lib: fix possible assert() fail in zlog_fd()

If the last message in a batched logging operation isn't printed due to
priority, this skips the code that flushes prepared messages through
writev() and can trigger the assert() at the end of zlog_fd().

Since any logmsg above info priority triggers a buffer flush, running
into this situation requires a log file target configured for info
priority, at least 1 message of info priority buffered, a debug message
buffered after that, and then a buffer flush (explicit or due to buffer
full).

I haven't seen this chain of events happen in the wild, but it needs
fixing anyway.

Signed-off-by: David Lamparter <equinox@diac24.net>

lib/zlog_targets.c

@@ -78,40 +78,48 @@ void zlog_fd(struct zlog_target *zt, struct zlog_msg *msgs[], size_t nmsgs)
 		struct zlog_msg *msg = msgs[i];
 		int prio = zlog_msg_prio(msg);
 
-		if (prio > zt->prio_min)
-			continue;
-
-		iov[iovpos].iov_base = ts_pos;
-		if (iovpos > 0)
-			*ts_pos++ = '\n';
-		ts_pos += zlog_msg_ts(msg, ts_pos, sizeof(ts_buf) - 1
-				      - (ts_pos - ts_buf),
-				      ZLOG_TS_LEGACY | zte->ts_subsec);
-		*ts_pos++ = ' ';
-		iov[iovpos].iov_len = ts_pos - (char *)iov[iovpos].iov_base;
+		if (prio <= zt->prio_min) {
+			iov[iovpos].iov_base = ts_pos;
+			if (iovpos > 0)
+				*ts_pos++ = '\n';
+			ts_pos += zlog_msg_ts(msg, ts_pos,
+					      sizeof(ts_buf) - 1
+						      - (ts_pos - ts_buf),
+					      ZLOG_TS_LEGACY | zte->ts_subsec);
+			*ts_pos++ = ' ';
+			iov[iovpos].iov_len =
+				ts_pos - (char *)iov[iovpos].iov_base;
 
-		iovpos++;
+			iovpos++;
 
-		if (zte->record_priority) {
-			iov[iovpos].iov_base = (char *)prionames[prio];
-			iov[iovpos].iov_len = strlen(iov[iovpos].iov_base);
+			if (zte->record_priority) {
+				iov[iovpos].iov_base = (char *)prionames[prio];
+				iov[iovpos].iov_len =
+					strlen(iov[iovpos].iov_base);
 
-			iovpos++;
-		}
+				iovpos++;
+			}
 
-		iov[iovpos].iov_base = zlog_prefix;
-		iov[iovpos].iov_len = zlog_prefixsz;
+			iov[iovpos].iov_base = zlog_prefix;
+			iov[iovpos].iov_len = zlog_prefixsz;
 
-		iovpos++;
+			iovpos++;
 
-		iov[iovpos].iov_base = (char *)zlog_msg_text(msg, &textlen);
-		iov[iovpos].iov_len = textlen;
+			iov[iovpos].iov_base =
+				(char *)zlog_msg_text(msg, &textlen);
+			iov[iovpos].iov_len = textlen;
 
-		iovpos++;
+			iovpos++;
+		}
 
-		if (ts_buf + sizeof(ts_buf) - ts_pos < TS_LEN
-		    || i + 1 == nmsgs
-		    || array_size(iov) - iovpos < 5) {
+		/* conditions that trigger writing:
+		 *  - out of space for more timestamps/headers
+		 *  - this being the last message in the batch
+		 *  - not enough remaining iov entries
+		 */
+		if (iovpos > 0 && (ts_buf + sizeof(ts_buf) - ts_pos < TS_LEN
+				   || i + 1 == nmsgs
+				   || array_size(iov) - iovpos < 5)) {
 			iov[iovpos].iov_base = (char *)"\n";
 			iov[iovpos].iov_len = 1;