Faster manager_process_barrier_fd and drop message if BARRIER=1 found

 - This function is called for each message, and almost all of them will not
   contain BARRIER=1. So 5 comparaisons are realized in previous code. Now only
   one strstr is used.

 - If BARRIER=1 is found, drop the message in all case. So always return true
   when BARRIER=1 is found at the beginning of a new line

Follow up of systemd#15547

src/core/manager.c

@@ -2285,16 +2285,35 @@ static int manager_dispatch_cgroups_agent_fd(sd_event_source *source, int fd, ui
 }
 
 static bool manager_process_barrier_fd(const char *buf, FDSet *fds) {
+        const char *p;
+
         assert(buf);
 
         /* nothing else must be sent when using BARRIER=1 */
-        if (STR_IN_SET(buf, "BARRIER=1", "BARRIER=1\n")) {
-                if (fdset_size(fds) != 1)
-                        log_warning("Got incorrect number of fds with BARRIER=1, closing them.");
-                return true;
-        } else if (startswith(buf, "BARRIER=1\n") || strstr(buf, "\nBARRIER=1\n") || endswith(buf, "\nBARRIER=1"))
-                log_warning("Extra notification messages sent with BARRIER=1, ignoring everything.");
+        p = strstr(buf, "BARRIER=1");
+        if (p != NULL) {
+                int barrier_found = -1;
+
+                if (p == buf) {
+                        /* Check there is nothing after BARRIER=1 */
+                        p += STRLEN("BARRIER=1");
+                        if ((p[0] == '\0') || (p[0] == '\n' && p[1] == '\0'))
+                                barrier_found = 1;
+                } else {
+                        /* Check BARRIER=1 is not at the beginning of a new line */
+                        if (p[-1] != '\n')
+                                barrier_found = 0;
+                }
+
+                if (barrier_found > 0) {
+                        if (fdset_size(fds) != 1)
+                                log_warning("Got incorrect number of fds with BARRIER=1, closing them.");
+                } else if (barrier_found < 0)
+                        log_warning("Extra notification messages sent with BARRIER=1, ignoring everything.");
 
+                /* Drop the message if BARRIER=1 was found */
+                return (barrier_found != 0);
+        }
         return false;
 }