daemon: --access-hook option

The --access-hook option to "git daemon" specifies an external
command to be run every time a client connects, with

 - service name (e.g. "upload-pack", etc.),
 - path to the repository,
 - hostname (%H),
 - canonical hostname (%CH),
 - ip address (%IP),
 - tcp port (%P)

as its command line arguments.  The external command can decide to
decline the service by exiting with a non-zero status (or to allow it
by exiting with a zero status).  It can also look at the $REMOTE_ADDR
and $REMOTE_PORT environment variables to learn about the requestor
when making this decision.

The external command can optionally write a single line to its
standard output to be sent to the requestor as an error message when
it declines the service.

Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-daemon.txt

@@ -16,6 +16,7 @@ SYNOPSIS
 	     [--reuseaddr] [--detach] [--pid-file=<file>]
 	     [--enable=<service>] [--disable=<service>]
 	     [--allow-override=<service>] [--forbid-override=<service>]
+	     [--access-hook=<path>]
 	     [--inetd | [--listen=<host_or_ipaddr>] [--port=<n>] [--user=<user> [--group=<group>]]
 	     [<directory>...]
 
@@ -171,6 +172,21 @@ the facility of inet daemon to achieve the same before spawning
 	errors are not enabled, all errors report "access denied" to the
 	client. The default is --no-informative-errors.
 
+--access-hook=<path>::
+	Every time a client connects, first run an external command
+	specified by the <path> with service name (e.g. "upload-pack"),
+	path to the repository, hostname (%H), canonical hostname
+	(%CH), ip address (%IP), and tcp port (%P) as its command line
+	arguments. The external command can decide to decline the
+	service by exiting with a non-zero status (or to allow it by
+	exiting with a zero status).  It can also look at the $REMOTE_ADDR
+	and $REMOTE_PORT environment variables to learn about the
+	requestor when making this decision.
++
+The external command can optionally write a single line to its
+standard output to be sent to the requestor as an error message when
+it declines the service.
+
 <directory>::
 	A directory to add to the whitelist of allowed directories. Unless
 	--strict-paths is specified this will also include subdirectories

daemon.c

@@ -30,6 +30,7 @@ static const char daemon_usage[] =
 "           [--interpolated-path=<path>]\n"
 "           [--reuseaddr] [--pid-file=<file>]\n"
 "           [--(enable|disable|allow-override|forbid-override)=<service>]\n"
+"           [--access-hook=<path>]\n"
 "           [--inetd | [--listen=<host_or_ipaddr>] [--port=<n>]\n"
 "                      [--detach] [--user=<user> [--group=<group>]]\n"
 "           [<directory>...]";
@@ -256,6 +257,71 @@ static int daemon_error(const char *dir, const char *msg)
 	return -1;
 }
 
+static char *access_hook;
+
+static int run_access_hook(struct daemon_service *service, const char *dir, const char *path)
+{
+	struct child_process child;
+	struct strbuf buf = STRBUF_INIT;
+	const char *argv[8];
+	const char **arg = argv;
+	char *eol;
+	int seen_errors = 0;
+
+#define STRARG(x) ((x) ? (x) : "")
+	*arg++ = access_hook;
+	*arg++ = service->name;
+	*arg++ = path;
+	*arg++ = STRARG(hostname);
+	*arg++ = STRARG(canon_hostname);
+	*arg++ = STRARG(ip_address);
+	*arg++ = STRARG(tcp_port);
+	*arg = NULL;
+#undef STRARG
+
+	memset(&child, 0, sizeof(child));
+	child.use_shell = 1;
+	child.argv = argv;
+	child.no_stdin = 1;
+	child.no_stderr = 1;
+	child.out = -1;
+	if (start_command(&child)) {
+		logerror("daemon access hook '%s' failed to start",
+			 access_hook);
+		goto error_return;
+	}
+	if (strbuf_read(&buf, child.out, 0) < 0) {
+		logerror("failed to read from pipe to daemon access hook '%s'",
+			 access_hook);
+		strbuf_reset(&buf);
+		seen_errors = 1;
+	}
+	if (close(child.out) < 0) {
+		logerror("failed to close pipe to daemon access hook '%s'",
+			 access_hook);
+		seen_errors = 1;
+	}
+	if (finish_command(&child))
+		seen_errors = 1;
+
+	if (!seen_errors) {
+		strbuf_release(&buf);
+		return 0;
+	}
+
+error_return:
+	strbuf_ltrim(&buf);
+	if (!buf.len)
+		strbuf_addstr(&buf, "service rejected");
+	eol = strchr(buf.buf, '\n');
+	if (eol)
+		*eol = '\0';
+	errno = EACCES;
+	daemon_error(dir, buf.buf);
+	strbuf_release(&buf);
+	return -1;
+}
+
 static int run_service(char *dir, struct daemon_service *service)
 {
 	const char *path;
@@ -303,6 +369,13 @@ static int run_service(char *dir, struct daemon_service *service)
 		return daemon_error(dir, "service not enabled");
 	}
 
+	/*
+	 * Optionally, a hook can choose to deny access to the
+	 * repository depending on the phase of the moon.
+	 */
+	if (access_hook && run_access_hook(service, dir, path))
+		return -1;
+
 	/*
 	 * We'll ignore SIGTERM from now on, we have a
 	 * good client.
@@ -1142,6 +1215,10 @@ int main(int argc, char **argv)
 			export_all_trees = 1;
 			continue;
 		}
+		if (!prefixcmp(arg, "--access-hook=")) {
+			access_hook = arg + 14;
+			continue;
+		}
 		if (!prefixcmp(arg, "--timeout=")) {
 			timeout = atoi(arg+10);
 			continue;