From ef8a2428cfe2ba86715e8dc1f966f9532ff5d190 Mon Sep 17 00:00:00 2001
From: Jeremy Harris <jgh146exb@wizmail.org>
Date: Sat, 25 Dec 2021 17:48:29 +0000
Subject: [PATCH]     smtp transport: poll for trailing data to drain before
 close

---
 src/src/daemon.c          | 2 +-
 src/src/functions.h       | 1 +
 src/src/transports/smtp.c | 7 +++++--
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/src/daemon.c b/src/src/daemon.c
index b10974a7a..4a3cb6adb 100644
--- a/src/src/daemon.c
+++ b/src/src/daemon.c
@@ -87,7 +87,7 @@ sigchld_seen = TRUE;
 }
 
 
-/* SIGTERM handler.  Try to get the damon pid file removed
+/* SIGTERM handler.  Try to get the daemon pid file removed
 before exiting. */
 
 static void
diff --git a/src/src/functions.h b/src/src/functions.h
index ba02b82b2..efabef044 100644
--- a/src/src/functions.h
+++ b/src/src/functions.h
@@ -1254,6 +1254,7 @@ child_open(uschar **argv, uschar **envp, int newumask, int *infdptr,
   outfdptr, make_leader, purpose);
 }
 
+/* Return 1 if fd is usable per pollbits, else 0 */
 static inline int
 poll_one_fd(int fd, short pollbits, int tmo_millisec)
 {
diff --git a/src/src/transports/smtp.c b/src/src/transports/smtp.c
index ee07bcfe8..721056f27 100644
--- a/src/src/transports/smtp.c
+++ b/src/src/transports/smtp.c
@@ -4810,8 +4810,11 @@ if (sx->send_quit || tcw_done && !tcw)
     sx->cctx.tls_ctx = NULL;
     }
 #endif
-  millisleep(20);
-  if (fcntl(sx->cctx.sock, F_SETFL, O_NONBLOCK) == 0)
+
+  /* Drain any trailing data from the socket before close, to avoid sending a RST */
+
+  if (  poll_one_fd(sx->cctx.sock, POLLIN, 20) != 0		/* 20ms */
+     && fcntl(sx->cctx.sock, F_SETFL, O_NONBLOCK) == 0)
     for (int i = 16, n;						/* drain socket */
 	 (n = read(sx->cctx.sock, sx->inbuffer, sizeof(sx->inbuffer))) > 0 && i > 0;
 	 i--) HDEBUG(D_transport|D_acl|D_v)
-- 
2.30.2