Fix bugs in libpq's management of GSS encryption state.

GSS-related resources should be cleaned up in pqDropConnection,
not freePGconn, else the wrong things happen when resetting
a connection or trying to switch to a different server.
It's also critical to reset conn->gssenc there.

During connection setup, initialize conn->try_gss at the correct
place, else switching to a different server won't work right.

Remove now-redundant cleanup of GSS resources around one (and, for
some reason, only one) pqDropConnection call in connectDBStart.

Per report from Kyotaro Horiguchi that psql would freeze up,
rather than successfully resetting a GSS-encrypted connection
after a server restart.

This is YA oversight in commit b0b39f7, so back-patch to v12.

Discussion: https://postgr.es/m/20200710.173803.435804731896516388.horikyota.ntt@gmail.com

Author: tglsfdc

src/interfaces/libpq/fe-connect.c

@@ -477,6 +477,11 @@ pqDropConnection(PGconn *conn, bool flushInput)
 	{
 		OM_uint32	min_s;
 
+		if (conn->gcred != GSS_C_NO_CREDENTIAL)
+		{
+			gss_release_cred(&min_s, &conn->gcred);
+			conn->gcred = GSS_C_NO_CREDENTIAL;
+		}
 		if (conn->gctx)
 			gss_delete_sec_context(&min_s, &conn->gctx, GSS_C_NO_BUFFER);
 		if (conn->gtarg_nam)
@@ -496,6 +501,7 @@ pqDropConnection(PGconn *conn, bool flushInput)
 			free(conn->gss_ResultBuffer);
 			conn->gss_ResultBuffer = NULL;
 		}
+		conn->gssenc = false;
 	}
 #endif
 #ifdef ENABLE_SSPI
@@ -2023,11 +2029,6 @@ connectDBStart(PGconn *conn)
 	 */
 	resetPQExpBuffer(&conn->errorMessage);
 
-#ifdef ENABLE_GSS
-	if (conn->gssencmode[0] == 'd') /* "disable" */
-		conn->try_gss = false;
-#endif
-
 	/*
 	 * Set up to try to connect to the first host.  (Setting whichhost = -1 is
 	 * a bit of a cheat, but PQconnectPoll will advance it to 0 before
@@ -2464,6 +2465,9 @@ PQconnectPoll(PGconn *conn)
 		conn->allow_ssl_try = (conn->sslmode[0] != 'd');	/* "disable" */
 		conn->wait_ssl_try = (conn->sslmode[0] == 'a'); /* "allow" */
 #endif
+#ifdef ENABLE_GSS
+		conn->try_gss = (conn->gssencmode[0] != 'd');	/* "disable" */
+#endif
 
 		reset_connection_state_machine = false;
 		need_new_connection = true;
@@ -3345,12 +3349,8 @@ PQconnectPoll(PGconn *conn)
 					 */
 					if (conn->gssenc && conn->gssencmode[0] == 'p')
 					{
-						OM_uint32	minor;
-
 						/* postmaster expects us to drop the connection */
 						conn->try_gss = false;
-						conn->gssenc = false;
-						gss_delete_sec_context(&minor, &conn->gctx, NULL);
 						pqDropConnection(conn, true);
 						conn->status = CONNECTION_NEEDED;
 						goto keep_going;
@@ -3902,9 +3902,6 @@ makeEmptyPGconn(void)
 	conn->verbosity = PQERRORS_DEFAULT;
 	conn->show_context = PQSHOW_CONTEXT_ERRORS;
 	conn->sock = PGINVALID_SOCKET;
-#ifdef ENABLE_GSS
-	conn->try_gss = true;
-#endif
 
 	/*
 	 * We try to send at least 8K at a time, which is the usual size of pipe
@@ -4061,22 +4058,6 @@ freePGconn(PGconn *conn)
 		free(conn->gsslib);
 	if (conn->connip)
 		free(conn->connip);
-#ifdef ENABLE_GSS
-	if (conn->gcred != GSS_C_NO_CREDENTIAL)
-	{
-		OM_uint32	minor;
-
-		gss_release_cred(&minor, &conn->gcred);
-		conn->gcred = GSS_C_NO_CREDENTIAL;
-	}
-	if (conn->gctx)
-	{
-		OM_uint32	minor;
-
-		gss_delete_sec_context(&minor, &conn->gctx, GSS_C_NO_BUFFER);
-		conn->gctx = NULL;
-	}
-#endif
 	/* Note that conn->Pfdebug is not ours to close or free */
 	if (conn->last_query)
 		free(conn->last_query);