Re: Add pg_file_sync() to adminpack

On Thu, Jan 9, 2020 at 7:31 AM Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
> On Mon, Jan 6, 2020 at 3:42 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> >
> > On Mon, Jan 06, 2020 at 03:20:13PM +0900, Arthur Zakirov wrote:
> > > It isn't case if a file doesn't exist. But if there are no permissions on
> > > the file:
> > >
> > > PANIC: could not open file "testfile": Permissions denied
> > > server closed the connection unexpectedly
> > >
> > > It could be fixed by implementing a function like pg_file_sync_internal() or
> > > by making the function fsync_fname_ext() external.
> >
> > The patch uses stat() to make sure that the file exists and has no
> > issues. Though it could be a problem with any kind of TOCTOU-like
> > issues (looking at you, Windows, for ENOPERM), so I agree that it
> > would make more sense to use pg_fsync() here with a fd opened first.
>
> I agree that it's not good for pg_file_sync() to cause a PANIC.
> I updated the patch so that pg_file_sync() uses fsync_fname_ext()
> instead of fsync_fname() as Arthur suggested.
>
> It's one of ideas to make pg_file_sync() open the file and directly call
> pg_fsync(). But fsync_fname_ext() has already such code and
> I'd like to avoid the code duplication.

This looks good to me.

> Attached is the updated version of the patch.

+ <row>
+ <entry><function>pg_catalog.pg_file_sync(filename text)</function></entry>
+ <entry><type>boolean</type></entry>
+ <entry>
+ Sync a file or directory
+ </entry>
+ </row>

"Flush to disk" looks better than "sync" here.

+/* ------------------------------------
+ * pg_file_sync
+ *
+ * We REVOKE EXECUTE on the function from PUBLIC.
+ * Users can then grant access to it based on their policies.
+ */

I think that pg_write_server_files should be allowed to call that
function by default.