On 2020/10/02 0:46, Bharath Rupireddy wrote:
On Thu, Oct 1, 2020 at 8:10 PM Fujii Masao <masao.fu...@oss.nttdata.com> wrote:
pg_stat_clear_snapshot() can be used to reset the entry.
Thanks. I wasn't knowing it.
+ EXIT WHEN proccnt = 0;
+ END LOOP;
Isn't it better to sleep here, to avoid th busy loop?
+1.
So what I thought was something like
CREATE OR REPLACE PROCEDURE wait_for_backend_termination()
LANGUAGE plpgsql
AS $$
BEGIN
LOOP
PERFORM * FROM pg_stat_activity WHERE application_name =
'fdw_retry_check';
EXIT WHEN NOT FOUND;
PERFORM pg_sleep(1), pg_stat_clear_snapshot();
END LOOP;
END;
$$;
Changed.
Attaching v8 patch, please review it.. Both make check and make
check-world passes on v8.
Thanks for updating the patch! It basically looks good to me.
I tweaked the patch as follows.
+ if (!entry->conn ||
+ PQstatus(entry->conn) != CONNECTION_BAD ||
With the above change, if entry->conn is NULL, an error is thrown and no new
connection is reestablished. But why? IMO it's more natural to reestablish
new connection in that case. So I removed "!entry->conn" from the above
condition.
+ ereport(DEBUG3,
+ (errmsg("could not start remote transaction on
connection %p",
+ entry->conn)),
I replaced errmsg() with errmsg_internal() because the translation of
this debug message is not necessary.
+SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE
+backend_type = 'client backend' AND application_name = 'fdw_retry_check';
+CALL wait_for_backend_termination();
Since we always use pg_terminate_backend() and wait_for_backend_termination()
together, I merged them into one function.
I simplied the comments on the regression test.
Attached is the updated version of the patch. If this patch is ok,
I'd like to mark it as ready for committer.
I have another question not related to this patch: though we have
wait_pid() function, we are not able to use it like
pg_terminate_backend() in other modules, wouldn't be nice if we can
make it generic under the name pg_wait_pid() and usable across all pg
modules?
I thought that, too. But I could not come up with good idea for *real* use case
of that function. At least that's useful for the regression test, though.
Anyway, IMO it's worth proposing that and hearing more opinions about that
from other hackers.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
diff --git a/contrib/postgres_fdw/connection.c
b/contrib/postgres_fdw/connection.c
index 08daf26fdf..901d3a4661 100644
--- a/contrib/postgres_fdw/connection.c
+++ b/contrib/postgres_fdw/connection.c
@@ -108,6 +108,7 @@ PGconn *
GetConnection(UserMapping *user, bool will_prep_stmt)
{
bool found;
+ volatile bool retry_conn = false;
ConnCacheEntry *entry;
ConnCacheKey key;
@@ -159,23 +160,25 @@ GetConnection(UserMapping *user, bool will_prep_stmt)
/* Reject further use of connections which failed abort cleanup. */
pgfdw_reject_incomplete_xact_state_change(entry);
+retry:
/*
* If the connection needs to be remade due to invalidation, disconnect
as
- * soon as we're out of all transactions.
+ * soon as we're out of all transactions. Also, if previous attempt to
+ * start new remote transaction failed on the cached connection,
disconnect
+ * it to retry a new connection.
*/
- if (entry->conn != NULL && entry->invalidated && entry->xact_depth == 0)
+ if ((entry->conn != NULL && entry->invalidated &&
+ entry->xact_depth == 0) || retry_conn)
{
- elog(DEBUG3, "closing connection %p for option changes to take
effect",
- entry->conn);
+ if (retry_conn)
+ elog(DEBUG3, "closing connection %p to reestablish a
new one",
+ entry->conn);
+ else
+ 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
@@ -206,9 +209,35 @@ GetConnection(UserMapping *user, bool will_prep_stmt)
}
/*
- * Start a new transaction or subtransaction if needed.
+ * We check the health of the cached connection here when starting
+ * a new remote transaction. If broken connection is detected,
+ * we try to reestablish a new connection. If broken connection is
+ * detected again here, we give up getting a connection.
*/
- begin_remote_xact(entry);
+ PG_TRY();
+ {
+ /* Start a new transaction or subtransaction if needed. */
+ begin_remote_xact(entry);
+ retry_conn = false;
+ }
+ PG_CATCH();
+ {
+ if (PQstatus(entry->conn) != CONNECTION_BAD ||
+ entry->xact_depth > 0 ||
+ retry_conn)
+ PG_RE_THROW();
+ retry_conn = true;
+ }
+ PG_END_TRY();
+
+ if (retry_conn)
+ {
+ ereport(DEBUG3,
+ (errmsg_internal("could not start remote
transaction on connection %p",
+ entry->conn)),
+ errdetail_internal("%s",
pchomp(PQerrorMessage(entry->conn))));
+ goto retry;
+ }
/* Remember if caller will prepare statements */
entry->have_prep_stmt |= will_prep_stmt;
diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out
b/contrib/postgres_fdw/expected/postgres_fdw.out
index 10e23d02ed..2c5614073f 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -8987,3 +8987,51 @@ PREPARE TRANSACTION 'fdw_tpc';
ERROR: cannot PREPARE a transaction that has operated on postgres_fdw foreign
tables
ROLLBACK;
WARNING: there is no transaction in progress
+-- ===================================================================
+-- reestablish new connection
+-- ===================================================================
+-- Terminate the backend having the specified application_name and wait for
+-- the termination to complete.
+CREATE OR REPLACE PROCEDURE terminate_backend_and_wait(appname text) AS $$
+BEGIN
+ PERFORM pg_terminate_backend(pid) FROM pg_stat_activity
+ WHERE application_name = appname;
+ LOOP
+ PERFORM * FROM pg_stat_activity WHERE application_name = appname;
+ EXIT WHEN NOT FOUND;
+ PERFORM pg_sleep(1), pg_stat_clear_snapshot();
+ END LOOP;
+END;
+$$ LANGUAGE plpgsql;
+-- Change application_name of remote connection to special one
+-- so that we can easily terminate the connection later.
+ALTER SERVER loopback OPTIONS (application_name 'fdw_retry_check');
+SELECT 1 FROM ft1 LIMIT 1;
+ ?column?
+----------
+ 1
+(1 row)
+
+-- Terminate the remote connection.
+CALL terminate_backend_and_wait('fdw_retry_check');
+-- This query should detect the broken connection when starting new remote
+-- transaction, reestablish new connection, and then succeed.
+BEGIN;
+SELECT 1 FROM ft1 LIMIT 1;
+ ?column?
+----------
+ 1
+(1 row)
+
+-- If the query detects the broken connection when starting new remote
+-- subtransaction, it doesn't reestablish new connection and should fail.
+CALL terminate_backend_and_wait('fdw_retry_check');
+SAVEPOINT s;
+SELECT 1 FROM ft1 LIMIT 1; -- should fail
+ERROR: server closed the connection unexpectedly
+ This probably means the server terminated abnormally
+ before or while processing the request.
+CONTEXT: remote SQL command: SAVEPOINT s2
+COMMIT;
+-- Clean up
+DROP PROCEDURE terminate_backend_and_wait(text);
diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql
b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 78156d10b4..4da1f78956 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -2653,3 +2653,44 @@ SELECT count(*) FROM ft1;
-- error here
PREPARE TRANSACTION 'fdw_tpc';
ROLLBACK;
+
+-- ===================================================================
+-- reestablish new connection
+-- ===================================================================
+
+-- Terminate the backend having the specified application_name and wait for
+-- the termination to complete.
+CREATE OR REPLACE PROCEDURE terminate_backend_and_wait(appname text) AS $$
+BEGIN
+ PERFORM pg_terminate_backend(pid) FROM pg_stat_activity
+ WHERE application_name = appname;
+ LOOP
+ PERFORM * FROM pg_stat_activity WHERE application_name = appname;
+ EXIT WHEN NOT FOUND;
+ PERFORM pg_sleep(1), pg_stat_clear_snapshot();
+ END LOOP;
+END;
+$$ LANGUAGE plpgsql;
+
+-- Change application_name of remote connection to special one
+-- so that we can easily terminate the connection later.
+ALTER SERVER loopback OPTIONS (application_name 'fdw_retry_check');
+SELECT 1 FROM ft1 LIMIT 1;
+
+-- Terminate the remote connection.
+CALL terminate_backend_and_wait('fdw_retry_check');
+
+-- This query should detect the broken connection when starting new remote
+-- transaction, reestablish new connection, and then succeed.
+BEGIN;
+SELECT 1 FROM ft1 LIMIT 1;
+
+-- If the query detects the broken connection when starting new remote
+-- subtransaction, it doesn't reestablish new connection and should fail.
+CALL terminate_backend_and_wait('fdw_retry_check');
+SAVEPOINT s;
+SELECT 1 FROM ft1 LIMIT 1; -- should fail
+COMMIT;
+
+-- Clean up
+DROP PROCEDURE terminate_backend_and_wait(text);
