Fix #26: handle Linux EPIPE on /dev/kmsg

When Linux CONFIG_LOG_BUF_SHIFT is set too low, or too many messages are
generated by the kernel, /dev/kmsg will overflow.  This is signaled with
EPIPE to userspace.  We can use the seqnos to figure out how many we've
lost, but seqnos are currently ignored.

> In case records get overwritten while /dev/kmsg is held open, or
> records get faster overwritten than they are read, the next read()
> will return -EPIPE and the current reading position gets updated to
> the next available record. The passed sequence numbers allow the log
> consumer to calculate the amount of lost messages.

-- https://lwn.net/Articles/490690/

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/syslogd.c

@@ -469,9 +469,21 @@ static void kernel_cb(int fd, void *arg)
 		if (i > 0) {
 			line[i + len] = '\0';
 		} else {
-			if (i < 0 && errno != EINTR && errno != EAGAIN) {
+			if (i < 0) {
+				switch (errno) {
+				case EPIPE: /* linux, log buffer overrun */
+					ERRX("Kernel log buffer filling up too quick, "
+					     "or too small log buffer, "
+					     "adjust kernel CONFIG_LOG_BUF_SHIFT");
+				case EINTR:
+				case EAGAIN:
+					break;
+
+				default:
 					ERR("klog read()");
 					socket_close(fd);
+					break;
+				}
 			}
 			break;
 		}