connection.c

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/*-------------------------------------------------------------------------
 *
 * connection.c
 *        Connection management functions for postgres_fdw
 *
 * Portions Copyright (c) 2012-2016, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        contrib/postgres_fdw/connection.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "postgres_fdw.h"

#include "access/xact.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "storage/latch.h"
#include "utils/hsearch.h"
#include "utils/memutils.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 */
    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? */
} 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 check_conn_params(const char **keywords, const char **values);
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);


/*
 * 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.
 *
 * XXX Note that caching connections theoretically requires a mechanism to
 * detect change of FDW objects to invalidate already established connections.
 * We could manage that by watching for invalidation events on the relevant
 * syscaches.  For the moment, though, it's not clear that this would really
 * be useful and not mere pedantry.  We could not flush any active connections
 * mid-transaction anyway.
 */
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);
    }

    /* 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)
    {
        /* initialize new hashtable entry (key is already filled in) */
        entry->conn = NULL;
        entry->xact_depth = 0;
        entry->have_prep_stmt = false;
        entry->have_error = false;
    }

    /*
     * 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 be left in a valid empty state.)
     */
    if (entry->conn == NULL)
    {
        ForeignServer *server = GetForeignServer(user->serverid);

        entry->xact_depth = 0;  /* just to be sure */
        entry->have_prep_stmt = false;
        entry->have_error = false;
        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);

        conn = PQconnectdbParams(keywords, values, false);
        if (!conn || PQstatus(conn) != CONNECTION_OK)
        {
            char       *connmessage;
            int         msglen;

            /* libpq typically appends a newline, strip that */
            connmessage = pstrdup(PQerrorMessage(conn));
            msglen = strlen(connmessage);
            if (msglen > 0 && connmessage[msglen - 1] == '\n')
                connmessage[msglen - 1] = '\0';
            ereport(ERROR,
               (errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
                errmsg("could not connect to server \"%s\"",
                       server->servername),
                errdetail_internal("%s", connmessage)));
        }

        /*
         * 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() && !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;
}

/*
 * 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)
{
    int         i;

    /* no check required if superuser */
    if (superuser())
        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;

    res = PQexec(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";
        do_sql_command(entry->conn, sql);
        entry->xact_depth = 1;
    }

    /*
     * 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);
        do_sql_command(entry->conn, sql);
        entry->xact_depth++;
    }
}

/*
 * 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 the 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   *last_res = NULL;

    for (;;)
    {
        PGresult *res;

        while (PQisBusy(conn))
        {
            int     wc;

            /* Sleep until there's something to do */
            wc = WaitLatchOrSocket(MyLatch,
                                   WL_LATCH_SET | WL_SOCKET_READABLE,
                                   PQsocket(conn),
                                   -1L);
            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;
    }

    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 = PQerrorMessage(conn);

        ereport(elevel,
                (errcode(sqlstate),
                 message_primary ? errmsg_internal("%s", message_primary) :
                 errmsg("unknown 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.
 */
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)
        {
            elog(DEBUG3, "closing remote transaction on connection %p",
                 entry->conn);

            switch (event)
            {
                case XACT_EVENT_PARALLEL_PRE_COMMIT:
                case XACT_EVENT_PRE_COMMIT:
                    /* Commit all remote transactions during pre-commit */
                    do_sql_command(entry->conn, "COMMIT TRANSACTION");

                    /*
                     * 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 remote transactions that modified anything,
                     * 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 modified remote 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:
                    /* 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)
                    {
                        PGcancel   *cancel;
                        char        errbuf[256];

                        if ((cancel = PQgetCancel(entry->conn)))
                        {
                            if (!PQcancel(cancel, errbuf, sizeof(errbuf)))
                                ereport(WARNING,
                                        (errcode(ERRCODE_CONNECTION_FAILURE),
                                         errmsg("could not send cancel request: %s",
                                                errbuf)));
                            PQfreeCancel(cancel);
                        }
                    }

                    /* If we're aborting, abort all remote transactions too */
                    res = PQexec(entry->conn, "ABORT TRANSACTION");
                    /* Note: can't throw ERROR, it would be infinite loop */
                    if (PQresultStatus(res) != PGRES_COMMAND_OK)
                        pgfdw_report_error(WARNING, res, entry->conn, true,
                                           "ABORT TRANSACTION");
                    else
                    {
                        PQclear(res);
                        /* As above, make sure to clear any prepared stmts */
                        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;
            }
        }

        /* Reset state to show we're out of a transaction */
        entry->xact_depth = 0;

        /*
         * If the connection isn't in a good idle state, discard it to
         * recover. Next GetConnection will open a new connection.
         */
        if (PQstatus(entry->conn) != CONNECTION_OK ||
            PQtransactionStatus(entry->conn) != PQTRANS_IDLE)
        {
            elog(DEBUG3, "discarding connection %p", entry->conn);
            PQfinish(entry->conn);
            entry->conn = NULL;
        }
    }

    /*
     * 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)))
    {
        PGresult   *res;
        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)
        {
            /* Commit all remote subtransactions during pre-commit */
            snprintf(sql, sizeof(sql), "RELEASE SAVEPOINT s%d", curlevel);
            do_sql_command(entry->conn, sql);
        }
        else
        {
            /* 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)
            {
                PGcancel   *cancel;
                char        errbuf[256];

                if ((cancel = PQgetCancel(entry->conn)))
                {
                    if (!PQcancel(cancel, errbuf, sizeof(errbuf)))
                        ereport(WARNING,
                                (errcode(ERRCODE_CONNECTION_FAILURE),
                                 errmsg("could not send cancel request: %s",
                                        errbuf)));
                    PQfreeCancel(cancel);
                }
            }

            /* Rollback all remote subtransactions during abort */
            snprintf(sql, sizeof(sql),
                     "ROLLBACK TO SAVEPOINT s%d; RELEASE SAVEPOINT s%d",
                     curlevel, curlevel);
            res = PQexec(entry->conn, sql);
            if (PQresultStatus(res) != PGRES_COMMAND_OK)
                pgfdw_report_error(WARNING, res, entry->conn, true, sql);
            else
                PQclear(res);
        }

        /* OK, we're outta that level of subtransaction */
        entry->xact_depth--;
    }
}