From c30f54ad732ca5c8762bb68bbe0f51de9137dd72 Mon Sep 17 00:00:00 2001
From: Thomas Munro <tmunro@postgresql.org>
Date: Sat, 3 Apr 2021 08:52:30 +1300
Subject: Detect POLLHUP/POLLRDHUP while running queries.
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Provide a new GUC check_client_connection_interval that can be used to
check whether the client connection has gone away, while running very
long queries.  It is disabled by default.

For now this uses a non-standard Linux extension (also adopted by at
least one other OS).  POLLRDHUP is not defined by POSIX, and other OSes
don't have a reliable way to know if a connection was closed without
actually trying to read or write.

In future we might consider trying to send a no-op/heartbeat message
instead, but that could require protocol changes.

Author: Sergey Cherkashin <s.cherkashin@postgrespro.ru>
Author: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@sraoss.co.jp>
Reviewed-by: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Maksim Milyutin <milyutinma@gmail.com>
Reviewed-by: Tsunakawa, Takayuki/綱川 貴之 <tsunakawa.takay@fujitsu.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (much earlier version)
Discussion: https://postgr.es/m/77def86b27e41f0efcba411460e929ae%40postgrespro.ru
---
 src/backend/libpq/pqcomm.c | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

(limited to 'src/backend/libpq/pqcomm.c')

diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index 4c7b1e7bfdf..4cd6d6dfbb9 100644
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -54,6 +54,9 @@
  */
 #include "postgres.h"
 
+#ifdef HAVE_POLL_H
+#include <poll.h>
+#endif
 #include <signal.h>
 #include <fcntl.h>
 #include <grp.h>
@@ -1921,3 +1924,40 @@ pq_settcpusertimeout(int timeout, Port *port)
 
 	return STATUS_OK;
 }
+
+/*
+ * Check if the client is still connected.
+ */
+bool
+pq_check_connection(void)
+{
+#if defined(POLLRDHUP)
+	/*
+	 * POLLRDHUP is a Linux extension to poll(2) to detect sockets closed by
+	 * the other end.  We don't have a portable way to do that without
+	 * actually trying to read or write data on other systems.  We don't want
+	 * to read because that would be confused by pipelined queries and COPY
+	 * data. Perhaps in future we'll try to write a heartbeat message instead.
+	 */
+	struct pollfd pollfd;
+	int			rc;
+
+	pollfd.fd = MyProcPort->sock;
+	pollfd.events = POLLOUT | POLLIN | POLLRDHUP;
+	pollfd.revents = 0;
+
+	rc = poll(&pollfd, 1, 0);
+
+	if (rc < 0)
+	{
+		ereport(COMMERROR,
+				(errcode_for_socket_access(),
+				 errmsg("could not poll socket: %m")));
+		return false;
+	}
+	else if (rc == 1 && (pollfd.revents & (POLLHUP | POLLRDHUP)))
+		return false;
+#endif
+
+	return true;
+}
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