commit 487b38e86c08431bc5f48aac72c8d753ee23cb03
Author: Willy Tarreau <w@1wt.eu>
Date:   Fri Jul 26 15:10:39 2019 +0200

    BUG/MINOR: log: make sure writev() is not interrupted on a file output
    
    Since 1.9 we support sending logs to various non-blocking outputs like
    stdou/stderr or flies, by using writev() which guarantees that it only
    returns after having written everything or nothing. However the syscall
    may be interrupted while doing so, and this is visible when writing to
    a tty during debug sessions, as some logs occasionally appear interleaved
    if an xterm or SSH connection is not very fast. Performance here is not a
    critical concern, log correctness is. Let's simply take the logger's lock
    around the writev() call to prevent multiple senders from stepping onto
    each other's toes.
    
    This may be backported to 2.0 and 1.9.
    
    (cherry picked from commit 9fbcb7e2e9c32659ab11927394fec2e160be2d0b)
    Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>

diff --git a/src/log.c b/src/log.c
index ef999d13..99f185e4 100644
--- a/src/log.c
+++ b/src/log.c
@@ -1672,8 +1672,15 @@ send:
 	iovec[7].iov_len  = 1;
 
 	if (logsrv->addr.ss_family == AF_UNSPEC) {
-		/* the target is a direct file descriptor */
+		/* the target is a direct file descriptor. While writev() guarantees
+		 * to write everything, it doesn't guarantee that it will not be
+		 * interrupted while doing so. This occasionally results in interleaved
+		 * messages when the output is a tty, hence the lock. There's no real
+		 * performance concern here for such type of output.
+		 */
+		HA_SPIN_LOCK(LOGSRV_LOCK, &logsrv->lock);
 		sent = writev(*plogfd, iovec, 8);
+		HA_SPIN_UNLOCK(LOGSRV_LOCK, &logsrv->lock);
 	}
 	else {
 		msghdr.msg_name = (struct sockaddr *)&logsrv->addr;