postgres_fdw: Replace WAIT_EVENT_EXTENSION with custom wait events

Three custom wait events are added here:
- "PostgresFdwCleanupResult", waiting while cleaning up PQgetResult() on
transaction abort.
- "PostgresFdwConnect", waiting to establish a connection to a remote
server.
- "PostgresFdwGetResult", waiting to receive a result from a remote
server.

Author: Masahiro Ikeda
Discussion: https://postgr.es/m/197bce267fa691a0ac62c86c4ab904c4@oss.nttdata.com

contrib/postgres_fdw/connection.c

@@ -83,6 +83,11 @@ static unsigned int prep_stmt_number = 0;
 /* tracks whether any work is needed in callback functions */
 static bool xact_got_connection = false;
 
+/* custom wait event values, retrieved from shared memory */
+static uint32 pgfdw_we_cleanup_result = 0;
+static uint32 pgfdw_we_connect = 0;
+static uint32 pgfdw_we_get_result = 0;
+
 /*
  * Milliseconds to wait to cancel an in-progress query or execute a cleanup
  * query; if it takes longer than 30 seconds to do these, we assume the
@@ -527,10 +532,14 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 		/* verify the set of connection parameters */
 		check_conn_params(keywords, values, user);
 
+		/* first time, allocate or get the custom wait event */
+		if (pgfdw_we_connect == 0)
+			pgfdw_we_connect = WaitEventExtensionNew("PostgresFdwConnect");
+
 		/* OK to make connection */
 		conn = libpqsrv_connect_params(keywords, values,
 									   false,	/* expand_dbname */
-									   WAIT_EVENT_EXTENSION);
+									   pgfdw_we_connect);
 
 		if (!conn || PQstatus(conn) != CONNECTION_OK)
 			ereport(ERROR,
@@ -858,12 +867,16 @@ pgfdw_get_result(PGconn *conn, const char *query)
 			{
 				int			wc;
 
+				/* first time, allocate or get the custom wait event */
+				if (pgfdw_we_get_result == 0)
+					pgfdw_we_get_result = WaitEventExtensionNew("PostgresFdwGetResult");
+
 				/* Sleep until there's something to do */
 				wc = WaitLatchOrSocket(MyLatch,
 									   WL_LATCH_SET | WL_SOCKET_READABLE |
 									   WL_EXIT_ON_PM_DEATH,
 									   PQsocket(conn),
-									   -1L, WAIT_EVENT_EXTENSION);
+									   -1L, pgfdw_we_get_result);
 				ResetLatch(MyLatch);
 
 				CHECK_FOR_INTERRUPTS();
@@ -1562,12 +1575,16 @@ pgfdw_get_cleanup_result(PGconn *conn, TimestampTz endtime, PGresult **result,
 					goto exit;
 				}
 
+				/* first time, allocate or get the custom wait event */
+				if (pgfdw_we_cleanup_result == 0)
+					pgfdw_we_cleanup_result = WaitEventExtensionNew("PostgresFdwCleanupResult");
+
 				/* Sleep until there's something to do */
 				wc = WaitLatchOrSocket(MyLatch,
 									   WL_LATCH_SET | WL_SOCKET_READABLE |
 									   WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
 									   PQsocket(conn),
-									   cur_timeout, WAIT_EVENT_EXTENSION);
+									   cur_timeout, pgfdw_we_cleanup_result);
 				ResetLatch(MyLatch);
 
 				CHECK_FOR_INTERRUPTS();

doc/src/sgml/postgres-fdw.sgml

@@ -1042,6 +1042,44 @@ postgres=# SELECT postgres_fdw_disconnect_all();
   </para>
  </sect2>
 
+ <sect2 id="postgres-fdw-wait-events">
+  <title>Wait Events</title>
+
+  <para>
+   <filename>postgres_fdw</filename> can report the following wait events
+   under the wait event type <literal>Extension</literal>:
+  </para>
+
+  <variablelist>
+   <varlistentry>
+    <term><literal>PostgresFdwCleanupResult</literal></term>
+    <listitem>
+     <para>
+      Waiting for transaction abort on remote server.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>PostgresFdwConnect</literal></term>
+    <listitem>
+     <para>
+      Waiting to establish a connection to a remote server.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>PostgresFdwGetResult</literal></term>
+    <listitem>
+     <para>
+      Waiting to receive the results of a query from a remote server.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+ </sect2>
+
  <sect2 id="postgres-fdw-configuration-parameters">
   <title>Configuration Parameters</title>