Srun to keep running after receiving unrecognized message

This can happen if something outside of Slurm opens the srun socket
and writes to it, since the data will not be of a form that Slurm
can decode.
Bug 354

src/srun/libsrun/allocate.c

@@ -207,6 +207,16 @@ static bool _retry(void)
 		      slurm_strerror(ESLURM_NODES_BUSY));
 		error_exit = immediate_exit;
 		return false;
+	} else if ((errno == SLURM_PROTOCOL_AUTHENTICATION_ERROR) ||
+		   (errno == SLURM_UNEXPECTED_MSG_ERROR) ||
+		   (errno == SLURM_PROTOCOL_INSANE_MSG_LENGTH)) {
+		static int external_msg_count = 0;
+		error("Srun communication socket apparently being written to "
+		      "by something other than Slurm");
+		if (external_msg_count++ < 4)
+			return true;
+		error("Unable to allocate resources: %m");
+		return false;
 	} else {
 		error("Unable to allocate resources: %m");
 		return false;