Make isotptun more robust when starting up and handling errors

isotptun.c

@@ -368,10 +368,35 @@ int main(int argc, char **argv)
 			nbytes = read(s, buffer, BUF_LEN);
 			if (nbytes < 0) {
 				perror_syslog("read isotp socket");
-				return -1;
+				// Handle/ignore various RX/TX errors from isotp.
+				//
+				// Note, we must handle both RX and TX, since the RX path initiates TX flow control to the sender.
+				switch(errno) {
+					// Need to read again on next pass
+					case EAGAIN:
+					// RX path timeout of data reception leads to -ETIMEDOUT
+					case ETIMEDOUT:
+					// TX path flowcontrol reception timeout leads to -ECOMM
+					case ECOMM:
+					// RX path SN mismatch leads to -EILSEQ
+					case EILSEQ:
+					// TX path flowcontrol reception with wrong layout/padding leads to -EBADMSG
+					// RX path data reception with wrong padding leads to -EBADMSG
+					case EBADMSG:
+					// TX path flowcontrol reception overflow leads to -EMSGSIZE
+					case EMSGSIZE:
+						perror_syslog("read isotp socket ignore errno");
+						// ignore the above errors as they are recoverable
+						break;
+					default:
+						// otherwise, its an error we cannot recover from so exit
+						return nbytes;
 				}
-			if (nbytes > MAX_PDU_LENGTH)
-				return -1;
+			} else {
+				if (nbytes > MAX_PDU_LENGTH) {
+					syslogger(LOG_WARNING, "isotp received too many bytes %d for max PDU length %d\n",
+							  nbytes, MAX_PDU_LENGTH);
+				} else {
 					ret = write(t, buffer, nbytes);
 					if (verbose) {
 						if (ret < 0 && errno == EAGAIN)
@@ -381,15 +406,21 @@ int main(int argc, char **argv)
 						fflush(stdout);
 					}
 				}
+			}
+		}
 
 		if (FD_ISSET(t, &rdfs)) {
 			nbytes = read(t, buffer, BUF_LEN);
 			if (nbytes < 0) {
+				if (errno != EAGAIN) {
 					perror_syslog("read tunfd");
-				return -1;
+					return nbytes;
 				}
-			if (nbytes > MAX_PDU_LENGTH)
-				return -1;
+			} else {
+				if (nbytes > MAX_PDU_LENGTH) {
+					syslogger(LOG_WARNING, "tun received too many bytes %d for max PDU length %d\n",
+							  nbytes, MAX_PDU_LENGTH);
+				} else {
 					ret = write(s, buffer, nbytes);
 					if (verbose) {
 						if (ret < 0 && errno == EAGAIN)
@@ -400,6 +431,8 @@ int main(int argc, char **argv)
 					}
 				}
 			}
+		}
+	}
 
 	close(s);
 	close(t);