Re: [HACKERS] Race in tablespace test on Windows
On Thu, Nov 13, 2014 at 8:46 AM, Noah Misch n...@leadboat.com wrote: On Tue, Nov 11, 2014 at 10:21:26AM +0530, Amit Kapila wrote: On Sat, Nov 8, 2014 at 10:34 AM, Noah Misch n...@leadboat.com wrote: Here is a briefer command sequence exhibiting the same problem: To make this work as well on Windows as it does elsewhere, DROP TABLESPACE would need to wait for other backends to close relevant unlinked files. Perhaps implement wait_unlinked_files(const char *dirname) to poll unlinked, open files until they disappear. (An attempt to open an unlinked file reports ERROR_ACCESS_DENIED. It might be tricky to reliably distinguish this cause from other causes of that error, but it should be possible.) I think the proposed mechanism can work but the wait can be very long (untill the backend holding descriptor executes another command). The DROP TABLESPACE could send a catchup interrupt. Yeah, that can work. Can we think of some other solution like in Drop Tablespace instead of checking if directory is empty, check if there is no object that belongs to database/cluster, then allow to forcibly delete that directory someway. I'm not aware of a way to forcibly delete the directory. One could rename files to the tablespace top-level directory just before unlinking them. Since DROP TABLESPACE never removes that directory, their continued presence there would not pose a problem. (Compare use of the rename-before-unlink trick in RemoveOldXlogFiles().) That adds the overhead of an additional system call to every unlink, which might be acceptable. It may be possible to rename after unlink, as-needed in DROP TABLESPACE. Right. I think we can discuss further about which approach is better, once someone decides to work on this issue. With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
Re: [HACKERS] Race in tablespace test on Windows
On Sat, Nov 8, 2014 at 10:34 AM, Noah Misch n...@leadboat.com wrote: In my Windows development environment, the tablespace regression test fails approximately half the time. Buildfarm member frogmouth failed in the same manner at least once: http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=frogmouthdt=2014-05-21%2014%3A30%3A01 Here is a briefer command sequence exhibiting the same problem: CREATE TABLESPACE testspace LOCATION '...somewhere...'; CREATE TABLE atable (c int) tablespace testspace; SELECT COUNT(*) FROM atable;-- open heap \c - ALTER TABLE atable SET TABLESPACE pg_default; DROP TABLESPACE testspace; -- bug: fails sometimes DROP TABLESPACE testspace; -- second one ~always works DROP TABLE atable; For me, it doesn't get success even second time, I am getting the same error until I execute some command on first session which means till first session has processed the invalidation messages. postgres=# Drop tablespace tbs; ERROR: tablespace tbs is not empty postgres=# Drop tablespace tbs; ERROR: tablespace tbs is not empty I have tested this on Windows 7. When we unlink an open file, Windows retains it in the directory structure until all processes close it. ALTER TABLE SET TABLESPACE sends invalidation messages prompting backends to do so. The backend running the ALTER TABLE always processes invalidations before processing another command. The other backend, the one serving commands before \c -, may have neither exited nor processed the invalidation. When it yet holds a file descriptor for atable, the DROP TABLESPACE fails. I suspect it's possible, though more difficult, to see like trouble in dbcommands.c users of RequestCheckpoint(CHECKPOINT_WAIT). To make this work as well on Windows as it does elsewhere, DROP TABLESPACE would need to wait for other backends to close relevant unlinked files. Perhaps implement wait_unlinked_files(const char *dirname) to poll unlinked, open files until they disappear. (An attempt to open an unlinked file reports ERROR_ACCESS_DENIED. It might be tricky to reliably distinguish this cause from other causes of that error, but it should be possible.) I think the proposed mechanism can work but the wait can be very long (untill the backend holding descriptor executes another command). Can we think of some other solution like in Drop Tablespace instead of checking if directory is empty, check if there is no object that belongs to database/cluster, then allow to forcibly delete that directory someway. I propose to add this as a TODO, then bandage the test case with s/^\\c -$/RESET ROLE;/. Yeah, this make sense. With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
[HACKERS] Race in tablespace test on Windows
In my Windows development environment, the tablespace regression test fails approximately half the time. Buildfarm member frogmouth failed in the same manner at least once: http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=frogmouthdt=2014-05-21%2014%3A30%3A01 Here is a briefer command sequence exhibiting the same problem: CREATE TABLESPACE testspace LOCATION '...somewhere...'; CREATE TABLE atable (c int) tablespace testspace; SELECT COUNT(*) FROM atable;-- open heap \c - ALTER TABLE atable SET TABLESPACE pg_default; DROP TABLESPACE testspace; -- bug: fails sometimes DROP TABLESPACE testspace; -- second one ~always works DROP TABLE atable; When we unlink an open file, Windows retains it in the directory structure until all processes close it. ALTER TABLE SET TABLESPACE sends invalidation messages prompting backends to do so. The backend running the ALTER TABLE always processes invalidations before processing another command. The other backend, the one serving commands before \c -, may have neither exited nor processed the invalidation. When it yet holds a file descriptor for atable, the DROP TABLESPACE fails. I suspect it's possible, though more difficult, to see like trouble in dbcommands.c users of RequestCheckpoint(CHECKPOINT_WAIT). To make this work as well on Windows as it does elsewhere, DROP TABLESPACE would need to wait for other backends to close relevant unlinked files. Perhaps implement wait_unlinked_files(const char *dirname) to poll unlinked, open files until they disappear. (An attempt to open an unlinked file reports ERROR_ACCESS_DENIED. It might be tricky to reliably distinguish this cause from other causes of that error, but it should be possible.) I propose to add this as a TODO, then bandage the test case with s/^\\c -$/RESET ROLE;/. That reduces the number of relevant backends to one, making the race irrelevant. Thanks, nm -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
