
+test_expect_success 'nonexistent optional template file in config' '
+	test_config commit.template ":(optional)$PWD"/notexist &&
+	GIT_EDITOR="echo hello >" git commit --allow-empty &&
+	git cat-file commit HEAD | sed -e "1,/^$/d" >actual &&
+	echo hello >expect &&
+	test_cmp expect actual
+'
+
 # From now on we'll use a template file that exists.
 TEMPLATE="$PWD"/template
 

wrapper.c

diff --git a/wrapper.c b/wrapper.c
index 2f00d2a..3d507d4 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -721,6 +721,19 @@ int xgethostname(char *buf, size_t len)
 	return ret;
 }
 
+int is_missing_file(const char *filename)
+{
+	struct stat st;
+
+	if (stat(filename, &st) < 0) {
+		if (errno == ENOENT)
+			return 1;
+		die_errno(_("could not stat %s"), filename);
+	}
+
+	return 0;
+}
+
 int is_empty_or_missing_file(const char *filename)
 {
 	struct stat st;

wrapper.h

diff --git a/wrapper.h b/wrapper.h
index 7df824e..44a8597 100644
--- a/wrapper.h
+++ b/wrapper.h
@@ -66,7 +66,9 @@ void write_file_buf(const char *path, const char *buf, size_t len);
 __attribute__((format (printf, 2, 3)))
 void write_file(const char *path, const char *fmt, ...);
 
-/* Return 1 if the file is empty or does not exists, 0 otherwise. */
+/* Return 1 if the file does not exist, 0 otherwise. */
+int is_missing_file(const char *filename);
+/* Return 1 if the file is empty or does not exist, 0 otherwise. */
 int is_empty_or_missing_file(const char *filename);
 
 enum fsync_action {