[GSoC][PATCH v2 2/2] credential-cache: use XDG_CACHE_HOME for socket

[Devin Lehmacher <lehmacdj@gmail.com>]

git-credential-cache will now use a socket following the XDG base path
specification by default. This increases consistency with other
applications and helps keep clutter out of users' home directories.

We still check the old socket location, ~/.git-credential-cache/socket
first in case the user already has a socket at that location. This
ensures that a socket previously created will be used over forcibly
switching to the new socket location.

If there is not a socket at that location we create a new one at
$XDG_CACHE_HOME/git/credential/socket. This complies with the XDG
standard and good for the reasons previously mentioned. We use the
subdirectory credential/ in case we later want to store other files
under $XDG_CACHE_HOME/git/ and to make the purpose of the socket clear.

I also change to documentation to reflect the new default socket
location.
---
 Documentation/git-credential-cache.txt |  3 ++-
 credential-cache.c                     | 16 +++++++++++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-credential-cache.txt b/Documentation/git-credential-cache.txt
index 96208f822..4b9db3856 100644
--- a/Documentation/git-credential-cache.txt
+++ b/Documentation/git-credential-cache.txt
@@ -34,7 +34,8 @@ OPTIONS
 
 	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
+	`~/.git-credential-cache/socket` if it exists and otherwise
+    `$XDG_CACHE_HOME/git/credential/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.
 
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