[GSoC][PATCH/RFC v3 2/3] credential-cache: use XDG_CACHE_HOME for socket

[Devin Lehmacher <lehmacdj@gmail.com>]

Make git-credential-cache follow the XDG base path specification by
default. This increases consistency with other applications and helps
keep clutter out of users' home directories.

Check the old socket location, ~/.git-credential-cache/socket and use it
instead if there is already a socket at that location rather than
forcibly creating a new socket at the new location.
If there is not a socket at that location create a new one at
$XDG_CACHE_HOME/git/credential/socket following XDG base path
specification. Use the subdirectory credential/ in case other files are
stored under $XDG_CACHE_HOME/git/ in the future and to make the socket's
purpose clear.

Signed-off-by: Devin Lehmacher <lehmacdj@gmail.com>
Reviewed-by: Junio C Hamano, Jeff King
---
 Documentation/git-credential-cache.txt | 10 ++++++----
 credential-cache.c                     | 16 +++++++++++++++-
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-credential-cache.txt b/Documentation/git-credential-cache.txt
index 96208f822..fce6319e8 100644
--- a/Documentation/git-credential-cache.txt
+++ b/Documentation/git-credential-cache.txt
@@ -33,10 +33,12 @@ OPTIONS
 --socket <path>::
 
 	Use `<path>` to contact a running cache daemon (or start a new
-	cache daemon if one is not started). Defaults to
-	`~/.git-credential-cache/socket`. If your home directory is on a
-	network-mounted filesystem, you may need to change this to a
-	local filesystem. You must specify an absolute path.
+	cache daemon if one is not started).
+	Defaults to `~/.git-credential-cache/socket` if it exists and
+	`$XDG_CACHE_HOME/git/credential/socket` otherwise.
+	If your home directory is on a network-mounted filesystem, you
+	may need to change this to a local filesystem. You must specify
+	an absolute path.
 
 CONTROLLING THE DAEMON
 ----------------------
diff --git a/credential-cache.c b/credential-cache.c
index cc8a6ee19..db1343b46 100644
--- a/credential-cache.c
+++ b/credential-cache.c
@@ -83,6 +83,20 @@ static void do_cache(const char *socket, const char *action, int timeout,
 	strbuf_release(&buf);
 }
 
+static char *get_socket_path(void) {
+	char *home_socket;
+
+	home_socket = expand_user_path("~/.git-credential-cache/socket");
+	if (home_socket) {
+		if (file_exists(home_socket))
+			return home_socket;
+		else
+			free(home_socket);
+	}
+
+	return xdg_cache_home("credential/socket");
+}
+
 int cmd_main(int argc, const char **argv)
 {
 	char *socket_path = NULL;
@@ -106,7 +120,7 @@ int cmd_main(int argc, const char **argv)
 	op = argv[0];
 
 	if (!socket_path)
-		socket_path = expand_user_path("~/.git-credential-cache/socket");
+		socket_path = get_socket_path();
 	if (!socket_path)
 		die("unable to find a suitable socket path; use --socket");
 
-- 
2.11.0