iscsi-scstd: Return SVC_UNAVAILABLE while logins are suspended

SIGUSR1/SIGUSR2 set/clear logins_suspended. While set, any login
attempt is rejected with a retriable Target Error instead of the
permanent Initiator Error (TGT_NOT_FOUND) that causes initiators
to give up.

iscsi-scst/usr/iscsi_scstd.c

@@ -858,6 +858,18 @@ static void init_max_params(void)
 	return;
 }
 
+static void suspend_logins(int sig)
+{
+	(void)sig;
+	logins_suspended = 1;
+}
+
+static void resume_logins(int sig)
+{
+	(void)sig;
+	logins_suspended = 0;
+}
+
 int main(int argc, char **argv)
 {
 	int ch, longindex;
@@ -883,6 +895,14 @@ int main(int argc, char **argv)
 	int rc = sigaction(SIGPIPE, &act, NULL);
 	assert(rc == 0);
 
+	act = (struct sigaction) { .sa_handler = suspend_logins };
+	rc = sigaction(SIGUSR1, &act, NULL);
+	assert(rc == 0);
+
+	act = (struct sigaction) { .sa_handler = resume_logins };
+	rc = sigaction(SIGUSR2, &act, NULL);
+	assert(rc == 0);
+
 	while ((ch = getopt_long(argc, argv, "c:fd:s:u:g:a:p:vh", long_options, &longindex)) >= 0) {
 		switch (ch) {
 		case 'c':

iscsi-scst/usr/iscsid.c

@@ -28,6 +28,7 @@
 
 int iscsi_enabled;
 char *internal_portal;
+int logins_suspended;
 
 static u32 ttt;
 
@@ -811,6 +812,13 @@ static void login_start(struct connection *conn)
 		return;
 	}
 
+	if (logins_suspended) {
+		log_info("Initiator %s login rejected: service temporarily unavailable",
+			 conn->initiator);
+		login_rsp_tgt_err(conn, ISCSI_STATUS_SVC_UNAVAILABLE);
+		return;
+	}
+
 	session_type = text_key_find(conn, "SessionType");
 	target_name = text_key_find(conn, "TargetName");
 

iscsi-scst/usr/iscsid.h

@@ -263,6 +263,7 @@ extern const char *get_error_str(int error);
 /* iscsid.c */
 extern int iscsi_enabled;
 extern char *internal_portal;
+extern int logins_suspended;
 
 extern int cmnd_execute(struct connection *conn);
 extern void cmnd_finish(struct connection *conn);