[sheepdog] [PATCH] sheep: lock against concurrent access to the log file

[Christoph Hellwig]

I'm currently debugging a problem where a logger stays around even after the
sheep was killed.  Lock access to the log file to make debugging cases like
this easier.

Signed-off-by: Christoph Hellwig <hch at lst.de>

diff --git a/lib/logger.c b/lib/logger.c
index 5bce85c..b15ecc5 100644
--- a/lib/logger.c
+++ b/lib/logger.c
@@ -424,6 +424,18 @@ notrace int log_init(char *program_name, int size, int to_stdout, int level,
 				syslog(LOG_ERR, "failed to open %s\n", outfile);
 				return 1;
 			}
+
+			if (lockf(fd, F_TLOCK, 1) < 0) {
+				if (errno == EACCES || errno == EAGAIN) {
+					eprintf("another sheep daemon "
+						"is using %s\n", outfile);
+				} else {
+					eprintf("unable to get a lock "
+						"on the log file %s (%s)\n",
+						outfile, strerror(errno));
+				}
+				return 1;
+			}
 		} else {
 			fd = -1;
 			openlog(log_name, LOG_CONS | LOG_PID, LOG_DAEMON);