text: /*-------------------------------------------------------------------------
   *
   * connection.c
   *		  Connection management functions for postgres_fdw
   *
   * Portions Copyright (c) 2012-2019, PostgreSQL Global Development Group
   *
   * IDENTIFICATION
   *		  contrib/postgres_fdw/connection.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include "postgres_fdw.h"
  
  #include "access/htup_details.h"
  #include "access/xact.h"
  #include "catalog/pg_user_mapping.h"
  #include "mb/pg_wchar.h"
  #include "miscadmin.h"
  #include "pgstat.h"
  #include "storage/latch.h"
  #include "utils/hsearch.h"
  #include "utils/inval.h"
  #include "utils/memutils.h"
  #include "utils/syscache.h"
  
  
  /*
   * Connection cache hash table entry
   *
   * The lookup key in this hash table is the user mapping OID. We use just one
   * connection per user mapping ID, which ensures that all the scans use the
   * same snapshot during a query.  Using the user mapping OID rather than
   * the foreign server OID + user OID avoids creating multiple connections when
   * the public user mapping applies to all user OIDs.
   *
   * The "conn" pointer can be NULL if we don't currently have a live connection.
   * When we do have a connection, xact_depth tracks the current depth of
   * transactions and subtransactions open on the remote side.  We need to issue
   * commands at the same nesting depth on the remote as we're executing at
   * ourselves, so that rolling back a subtransaction will kill the right
   * queries and not the wrong ones.
   */
  typedef Oid ConnCacheKey;
  
  typedef struct ConnCacheEntry
  {
  	ConnCacheKey key;			/* hash key (must be first) */
  	PGconn	   *conn;			/* connection to foreign server, or NULL */
  	/* Remaining fields are invalid when conn is NULL: */
  	int			xact_depth;		/* 0 = no xact open, 1 = main xact open, 2 =
  								 * one level of subxact open, etc */
  	bool		have_prep_stmt; /* have we prepared any stmts in this xact? */
  	bool		have_error;		/* have any subxacts aborted in this xact? */
  	bool		changing_xact_state;	/* xact state change in process */
  	bool		invalidated;	/* true if reconnect is pending */
  	uint32		server_hashvalue;	/* hash value of foreign server OID */
  	uint32		mapping_hashvalue;	/* hash value of user mapping OID */
  } ConnCacheEntry;
  
  /*
   * Connection cache (initialized on first use)
   */
  static HTAB *ConnectionHash = NULL;
  
  /* for assigning cursor numbers and prepared statement numbers */
  static unsigned int cursor_number = 0;
  static unsigned int prep_stmt_number = 0;
  
  /* tracks whether any work is needed in callback functions */
  static bool xact_got_connection = false;
  
  /* prototypes of private functions */
  static PGconn *connect_pg_server(ForeignServer *server, UserMapping *user);
  static void disconnect_pg_server(ConnCacheEntry *entry);
  static void check_conn_params(const char **keywords, const char **values, UserMapping *user);
  static void configure_remote_session(PGconn *conn);
  static void do_sql_command(PGconn *conn, const char *sql);
  static void begin_remote_xact(ConnCacheEntry *entry);
  static void pgfdw_xact_callback(XactEvent event, void *arg);
  static void pgfdw_subxact_callback(SubXactEvent event,
  								   SubTransactionId mySubid,
  								   SubTransactionId parentSubid,
  								   void *arg);
  static void pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue);
  static void pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry);
  static bool pgfdw_cancel_query(PGconn *conn);
  static bool pgfdw_exec_cleanup_query(PGconn *conn, const char *query,
  									 bool ignore_errors);
  static bool pgfdw_get_cleanup_result(PGconn *conn, TimestampTz endtime,
  									 PGresult **result);
  
  
  /*
   * Get a PGconn which can be used to execute queries on the remote PostgreSQL
   * server with the user's authorization.  A new connection is established
   * if we don't already have a suitable one, and a transaction is opened at
   * the right subtransaction nesting depth if we didn't do that already.
   *
   * 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.
   */
  PGconn *
  GetConnection(UserMapping *user, bool will_prep_stmt)
  {
  	bool		found;
  	ConnCacheEntry *entry;
  	ConnCacheKey key;
  
  	/* First time through, initialize connection cache hashtable */
  	if (ConnectionHash == NULL)
  	{
  		HASHCTL		ctl;
  
  		MemSet(&ctl, 0, sizeof(ctl));
  		ctl.keysize = sizeof(ConnCacheKey);
  		ctl.entrysize = sizeof(ConnCacheEntry);
  		/* allocate ConnectionHash in the cache context */
  		ctl.hcxt = CacheMemoryContext;
  		ConnectionHash = hash_create("postgres_fdw connections", 8,
  									 &ctl,
  									 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
  
  		/*
  		 * Register some callback functions that manage connection cleanup.
  		 * This should be done just once in each backend.
  		 */
  		RegisterXactCallback(pgfdw_xact_callback, NULL);
  		RegisterSubXactCallback(pgfdw_subxact_callback, NULL);
  		CacheRegisterSyscacheCallback(FOREIGNSERVEROID,
  									  pgfdw_inval_callback, (Datum) 0);
  		CacheRegisterSyscacheCallback(USERMAPPINGOID,
  									  pgfdw_inval_callback, (Datum) 0);
  	}
  
  	/* Set flag that we did GetConnection during the current transaction */
  	xact_got_connection = true;
  
  	/* Create hash key for the entry.  Assume no pad bytes in key struct */
  	key = user->umid;
  
  	/*
  	 * Find or create cached entry for requested connection.
  	 */
  	entry = hash_search(ConnectionHash, &key, HASH_ENTER, &found);
  	if (!found)
  	{
  		/*
  		 * We need only clear "conn" here; remaining fields will be filled
  		 * later when "conn" is set.
  		 */
  		entry->conn = NULL;
  	}
  
  	/* Reject further use of connections which failed abort cleanup. */
  	pgfdw_reject_incomplete_xact_state_change(entry);
  
  	/*
  	 * If the connection needs to be remade due to invalidation, disconnect as
  	 * soon as we're out of all transactions.
  	 */
  	if (entry->conn != NULL && entry->invalidated && entry->xact_depth == 0)
  	{
  		elog(DEBUG3, "closing connection %p for option changes to take effect",
  			 entry->conn);
  		disconnect_pg_server(entry);
  	}
  
  	/*
  	 * We don't check the health of cached connection here, because it would
  	 * require some overhead.  Broken connection will be detected when the
  	 * connection is actually used.
  	 */
  
  	/*
  	 * If cache entry doesn't have a connection, we have to establish a new
  	 * connection.  (If connect_pg_server throws an error, the cache entry
  	 * will remain in a valid empty state, ie conn == NULL.)
  	 */
  	if (entry->conn == NULL)
  	{
  		ForeignServer *server = GetForeignServer(user->serverid);
  
  		/* Reset all transient state fields, to be sure all are clean */
  		entry->xact_depth = 0;
  		entry->have_prep_stmt = false;
  		entry->have_error = false;
  		entry->changing_xact_state = false;
  		entry->invalidated = false;
  		entry->server_hashvalue =
  			GetSysCacheHashValue1(FOREIGNSERVEROID,
  								  ObjectIdGetDatum(server->serverid));
  		entry->mapping_hashvalue =
  			GetSysCacheHashValue1(USERMAPPINGOID,
  								  ObjectIdGetDatum(user->umid));
  
  		/* Now try to make the connection */
  		entry->conn = connect_pg_server(server, user);
  
  		elog(DEBUG3, "new postgres_fdw connection %p for server \"%s\" (user mapping oid %u, userid %u)",
  			 entry->conn, server->servername, user->umid, user->userid);
  	}
  
  	/*
  	 * Start a new transaction or subtransaction if needed.
  	 */
  	begin_remote_xact(entry);
  
  	/* Remember if caller will prepare statements */
  	entry->have_prep_stmt |= will_prep_stmt;
  
  	return entry->conn;
  }
  
  /*
   * Connect to remote server using specified server and user mapping properties.
   */
  static PGconn *
  connect_pg_server(ForeignServer *server, UserMapping *user)
  {
  	PGconn	   *volatile conn = NULL;
  
  	/*
  	 * Use PG_TRY block to ensure closing connection on error.
  	 */
  	PG_TRY();
  	{
  		const char **keywords;
  		const char **values;
  		int			n;
  
  		/*
  		 * Construct connection params from generic options of ForeignServer
  		 * and UserMapping.  (Some of them might not be libpq options, in
  		 * which case we'll just waste a few array slots.)  Add 3 extra slots
  		 * for fallback_application_name, client_encoding, end marker.
  		 */
  		n = list_length(server->options) + list_length(user->options) + 3;
  		keywords = (const char **) palloc(n * sizeof(char *));
  		values = (const char **) palloc(n * sizeof(char *));
  
  		n = 0;
  		n += ExtractConnectionOptions(server->options,
  									  keywords + n, values + n);
  		n += ExtractConnectionOptions(user->options,
  									  keywords + n, values + n);
  
  		/* Use "postgres_fdw" as fallback_application_name. */
  		keywords[n] = "fallback_application_name";
  		values[n] = "postgres_fdw";
  		n++;
  
  		/* Set client_encoding so that libpq can convert encoding properly. */
  		keywords[n] = "client_encoding";
  		values[n] = GetDatabaseEncodingName();
  		n++;
  
  		keywords[n] = values[n] = NULL;
  
  		/* verify connection parameters and make connection */
  		check_conn_params(keywords, values, user);
  
  		conn = PQconnectdbParams(keywords, values, false);
  		if (!conn || PQstatus(conn) != CONNECTION_OK)
  			ereport(ERROR,
  					(errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
  					 errmsg("could not connect to server \"%s\"",
  							server->servername),
  					 errdetail_internal("%s", pchomp(PQerrorMessage(conn)))));
  
  		/*
  		 * Check that non-superuser has used password to establish connection;
  		 * otherwise, he's piggybacking on the postgres server's user
  		 * identity. See also dblink_security_check() in contrib/dblink.
  		 */
  		if (!superuser_arg(user->userid) && !PQconnectionUsedPassword(conn))
  			ereport(ERROR,
  					(errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
  					 errmsg("password is required"),
  					 errdetail("Non-superuser cannot connect if the server does not request a password."),
  					 errhint("Target server's authentication method must be changed.")));
  
  		/* Prepare new session for use */
  		configure_remote_session(conn);
  
  		pfree(keywords);
  		pfree(values);
  	}
  	PG_CATCH();
  	{
  		/* Release PGconn data structure if we managed to create one */
  		if (conn)
  			PQfinish(conn);
  		PG_RE_THROW();
  	}
  	PG_END_TRY();
  
  	return conn;
  }
  
  /*
   * Disconnect any open connection for a connection cache entry.
   */
  static void
  disconnect_pg_server(ConnCacheEntry *entry)
  {
  	if (entry->conn != NULL)
  	{
  		PQfinish(entry->conn);
  		entry->conn = NULL;
  	}
  }
  
  /*
   * For non-superusers, insist that the connstr specify a password.  This
   * prevents a password from being picked up from .pgpass, a service file,
   * the environment, etc.  We don't want the postgres user's passwords
   * to be accessible to non-superusers.  (See also dblink_connstr_check in
   * contrib/dblink.)
   */
  static void
  check_conn_params(const char **keywords, const char **values, UserMapping *user)
  {
  	int			i;
  
  	/* no check required if superuser */
  	if (superuser_arg(user->userid))
  		return;
  
  	/* ok if params contain a non-empty password */
  	for (i = 0; keywords[i] != NULL; i++)
  	{
  		if (strcmp(keywords[i], "password") == 0 && values[i][0] != '\0')
  			return;
  	}
  
  	ereport(ERROR,
  			(errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
  			 errmsg("password is required"),
  			 errdetail("Non-superusers must provide a password in the user mapping.")));
  }
  
  /*
   * Issue SET commands to make sure remote session is configured properly.
   *
   * We do this just once at connection, assuming nothing will change the
   * values later.  Since we'll never send volatile function calls to the
   * remote, there shouldn't be any way to break this assumption from our end.
   * It's possible to think of ways to break it at the remote end, eg making
   * a foreign table point to a view that includes a set_config call ---
   * but once you admit the possibility of a malicious view definition,
   * there are any number of ways to break things.
   */
  static void
  configure_remote_session(PGconn *conn)
  {
  	int			remoteversion = PQserverVersion(conn);
  
  	/* Force the search path to contain only pg_catalog (see deparse.c) */
  	do_sql_command(conn, "SET search_path = pg_catalog");
  
  	/*
  	 * Set remote timezone; this is basically just cosmetic, since all
  	 * transmitted and returned timestamptzs should specify a zone explicitly
  	 * anyway.  However it makes the regression test outputs more predictable.
  	 *
  	 * We don't risk setting remote zone equal to ours, since the remote
  	 * server might use a different timezone database.  Instead, use UTC
  	 * (quoted, because very old servers are picky about case).
  	 */
  	do_sql_command(conn, "SET timezone = 'UTC'");
  
  	/*
  	 * Set values needed to ensure unambiguous data output from remote.  (This
  	 * logic should match what pg_dump does.  See also set_transmission_modes
  	 * in postgres_fdw.c.)
  	 */
  	do_sql_command(conn, "SET datestyle = ISO");
  	if (remoteversion >= 80400)
  		do_sql_command(conn, "SET intervalstyle = postgres");
  	if (remoteversion >= 90000)
  		do_sql_command(conn, "SET extra_float_digits = 3");
  	else
  		do_sql_command(conn, "SET extra_float_digits = 2");
  }
  
  /*
   * Convenience subroutine to issue a non-data-returning SQL command to remote
   */
  static void
  do_sql_command(PGconn *conn, const char *sql)
  {
  	PGresult   *res;
  
  	if (!PQsendQuery(conn, sql))
  		pgfdw_report_error(ERROR, NULL, conn, false, sql);
  	res = pgfdw_get_result(conn, sql);
  	if (PQresultStatus(res) != PGRES_COMMAND_OK)
  		pgfdw_report_error(ERROR, res, conn, true, sql);
  	PQclear(res);
  }
  
  /*
   * Start remote transaction or subtransaction, if needed.
   *
   * Note that we always use at least REPEATABLE READ in the remote session.
   * This is so that, if a query initiates multiple scans of the same or
   * different foreign tables, we will get snapshot-consistent results from
   * those scans.  A disadvantage is that we can't provide sane emulation of
   * READ COMMITTED behavior --- it would be nice if we had some other way to
   * control which remote queries share a snapshot.
   */
  static void
  begin_remote_xact(ConnCacheEntry *entry)
  {
  	int			curlevel = GetCurrentTransactionNestLevel();
  
  	/* Start main transaction if we haven't yet */
  	if (entry->xact_depth <= 0)
  	{
  		const char *sql;
  
  		elog(DEBUG3, "starting remote transaction on connection %p",
  			 entry->conn);
  
  		if (IsolationIsSerializable())
  			sql = "START TRANSACTION ISOLATION LEVEL SERIALIZABLE";
  		else
  			sql = "START TRANSACTION ISOLATION LEVEL REPEATABLE READ";
  		entry->changing_xact_state = true;
  		do_sql_command(entry->conn, sql);
  		entry->xact_depth = 1;
  		entry->changing_xact_state = false;
  	}
  
  	/*
  	 * If we're in a subtransaction, stack up savepoints to match our level.
  	 * This ensures we can rollback just the desired effects when a
  	 * subtransaction aborts.
  	 */
  	while (entry->xact_depth < curlevel)
  	{
  		char		sql[64];
  
  		snprintf(sql, sizeof(sql), "SAVEPOINT s%d", entry->xact_depth + 1);
  		entry->changing_xact_state = true;
  		do_sql_command(entry->conn, sql);
  		entry->xact_depth++;
  		entry->changing_xact_state = false;
  	}
  }
  
  /*
   * Release connection reference count created by calling GetConnection.
   */
  void
  ReleaseConnection(PGconn *conn)
  {
  	/*
  	 * Currently, we don't actually track connection references because all
  	 * cleanup is managed on a transaction or subtransaction basis instead. So
  	 * there's nothing to do here.
  	 */
  }
  
  /*
   * Assign a "unique" number for a cursor.
   *
   * These really only need to be unique per connection within a transaction.
   * For the moment we ignore the per-connection point and assign them across
   * all connections in the transaction, but we ask for the connection to be
   * supplied in case we want to refine that.
   *
   * Note that even if wraparound happens in a very long transaction, actual
   * collisions are highly improbable; just be sure to use %u not %d to print.
   */
  unsigned int
  GetCursorNumber(PGconn *conn)
  {
  	return ++cursor_number;
  }
  
  /*
   * Assign a "unique" number for a prepared statement.
   *
   * This works much like GetCursorNumber, except that we never reset the counter
   * within a session.  That's because we can't be 100% sure we've gotten rid
   * of all prepared statements on all connections, and it's not really worth
   * increasing the risk of prepared-statement name collisions by resetting.
   */
  unsigned int
  GetPrepStmtNumber(PGconn *conn)
  {
  	return ++prep_stmt_number;
  }
  
  /*
   * Submit a query and wait for the result.
   *
   * This function is interruptible by signals.
   *
   * Caller is responsible for the error handling on the result.
   */
  PGresult *
  pgfdw_exec_query(PGconn *conn, const char *query)
  {
  	/*
  	 * 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.
  	 */
  	if (!PQsendQuery(conn, query))
  		pgfdw_report_error(ERROR, NULL, conn, false, query);
  
  	/* Wait for the result. */
  	return pgfdw_get_result(conn, query);
  }
  
  /*
   * Wait for the result from a prior asynchronous execution function call.
   *
   * This function offers quick responsiveness by checking for any interruptions.
   *
   * This function emulates PQexec()'s behavior of returning the last result
   * when there are many.
   *
   * Caller is responsible for the error handling on the result.
   */
  PGresult *
  pgfdw_get_result(PGconn *conn, const char *query)
  {
  	PGresult   *volatile last_res = NULL;
  
  	/* In what follows, do not leak any PGresults on an error. */
  	PG_TRY();
  	{
  		for (;;)
  		{
  			PGresult   *res;
  
  			while (PQisBusy(conn))
  			{
  				int			wc;
  
  				/* Sleep until there's something to do */
  				wc = WaitLatchOrSocket(MyLatch,
  									   WL_LATCH_SET | WL_SOCKET_READABLE |
  									   WL_EXIT_ON_PM_DEATH,
  									   PQsocket(conn),
  									   -1L, PG_WAIT_EXTENSION);
  				ResetLatch(MyLatch);
  
  				CHECK_FOR_INTERRUPTS();
  
  				/* Data available in socket? */
  				if (wc & WL_SOCKET_READABLE)
  				{
  					if (!PQconsumeInput(conn))
  						pgfdw_report_error(ERROR, NULL, conn, false, query);
  				}
  			}
  
  			res = PQgetResult(conn);
  			if (res == NULL)
  				break;			/* query is complete */
  
  			PQclear(last_res);
  			last_res = res;
  		}
  	}
  	PG_CATCH();
  	{
  		PQclear(last_res);
  		PG_RE_THROW();
  	}
  	PG_END_TRY();
  
  	return last_res;
  }
  
  /*
   * Report an error we got from the remote server.
   *
   * elevel: error level to use (typically ERROR, but might be less)
   * res: PGresult containing the error
   * conn: connection we did the query on
   * clear: if true, PQclear the result (otherwise caller will handle it)
   * sql: NULL, or text of remote command we tried to execute
   *
   * Note: callers that choose not to throw ERROR for a remote error are
   * responsible for making sure that the associated ConnCacheEntry gets
   * marked with have_error = true.
   */
  void
  pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
  				   bool clear, const char *sql)
  {
  	/* If requested, PGresult must be released before leaving this function. */
  	PG_TRY();
  	{
  		char	   *diag_sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
  		char	   *message_primary = PQresultErrorField(res, PG_DIAG_MESSAGE_PRIMARY);
  		char	   *message_detail = PQresultErrorField(res, PG_DIAG_MESSAGE_DETAIL);
  		char	   *message_hint = PQresultErrorField(res, PG_DIAG_MESSAGE_HINT);
  		char	   *message_context = PQresultErrorField(res, PG_DIAG_CONTEXT);
  		int			sqlstate;
  
  		if (diag_sqlstate)
  			sqlstate = MAKE_SQLSTATE(diag_sqlstate[0],
  									 diag_sqlstate[1],
  									 diag_sqlstate[2],
  									 diag_sqlstate[3],
  									 diag_sqlstate[4]);
  		else
  			sqlstate = ERRCODE_CONNECTION_FAILURE;
  
  		/*
  		 * If we don't get a message from the PGresult, try the PGconn.  This
  		 * is needed because for connection-level failures, PQexec may just
  		 * return NULL, not a PGresult at all.
  		 */
  		if (message_primary == NULL)
  			message_primary = pchomp(PQerrorMessage(conn));
  
  		ereport(elevel,
  				(errcode(sqlstate),
  				 message_primary ? errmsg_internal("%s", message_primary) :
  				 errmsg("could not obtain message string for remote error"),
  				 message_detail ? errdetail_internal("%s", message_detail) : 0,
  				 message_hint ? errhint("%s", message_hint) : 0,
  				 message_context ? errcontext("%s", message_context) : 0,
  				 sql ? errcontext("remote SQL command: %s", sql) : 0));
  	}
  	PG_CATCH();
  	{
  		if (clear)
  			PQclear(res);
  		PG_RE_THROW();
  	}
  	PG_END_TRY();
  	if (clear)
  		PQclear(res);
  }
  
  /*
   * pgfdw_xact_callback --- cleanup at main-transaction end.
   *
   * This runs just late enough that it must not enter user-defined code
   * locally.  (Entering such code on the remote side is fine.  Its remote
   * COMMIT TRANSACTION may run deferred triggers.)
   */
  static void
  pgfdw_xact_callback(XactEvent event, void *arg)
  {
  	HASH_SEQ_STATUS scan;
  	ConnCacheEntry *entry;
  
  	/* Quick exit if no connections were touched in this transaction. */
  	if (!xact_got_connection)
  		return;
  
  	/*
  	 * Scan all connection cache entries to find open remote transactions, and
  	 * close them.
  	 */
  	hash_seq_init(&scan, ConnectionHash);
  	while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
  	{
  		PGresult   *res;
  
  		/* Ignore cache entry if no open connection right now */
  		if (entry->conn == NULL)
  			continue;
  
  		/* If it has an open remote transaction, try to close it */
  		if (entry->xact_depth > 0)
  		{
  			bool		abort_cleanup_failure = false;
  
  			elog(DEBUG3, "closing remote transaction on connection %p",
  				 entry->conn);
  
  			switch (event)
  			{
  				case XACT_EVENT_PARALLEL_PRE_COMMIT:
  				case XACT_EVENT_PRE_COMMIT:
  
  					/*
  					 * If abort cleanup previously failed for this connection,
  					 * we can't issue any more commands against it.
  					 */
  					pgfdw_reject_incomplete_xact_state_change(entry);
  
  					/* Commit all remote transactions during pre-commit */
  					entry->changing_xact_state = true;
  					do_sql_command(entry->conn, "COMMIT TRANSACTION");
  					entry->changing_xact_state = false;
  
  					/*
  					 * If there were any errors in subtransactions, and we
  					 * made prepared statements, do a DEALLOCATE ALL to make
  					 * sure we get rid of all prepared statements. This is
  					 * annoying and not terribly bulletproof, but it's
  					 * probably not worth trying harder.
  					 *
  					 * DEALLOCATE ALL only exists in 8.3 and later, so this
  					 * constrains how old a server postgres_fdw can
  					 * communicate with.  We intentionally ignore errors in
  					 * the DEALLOCATE, so that we can hobble along to some
  					 * extent with older servers (leaking prepared statements
  					 * as we go; but we don't really support update operations
  					 * pre-8.3 anyway).
  					 */
  					if (entry->have_prep_stmt && entry->have_error)
  					{
  						res = PQexec(entry->conn, "DEALLOCATE ALL");
  						PQclear(res);
  					}
  					entry->have_prep_stmt = false;
  					entry->have_error = false;
  					break;
  				case XACT_EVENT_PRE_PREPARE:
  
  					/*
  					 * We disallow any remote transactions, since it's not
  					 * very reasonable to hold them open until the prepared
  					 * transaction is committed.  For the moment, throw error
  					 * unconditionally; later we might allow read-only cases.
  					 * Note that the error will cause us to come right back
  					 * here with event == XACT_EVENT_ABORT, so we'll clean up
  					 * the connection state at that point.
  					 */
  					ereport(ERROR,
  							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  							 errmsg("cannot PREPARE a transaction that has operated on postgres_fdw foreign tables")));
  					break;
  				case XACT_EVENT_PARALLEL_COMMIT:
  				case XACT_EVENT_COMMIT:
  				case XACT_EVENT_PREPARE:
  					/* Pre-commit should have closed the open transaction */
  					elog(ERROR, "missed cleaning up connection during pre-commit");
  					break;
  				case XACT_EVENT_PARALLEL_ABORT:
  				case XACT_EVENT_ABORT:
  
  					/*
  					 * Don't try to clean up the connection if we're already
  					 * in error recursion trouble.
  					 */
  					if (in_error_recursion_trouble())
  						entry->changing_xact_state = true;
  
  					/*
  					 * If connection is already unsalvageable, don't touch it
  					 * further.
  					 */
  					if (entry->changing_xact_state)
  						break;
  
  					/*
  					 * Mark this connection as in the process of changing
  					 * transaction state.
  					 */
  					entry->changing_xact_state = true;
  
  					/* Assume we might have lost track of prepared statements */
  					entry->have_error = true;
  
  					/*
  					 * If a command has been submitted to the remote server by
  					 * using an asynchronous execution function, the command
  					 * might not have yet completed.  Check to see if a
  					 * command is still being processed by the remote server,
  					 * and if so, request cancellation of the command.
  					 */
  					if (PQtransactionStatus(entry->conn) == PQTRANS_ACTIVE &&
  						!pgfdw_cancel_query(entry->conn))
  					{
  						/* Unable to cancel running query. */
  						abort_cleanup_failure = true;
  					}
  					else if (!pgfdw_exec_cleanup_query(entry->conn,
  													   "ABORT TRANSACTION",
  													   false))
  					{
  						/* Unable to abort remote transaction. */
  						abort_cleanup_failure = true;
  					}
  					else if (entry->have_prep_stmt && entry->have_error &&
  							 !pgfdw_exec_cleanup_query(entry->conn,
  													   "DEALLOCATE ALL",
  													   true))
  					{
  						/* Trouble clearing prepared statements. */
  						abort_cleanup_failure = true;
  					}
  					else
  					{
  						entry->have_prep_stmt = false;
  						entry->have_error = false;
  					}
  
  					/* Disarm changing_xact_state if it all worked. */
  					entry->changing_xact_state = abort_cleanup_failure;
  					break;
  			}
  		}
  
  		/* Reset state to show we're out of a transaction */
  		entry->xact_depth = 0;
  
  		/*
  		 * If the connection isn't in a good idle state or it is marked as
  		 * invalid, then discard it to recover. Next GetConnection will open a
  		 * new connection.
  		 */
  		if (PQstatus(entry->conn) != CONNECTION_OK ||
  			PQtransactionStatus(entry->conn) != PQTRANS_IDLE ||
  			entry->changing_xact_state ||
  			entry->invalidated)
  		{
  			elog(DEBUG3, "discarding connection %p", entry->conn);
  			disconnect_pg_server(entry);
  		}
  	}
  
  	/*
  	 * Regardless of the event type, we can now mark ourselves as out of the
  	 * transaction.  (Note: if we are here during PRE_COMMIT or PRE_PREPARE,
  	 * this saves a useless scan of the hashtable during COMMIT or PREPARE.)
  	 */
  	xact_got_connection = false;
  
  	/* Also reset cursor numbering for next transaction */
  	cursor_number = 0;
  }
  
  /*
   * pgfdw_subxact_callback --- cleanup at subtransaction end.
   */
  static void
  pgfdw_subxact_callback(SubXactEvent event, SubTransactionId mySubid,
  					   SubTransactionId parentSubid, void *arg)
  {
  	HASH_SEQ_STATUS scan;
  	ConnCacheEntry *entry;
  	int			curlevel;
  
  	/* Nothing to do at subxact start, nor after commit. */
  	if (!(event == SUBXACT_EVENT_PRE_COMMIT_SUB ||
  		  event == SUBXACT_EVENT_ABORT_SUB))
  		return;
  
  	/* Quick exit if no connections were touched in this transaction. */
  	if (!xact_got_connection)
  		return;
  
  	/*
  	 * Scan all connection cache entries to find open remote subtransactions
  	 * of the current level, and close them.
  	 */
  	curlevel = GetCurrentTransactionNestLevel();
  	hash_seq_init(&scan, ConnectionHash);
  	while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
  	{
  		char		sql[100];
  
  		/*
  		 * We only care about connections with open remote subtransactions of
  		 * the current level.
  		 */
  		if (entry->conn == NULL || entry->xact_depth < curlevel)
  			continue;
  
  		if (entry->xact_depth > curlevel)
  			elog(ERROR, "missed cleaning up remote subtransaction at level %d",
  				 entry->xact_depth);
  
  		if (event == SUBXACT_EVENT_PRE_COMMIT_SUB)
  		{
  			/*
  			 * If abort cleanup previously failed for this connection, we
  			 * can't issue any more commands against it.
  			 */
  			pgfdw_reject_incomplete_xact_state_change(entry);
  
  			/* Commit all remote subtransactions during pre-commit */
  			snprintf(sql, sizeof(sql), "RELEASE SAVEPOINT s%d", curlevel);
  			entry->changing_xact_state = true;
  			do_sql_command(entry->conn, sql);
  			entry->changing_xact_state = false;
  		}
  		else if (in_error_recursion_trouble())
  		{
  			/*
  			 * Don't try to clean up the connection if we're already in error
  			 * recursion trouble.
  			 */
  			entry->changing_xact_state = true;
  		}
  		else if (!entry->changing_xact_state)
  		{
  			bool		abort_cleanup_failure = false;
  
  			/* Remember that abort cleanup is in progress. */
  			entry->changing_xact_state = true;
  
  			/* Assume we might have lost track of prepared statements */
  			entry->have_error = true;
  
  			/*
  			 * If a command has been submitted to the remote server by using
  			 * an asynchronous execution function, the command might not have
  			 * yet completed.  Check to see if a command is still being
  			 * processed by the remote server, and if so, request cancellation
  			 * of the command.
  			 */
  			if (PQtransactionStatus(entry->conn) == PQTRANS_ACTIVE &&
  				!pgfdw_cancel_query(entry->conn))
  				abort_cleanup_failure = true;
  			else
  			{
  				/* Rollback all remote subtransactions during abort */
  				snprintf(sql, sizeof(sql),
  						 "ROLLBACK TO SAVEPOINT s%d; RELEASE SAVEPOINT s%d",
  						 curlevel, curlevel);
  				if (!pgfdw_exec_cleanup_query(entry->conn, sql, false))
  					abort_cleanup_failure = true;
  			}
  
  			/* Disarm changing_xact_state if it all worked. */
  			entry->changing_xact_state = abort_cleanup_failure;
  		}
  
  		/* OK, we're outta that level of subtransaction */
  		entry->xact_depth--;
  	}
  }
  
  /*
   * Connection invalidation callback function
   *
   * After a change to a pg_foreign_server or pg_user_mapping catalog entry,
   * close connections depending on that entry immediately if current transaction
   * has not used those connections yet. Otherwise, mark those connections as
   * invalid and then make pgfdw_xact_callback() close them at the end of current
   * transaction, since they cannot be closed in the midst of the transaction
   * using them. Closed connections will be remade at the next opportunity if
   * necessary.
   *
   * Although most cache invalidation callbacks blow away all the related stuff
   * regardless of the given hashvalue, connections are expensive enough that
   * it's worth trying to avoid that.
   *
   * NB: We could avoid unnecessary disconnection more strictly by examining
   * individual option values, but it seems too much effort for the gain.
   */
  static void
  pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue)
  {
  	HASH_SEQ_STATUS scan;
  	ConnCacheEntry *entry;
  
  	Assert(cacheid == FOREIGNSERVEROID || cacheid == USERMAPPINGOID);
  
  	/* ConnectionHash must exist already, if we're registered */
  	hash_seq_init(&scan, ConnectionHash);
  	while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
  	{
  		/* Ignore invalid entries */
  		if (entry->conn == NULL)
  			continue;
  
  		/* hashvalue == 0 means a cache reset, must clear all state */
  		if (hashvalue == 0 ||
  			(cacheid == FOREIGNSERVEROID &&
  			 entry->server_hashvalue == hashvalue) ||
  			(cacheid == USERMAPPINGOID &&
  			 entry->mapping_hashvalue == hashvalue))
  		{
  			/*
  			 * Close the connection immediately if it's not used yet in this
  			 * transaction. Otherwise mark it as invalid so that
  			 * pgfdw_xact_callback() can close it at the end of this
  			 * transaction.
  			 */
  			if (entry->xact_depth == 0)
  			{
  				elog(DEBUG3, "discarding connection %p", entry->conn);
  				disconnect_pg_server(entry);
  			}
  			else
  				entry->invalidated = true;
  		}
  	}
  }
  
  /*
   * Raise an error if the given connection cache entry is marked as being
   * in the middle of an xact state change.  This should be called at which no
   * such change is expected to be in progress; if one is found to be in
   * progress, it means that we aborted in the middle of a previous state change
   * and now don't know what the remote transaction state actually is.
   * Such connections can't safely be further used.  Re-establishing the
   * connection would change the snapshot and roll back any writes already
   * performed, so that's not an option, either. Thus, we must abort.
   */
  static void
  pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry)
  {
  	HeapTuple	tup;
  	Form_pg_user_mapping umform;
  	ForeignServer *server;
  
  	/* nothing to do for inactive entries and entries of sane state */
  	if (entry->conn == NULL || !entry->changing_xact_state)
  		return;
  
  	/* make sure this entry is inactive */
  	disconnect_pg_server(entry);
  
  	/* find server name to be shown in the message below */
  	tup = SearchSysCache1(USERMAPPINGOID,
  						  ObjectIdGetDatum(entry->key));
  	if (!HeapTupleIsValid(tup))
  		elog(ERROR, "cache lookup failed for user mapping %u", entry->key);
  	umform = (Form_pg_user_mapping) GETSTRUCT(tup);
  	server = GetForeignServer(umform->umserver);
  	ReleaseSysCache(tup);
  
  	ereport(ERROR,
  			(errcode(ERRCODE_CONNECTION_EXCEPTION),
  			 errmsg("connection to server \"%s\" was lost",
  					server->servername)));
  }
  
  /*
   * 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.
   */
  static bool
  pgfdw_cancel_query(PGconn *conn)
  {
  	PGcancel   *cancel;
  	char		errbuf[256];
  	PGresult   *result = NULL;
  	TimestampTz endtime;
  
  	/*
  	 * If it takes too long to cancel the query and discard the result, assume
  	 * the connection is dead.
  	 */
  	endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(), 30000);
  
  	/*
  	 * Issue cancel request.  Unfortunately, there's no good way to limit the
  	 * amount of time that we might block inside PQgetCancel().
  	 */
  	if ((cancel = PQgetCancel(conn)))
  	{
  		if (!PQcancel(cancel, errbuf, sizeof(errbuf)))
  		{
  			ereport(WARNING,
  					(errcode(ERRCODE_CONNECTION_FAILURE),
  					 errmsg("could not send cancel request: %s",
  							errbuf)));
  			PQfreeCancel(cancel);
  			return false;
  		}
  		PQfreeCancel(cancel);
  	}
  
  	/* Get and discard the result of the query. */
  	if (pgfdw_get_cleanup_result(conn, endtime, &result))
  		return false;
  	PQclear(result);
  
  	return true;
  }
  
  /*
   * Submit a query during (sub)abort cleanup and wait up to 30 seconds for the
   * result.  If the query is executed without error, the return value is true.
   * If the query is executed successfully but returns an error, the return
   * value is true if and only if ignore_errors is set.  If the query can't be
   * sent or times out, the return value is false.
   */
  static bool
  pgfdw_exec_cleanup_query(PGconn *conn, const char *query, bool ignore_errors)
  {
  	PGresult   *result = NULL;
  	TimestampTz endtime;
  
  	/*
  	 * If it takes too long to execute a cleanup query, assume the connection
  	 * is dead.  It's fairly likely that this is why we aborted in the first
  	 * place (e.g. statement timeout, user cancel), so the timeout shouldn't
  	 * be too long.
  	 */
  	endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(), 30000);
  
  	/*
  	 * 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.
  	 */
  	if (!PQsendQuery(conn, query))
  	{
  		pgfdw_report_error(WARNING, NULL, conn, false, query);
  		return false;
  	}
  
  	/* Get the result of the query. */
  	if (pgfdw_get_cleanup_result(conn, endtime, &result))
  		return false;
  
  	/* Issue a warning if not successful. */
  	if (PQresultStatus(result) != PGRES_COMMAND_OK)
  	{
  		pgfdw_report_error(WARNING, result, conn, true, query);
  		return ignore_errors;
  	}
  	PQclear(result);
  
  	return true;
  }
  
  /*
   * Get, during abort cleanup, the result of a query that is in progress.  This
   * might be a query that is being interrupted by transaction abort, or it might
   * be a query that was initiated as part of transaction abort to get the remote
   * side back to the appropriate state.
   *
   * It's not a huge problem if we throw an ERROR here, but if we get into error
   * recursion trouble, we'll end up slamming the connection shut, which will
   * necessitate failing the entire toplevel transaction even if subtransactions
   * were used.  Try to use WARNING where we can.
   *
   * endtime is the time at which we should give up and assume the remote
   * side is dead.  Returns true if the timeout expired, otherwise false.
   * Sets *result except in case of a timeout.
   */
  static bool
  pgfdw_get_cleanup_result(PGconn *conn, TimestampTz endtime, PGresult **result)
  {
  	volatile bool timed_out = false;
  	PGresult   *volatile last_res = NULL;
  
  	/* In what follows, do not leak any PGresults on an error. */
  	PG_TRY();
  	{
  		for (;;)
  		{
  			PGresult   *res;
  
  			while (PQisBusy(conn))
  			{
  				int			wc;
  				TimestampTz now = GetCurrentTimestamp();
  				long		cur_timeout;
  
  				/* If timeout has expired, give up, else get sleep time. */
  				cur_timeout = TimestampDifferenceMilliseconds(now, endtime);
  				if (cur_timeout <= 0)
  				{
  					timed_out = true;
  					goto exit;
  				}
  
  				/* 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, PG_WAIT_EXTENSION);
  				ResetLatch(MyLatch);
  
  				CHECK_FOR_INTERRUPTS();
  
  				/* Data available in socket? */
  				if (wc & WL_SOCKET_READABLE)
  				{
  					if (!PQconsumeInput(conn))
  					{
  						/* connection trouble; treat the same as a timeout */
  						timed_out = true;
  						goto exit;
  					}
  				}
  			}
  
  			res = PQgetResult(conn);
  			if (res == NULL)
  				break;			/* query is complete */
  
  			PQclear(last_res);
  			last_res = res;
  		}
  exit:	;
  	}
  	PG_CATCH();
  	{
  		PQclear(last_res);
  		PG_RE_THROW();
  	}
  	PG_END_TRY();
  
  	if (timed_out)
  		PQclear(last_res);
  	else
  		*result = last_res;
  	return timed_out;
  }
  