Add support for asynchronous execution.

This implements asynchronous execution, which runs multiple parts of a
non-parallel-aware Append concurrently rather than serially to improve
performance when possible.  Currently, the only node type that can be
run concurrently is a ForeignScan that is an immediate child of such an
Append.  In the case where such ForeignScans access data on different
remote servers, this would run those ForeignScans concurrently, and
overlap the remote operations to be performed simultaneously, so it'll
improve the performance especially when the operations involve
time-consuming ones such as remote join and remote aggregation.

We may extend this to other node types such as joins or aggregates over
ForeignScans in the future.

This also adds the support for postgres_fdw, which is enabled by the
table-level/server-level option "async_capable".  The default is false.

Robert Haas, Kyotaro Horiguchi, Thomas Munro, and myself.  This commit
is mostly based on the patch proposed by Robert Haas, but also uses
stuff from the patch proposed by Kyotaro Horiguchi and from the patch
proposed by Thomas Munro.  Reviewed by Kyotaro Horiguchi, Konstantin
Knizhnik, Andrey Lepikhov, Movead Li, Thomas Munro, Justin Pryzby, and
others.

Discussion: https://postgr.es/m/CA%2BTgmoaXQEt4tZ03FtQhnzeDEMzBck%2BLrni0UWHVVgOTnA6C1w%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGLBRyu0rHrDCMC4%3DRn3252gogyp1SjOgG8SEKKZv%3DFwfQ%40mail.gmail.com
Discussion: https://postgr.es/m/20200228.170650.667613673625155850.horikyota.ntt%40gmail.com

Author: Etsuro Fujita

contrib/postgres_fdw/connection.c

@@ -62,6 +62,7 @@ typedef struct ConnCacheEntry
 	Oid			serverid;		/* foreign server OID used to get server name */
 	uint32		server_hashvalue;	/* hash value of foreign server OID */
 	uint32		mapping_hashvalue;	/* hash value of user mapping OID */
+	PgFdwConnState state;		/* extra per-connection state */
 } ConnCacheEntry;
 
 /*
@@ -115,9 +116,12 @@ static bool disconnect_cached_connections(Oid serverid);
  * will_prep_stmt must be true if caller intends to create any prepared
  * statements.  Since those don't go away automatically at transaction end
  * (not even on error), we need this flag to cue manual cleanup.
+ *
+ * If state is not NULL, *state receives the per-connection state associated
+ * with the PGconn.
  */
 PGconn *
-GetConnection(UserMapping *user, bool will_prep_stmt)
+GetConnection(UserMapping *user, bool will_prep_stmt, PgFdwConnState **state)
 {
 	bool		found;
 	bool		retry = false;
@@ -196,6 +200,9 @@ GetConnection(UserMapping *user, bool will_prep_stmt)
 	 */
 	PG_TRY();
 	{
+		/* Process a pending asynchronous request if any. */
+		if (entry->state.pendingAreq)
+			process_pending_request(entry->state.pendingAreq);
 		/* Start a new transaction or subtransaction if needed. */
 		begin_remote_xact(entry);
 	}
@@ -264,6 +271,10 @@ GetConnection(UserMapping *user, bool will_prep_stmt)
 	/* Remember if caller will prepare statements */
 	entry->have_prep_stmt |= will_prep_stmt;
 
+	/* If caller needs access to the per-connection state, return it. */
+	if (state)
+		*state = &entry->state;
+
 	return entry->conn;
 }
 
@@ -291,6 +302,7 @@ make_new_connection(ConnCacheEntry *entry, UserMapping *user)
 	entry->mapping_hashvalue =
 		GetSysCacheHashValue1(USERMAPPINGOID,
 							  ObjectIdGetDatum(user->umid));
+	memset(&entry->state, 0, sizeof(entry->state));
 
 	/* Now try to make the connection */
 	entry->conn = connect_pg_server(server, user);
@@ -648,8 +660,12 @@ GetPrepStmtNumber(PGconn *conn)
  * Caller is responsible for the error handling on the result.
  */
 PGresult *
-pgfdw_exec_query(PGconn *conn, const char *query)
+pgfdw_exec_query(PGconn *conn, const char *query, PgFdwConnState *state)
 {
+	/* First, process a pending asynchronous request, if any. */
+	if (state && state->pendingAreq)
+		process_pending_request(state->pendingAreq);
+
 	/*
 	 * Submit a query.  Since we don't use non-blocking mode, this also can
 	 * block.  But its risk is relatively small, so we ignore that for now.
@@ -940,6 +956,8 @@ pgfdw_xact_callback(XactEvent event, void *arg)
 					{
 						entry->have_prep_stmt = false;
 						entry->have_error = false;
+						/* Also reset per-connection state */
+						memset(&entry->state, 0, sizeof(entry->state));
 					}
 
 					/* Disarm changing_xact_state if it all worked. */
@@ -1172,6 +1190,10 @@ pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry)
  * Cancel the currently-in-progress query (whose query text we do not have)
  * and ignore the result.  Returns true if we successfully cancel the query
  * and discard any pending result, and false if not.
+ *
+ * XXX: if the query was one sent by fetch_more_data_begin(), we could get the
+ * query text from the pendingAreq saved in the per-connection state, then
+ * report the query using it.
  */
 static bool
 pgfdw_cancel_query(PGconn *conn)