gc: fix flushing of messages to logs

There were the usual effects: very incorrect timestamps (even by days),
and I suspect we could even lose some logs completely.

NEWS

@@ -6,6 +6,7 @@ Bugfixes
 - hints module: NODATA answers also for non-address queries (!1005)
 - tls: send alert to peer if handshake fails (!1007)
 - cache: fix interaction between LMDB locks and preallocation (!1013)
+- cache garbage collector: fix flushing of messages to logs (!1009)
 
 
 Knot Resolver 5.1.1 (2020-05-19)

utils/cache_gc/main.c

@@ -56,6 +56,11 @@ static long get_nonneg_optarg()
 int main(int argc, char *argv[])
 {
 	printf("Knot Resolver Cache Garbage Collector v. %s\n", KR_CACHE_GC_VERSION);
+	if (setvbuf(stdout, NULL, _IONBF, 0) || setvbuf(stderr, NULL, _IONBF, 0)) {
+		fprintf(stderr, "Failed to to set output buffering (ignored): %s\n",
+				strerror(errno));
+		fflush(stderr);
+	}
 
 	signal(SIGTERM, got_killed);
 	signal(SIGKILL, got_killed);