Re: Cygwin cleanup

On Thu, Jan 12, 2023 at 10:17:55PM -0600, Justin Pryzby wrote:
> On Thu, Jan 12, 2023 at 06:43:54PM -0800, Andres Freund wrote:
> > > It looks like logical decoding may be the "most wrong" place that
> > > wal_sync_method is being used, so maybe my change is reasonable to
> > > consider, and not just a workaround.
> >
> > I don't follow. What does using fsync_fname() vs fsync_fname_ext() have to do
> > with pg_fsync() using wal_sync_method? fsync_fname() is just a wrapper around
> > fsync_fname_ext(). Both end up in pg_fsync().
>
> My patch used fsync_fname_ext() which would cause an ERROR rather than a
> PANIC when failing to fsync the logical decoding pathname.
>
> > Are you actually proposing that we don't PANIC after an fsync for the category
> > of files that you list here, even with data_sync_retry set?
>
> Yes, but I'm referring only to my change to SnapBuildSerialize().
>
> The rest of the verbage was me trying to figure out the
> history/evolution of pg_fsync usage.

Also note the existing comment (originating from Thomas' "fsync-gate"
commit, which introduced data_sync_retry):

+ * It's safe to just ERROR on fsync() here because we'll retry the whole
+ * operation including the writes.

Also, it seems to work fine if one calls pg_fsync() again, rather than
calling fsync_fname(), which re-opens the file.

It also seems to work fine if one omits the initial call to
fsync_fname("pg_logical/snapshots", true);

Since SnapBuildSerialize() isn't atomic (the system could crash at any
point), I'm not seeing why these wouldn't be adequately safe. But also
hoping Thomas will comment on that.

--
Justin