Do not attempt to manage OpenSSL locking callbacks in libpq

A deadlock occurs when connecting to Postgresql using SSL with postgresql-libs in
a multi-threaded environment with other threads performing SSL independently.

This issue has been causing Module Build Service (https://pagure.io/fm-orchestrator)
outages, which is deployed on RHEL 7.4 but with the latest postgresql-libs package installed.

You can find a reproducer script and a more in-depth description of the issue here:
https://postgrespro.com/list/thread-id/1861629

The upstream patch that resolves this issue is here:
https://commitfest.postgresql.org/4/140/

RHBZ: #1755154

diff -ur postgresql-9.2.24/src/interfaces/libpq/fe-secure.c postgresql-patch/src/interfaces/libpq/fe-secure.c
--- postgresql-9.2.24/src/interfaces/libpq/fe-secure.c	2017-11-06 23:17:39.000000000 +0100
+++ postgresql-patch/src/interfaces/libpq/fe-secure.c	2019-10-29 15:25:28.448918186 +0100
@@ -941,9 +941,12 @@
 
 		if (ssl_open_connections++ == 0)
 		{
-			/* These are only required for threaded libcrypto applications */
-			CRYPTO_set_id_callback(pq_threadidcallback);
-			CRYPTO_set_locking_callback(pq_lockingcallback);
+			/* These are only required for threaded libcrypto applications, but
+                        * make sure we don't stomp on them if they're already set. */
+                       if (CRYPTO_get_id_callback() == NULL)
+                               CRYPTO_set_id_callback(pq_threadidcallback);
+                       if (CRYPTO_get_locking_callback() == NULL)
+                               CRYPTO_set_locking_callback(pq_lockingcallback);
 		}
 	}
 #endif   /* HAVE_CRYPTO_LOCK */
@@ -997,10 +1000,13 @@
 
 	if (pq_init_crypto_lib && ssl_open_connections == 0)
 	{
-		/* No connections left, unregister libcrypto callbacks */
-		CRYPTO_set_locking_callback(NULL);
-		CRYPTO_set_id_callback(NULL);
 
+		/* No connections left, unregister libcrypto callbacks, if no one
+                * registered different ones in the meantime. */
+               if (CRYPTO_get_id_callback() == pq_threadidcallback)
+                       CRYPTO_set_id_callback(NULL);
+               if (CRYPTO_get_locking_callback() == pq_lockingcallback)
+                       CRYPTO_set_locking_callback(NULL);
 		/*
 		 * We don't free the lock array. If we get another connection in
 		 * this process, we will just re-use them with the existing mutexes.