daemon: give friendlier error messages to clients

When the git-daemon is asked about an inaccessible repository, it simply
hangs up the connection without saying anything further. This makes it
hard to distinguish between a repository we cannot access (e.g., due to
typo), and a service or network outage.

Instead, let's print an "ERR" line, which git clients understand since
v1.6.1 (2008-12-24).

Because there is a risk of leaking information about non-exported
repositories, by default all errors simply say "access denied or
repository not exported". Sites which don't have hidden repositories, or
don't care, can pass a flag to turn on more specific messages.

Signed-off-by: Jeff King <peff@peff.net>
Helped-by: Sitaram Chamarty <sitaramc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-daemon.txt

@@ -161,6 +161,16 @@ the facility of inet daemon to achieve the same before spawning
 	repository configuration.  By default, all the services
 	are overridable.
 
+--informative-errors::
+--no-informative-errors::
+	When informative errors are turned on, git-daemon will report
+	more verbose errors to the client, differentiating conditions
+	like "no such repository" from "repository not exported". This
+	is more convenient for clients, but may leak information about
+	the existence of unexported repositories.  When informative
+	errors are not enabled, all errors report "access denied" to the
+	client. The default is --no-informative-errors.
+
 <directory>::
 	A directory to add to the whitelist of allowed directories. Unless
 	--strict-paths is specified this will also include subdirectories

daemon.c

@@ -20,6 +20,7 @@
 static int log_syslog;
 static int verbose;
 static int reuseaddr;
+static int informative_errors;
 
 static const char daemon_usage[] =
 "git daemon [--verbose] [--syslog] [--export-all]\n"
@@ -247,6 +248,14 @@ static int git_daemon_config(const char *var, const char *value, void *cb)
 	return 0;
 }
 
+static int daemon_error(const char *dir, const char *msg)
+{
+	if (!informative_errors)
+		msg = "access denied or repository not exported";
+	packet_write(1, "ERR %s: %s", msg, dir);
+	return -1;
+}
+
 static int run_service(char *dir, struct daemon_service *service)
 {
 	const char *path;
@@ -257,11 +266,11 @@ static int run_service(char *dir, struct daemon_service *service)
 	if (!enabled && !service->overridable) {
 		logerror("'%s': service not enabled.", service->name);
 		errno = EACCES;
-		return -1;
+		return daemon_error(dir, "service not enabled");
 	}
 
 	if (!(path = path_ok(dir)))
-		return -1;
+		return daemon_error(dir, "no such repository");
 
 	/*
 	 * Security on the cheap.
@@ -277,7 +286,7 @@ static int run_service(char *dir, struct daemon_service *service)
 	if (!export_all_trees && access("git-daemon-export-ok", F_OK)) {
 		logerror("'%s': repository not exported.", path);
 		errno = EACCES;
-		return -1;
+		return daemon_error(dir, "repository not exported");
 	}
 
 	if (service->overridable) {
@@ -291,7 +300,7 @@ static int run_service(char *dir, struct daemon_service *service)
 		logerror("'%s': service not enabled for '%s'",
 			 service->name, path);
 		errno = EACCES;
-		return -1;
+		return daemon_error(dir, "service not enabled");
 	}
 
 	/*
@@ -1167,6 +1176,14 @@ int main(int argc, char **argv)
 			make_service_overridable(arg + 18, 0);
 			continue;
 		}
+		if (!prefixcmp(arg, "--informative-errors")) {
+			informative_errors = 1;
+			continue;
+		}
+		if (!prefixcmp(arg, "--no-informative-errors")) {
+			informative_errors = 0;
+			continue;
+		}
 		if (!strcmp(arg, "--")) {
 			ok_paths = &argv[i+1];
 			break;