Обсуждение: A couple of cosmetic changes around shared memory code
Hello, while investigating the shm_mq code and its testing module I made some cosmetic improvements there. You can see them in the attached diff file.
Вложения
On Tue, May 17, 2016 at 4:40 AM, Piotr Stefaniak <postgres@piotr-stefaniak.me> wrote: > while investigating the shm_mq code and its testing module I made some > cosmetic improvements there. You can see them in the attached diff file. - toc_bytes = offsetof(shm_toc, toc_entry) +nentry * sizeof(shm_toc_entry) + toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry) + allocated_bytes; I don't recall the exact reason, but this is intentional style (memories from a patchwork with Tom). See for example geo_ops.c or pl_funcs.c. Though it is true that things are not completely consistent in the code with offset. - seg = dsm_create(shm_toc_estimate(&e), 0); + seg = dsm_create(segsize, 0); Yep. - proc_exit(1); + proc_exit(0); Agreed here. I don't see why this should not exit with 0 if there have not been any errors. -- Michael
Michael Paquier <michael.paquier@gmail.com> writes:
> On Tue, May 17, 2016 at 4:40 AM, Piotr Stefaniak
> <postgres@piotr-stefaniak.me> wrote:
> - toc_bytes = offsetof(shm_toc, toc_entry) +nentry * sizeof(shm_toc_entry)
> + toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry)
> + allocated_bytes;
> I don't recall the exact reason, but this is intentional style
> (memories from a patchwork with Tom).
Well, it's not so much intentional as that pgindent will make it look like
that no matter what you do --- it's got some weird interaction with
sizeof, offsetof, and typedef names versus operators later on the same
line. I'd call that a pgindent bug myself, but have no particular desire
to try to fix it.
You could possibly make it look nicer by splitting into multiple lines,
but you'll need to try pgindent'ing to see if you've actually improved the
end result at all.
regards, tom lane
On 2016-05-17 19:05, Tom Lane wrote: > Michael Paquier <michael.paquier@gmail.com> writes: >> On Tue, May 17, 2016 at 4:40 AM, Piotr Stefaniak >> <postgres@piotr-stefaniak.me> wrote: >> - toc_bytes = offsetof(shm_toc, toc_entry) +nentry * sizeof(shm_toc_entry) >> + toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry) >> + allocated_bytes; > >> I don't recall the exact reason, but this is intentional style >> (memories from a patchwork with Tom). > > Well, it's not so much intentional as that pgindent will make it look like > that no matter what you do --- it's got some weird interaction with > sizeof, offsetof, and typedef names versus operators later on the same > line. I'd call that a pgindent bug myself, but have no particular desire > to try to fix it. From my understanding, it's because pg_bsd_indent thinks that "(shm_toc)" is a cast since shm_toc is a keyword (type alias fetched from the "list of typedefs" file in this case, to be precise), forcing the "+" to be a unary operator instead of binary. One easy way to work around this bug is putting shm_toc into its own parentheses but I'd prefer dropping this particular fix from my "cosmetic changes" patch until pg_bsd_indent is fixed. I may come up with a possible fix for this bug, but don't hold your breath.
On 2016-05-16 21:40, Piotr Stefaniak wrote: > Hello, > > while investigating the shm_mq code and its testing module I made some > cosmetic improvements there. You can see them in the attached diff file. Revised patch attached.
Вложения
On Sun, Jun 26, 2016 at 6:19 AM, Piotr Stefaniak <postgres@piotr-stefaniak.me> wrote: >> while investigating the shm_mq code and its testing module I made some >> cosmetic improvements there. You can see them in the attached diff file. > > Revised patch attached. The first hunk of this corrects an outdated comment, so we should certainly apply that. I'm not seeing what the value of the other bits is. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
On Tue, Jun 28, 2016 at 6:49 AM, Robert Haas <robertmhaas@gmail.com> wrote: > On Sun, Jun 26, 2016 at 6:19 AM, Piotr Stefaniak > <postgres@piotr-stefaniak.me> wrote: >>> while investigating the shm_mq code and its testing module I made some >>> cosmetic improvements there. You can see them in the attached diff file. >> >> Revised patch attached. > > The first hunk of this corrects an outdated comment, so we should > certainly apply that. I'm not seeing what the value of the other bits > is. - proc_exit(1); + proc_exit(0); Looking again at this thread with fresh eyes, isn't the origin of the confusion the fact that we do need to have a non-zero error code so as the worker is never restarted thanks to BGW_NEVER_RESTART? Even with that, it is a strange concept to leave with proc_exit(1) in the case where a worker left correctly.. -- Michael
On Mon, Jun 27, 2016 at 11:40 PM, Michael Paquier <michael.paquier@gmail.com> wrote: > On Tue, Jun 28, 2016 at 6:49 AM, Robert Haas <robertmhaas@gmail.com> wrote: >> On Sun, Jun 26, 2016 at 6:19 AM, Piotr Stefaniak >> <postgres@piotr-stefaniak.me> wrote: >>>> while investigating the shm_mq code and its testing module I made some >>>> cosmetic improvements there. You can see them in the attached diff file. >>> >>> Revised patch attached. >> >> The first hunk of this corrects an outdated comment, so we should >> certainly apply that. I'm not seeing what the value of the other bits >> is. > > - proc_exit(1); > + proc_exit(0); > Looking again at this thread with fresh eyes, isn't the origin of the > confusion the fact that we do need to have a non-zero error code so as > the worker is never restarted thanks to BGW_NEVER_RESTART? Even with > that, it is a strange concept to leave with proc_exit(1) in the case > where a worker left correctly.. This code predates be7558162acc5578d0b2cf0c8d4c76b6076ce352, prior to which proc_exit(0) forced an immediate, unconditional restart. It's true that, given that commit, changing this code to do proc_exit(0) instead of proc_exit(1) would be harmless. However, people writing background workers that might need to work with 9.3 would be best advised to stick with proc_exit(1). Therefore, I maintain that this is not broken and doesn't need to be fixed. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
On 2016-06-29 18:58, Robert Haas wrote: > This code predates be7558162acc5578d0b2cf0c8d4c76b6076ce352, prior to > which proc_exit(0) forced an immediate, unconditional restart. It's > true that, given that commit, changing this code to do proc_exit(0) > instead of proc_exit(1) would be harmless. However, people writing > background workers that might need to work with 9.3 would be best > advised to stick with proc_exit(1). Therefore, I maintain that this > is not broken and doesn't need to be fixed. Then I'd argue that it ought to be documented in form of a C comment for people writing background workers and for those who might want to "fix" this in the future.
On Wed, Jun 29, 2016 at 4:35 PM, Piotr Stefaniak <postgres@piotr-stefaniak.me> wrote: > On 2016-06-29 18:58, Robert Haas wrote: >> This code predates be7558162acc5578d0b2cf0c8d4c76b6076ce352, prior to >> which proc_exit(0) forced an immediate, unconditional restart. It's >> true that, given that commit, changing this code to do proc_exit(0) >> instead of proc_exit(1) would be harmless. However, people writing >> background workers that might need to work with 9.3 would be best >> advised to stick with proc_exit(1). Therefore, I maintain that this >> is not broken and doesn't need to be fixed. > > Then I'd argue that it ought to be documented in form of a C comment for > people writing background workers and for those who might want to "fix" this > in the future. Well, I suppose we could do that. Would we then add the same comment to worker_spi, which does the same thing for the same reason, and every future contrib module that does stuff with background workers which we might accept? It might be better to document this in bgworker.sgml instead. That already documents some facts about exiting: <para> If <structfield>bgw_restart_time</structfield> for a background worker is configured as <literal>BGW_NEVER_RESTART</>,or if it exits with an exit code of 0 or is terminated by <function>TerminateBackgroundWorker</>, it will be automatically unregistered by the postmaster on exit. Otherwise, it willbe restarted after the time period configured via <structfield>bgw_restart_time</>, or immediately if the postmaster reinitializes the cluster due to a backend failure. Backends which need to suspend execution only temporarilyshould use an interruptible sleep rather than exiting; this can be achieved by calling <function>WaitLatch()</function>.Make sure the <literal>WL_POSTMASTER_DEATH</> flag is set when calling that function, and verify the return code for a prompt exit in the emergency case that <command>postgres</> itself has terminated. </para> That paragraph leaves out a number of important details, like: 1. A background worker that exits with any exit code other than 0 or 1 will cause a postmaster crash-and-restart cycle. 2. Therefore, an exit of code 1 should be used whenever the process wants to be restarted in accordance with bgw_restart_time, and is therefore in some sense the "normal" way for a background worker to exit. 3. The aforementioned details about how 9.3 behavior was different from current behavior. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
Robert Haas wrote: > It might be better to document this in bgworker.sgml instead. That > already documents some facts about exiting: > > <para> > If <structfield>bgw_restart_time</structfield> for a background worker is > configured as <literal>BGW_NEVER_RESTART</>, or if it exits with an exit > code of 0 or is terminated by <function>TerminateBackgroundWorker</>, > it will be automatically unregistered by the postmaster on exit. > Otherwise, it will be restarted after the time period configured via > <structfield>bgw_restart_time</>, or immediately if the postmaster > reinitializes the cluster due to a backend failure. Backends which need > to suspend execution only temporarily should use an interruptible sleep > rather than exiting; this can be achieved by calling > <function>WaitLatch()</function>. Make sure the > <literal>WL_POSTMASTER_DEATH</> flag is set when calling that function, and > verify the return code for a prompt exit in the emergency case that > <command>postgres</> itself has terminated. > </para> > > That paragraph leaves out a number of important details, like: > > 1. A background worker that exits with any exit code other than 0 or 1 > will cause a postmaster crash-and-restart cycle. > > 2. Therefore, an exit of code 1 should be used whenever the process > wants to be restarted in accordance with bgw_restart_time, and is > therefore in some sense the "normal" way for a background worker to > exit. Yeah, I think bgworker.sgml is the right place to document these details. > 3. The aforementioned details about how 9.3 behavior was different > from current behavior. Not really sure about this. I think patching the 9.3 docs to state the details differently from the 9.4--master details would not be sufficient, as people moving code would not read the 9.4 docs again to ensure the semantics remain the same (and since 9.4 has been out for awhile, patching the release notes wouldn't suffice either). Patching the 9.3 docs to say "9.4 is different" would be odd, since 9.4 is "in the future" for the POV of 9.3 docs. I think the best option is to backpatch a doc change from 9.4 onwards stating what is the new behavior, and add a note stating that 9.3 was different. -- Álvaro Herrera http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services