Re: .ready and .done files considered harmful

Thanks for the feedback.

> + StatusFilePath(archiveStatusPath, xlog, ".ready");
> + if (stat(archiveStatusPath, &stat_buf) == 0)
> + PgArchEnableDirScan();

> We may want to call PgArchWakeup() after setting the flag.

Yes, added a call to wake up archiver.

> > + * - The next anticipated log segment is not available.
> >
> > I wonder if we really need to perform a directory scan in this case.
> > Unless there are other cases where the .ready files are created out of
> > order, I think this just causes an unnecessary directory scan every
> > time the archiver catches up.

> Thinking further, I suppose this is necessary for when lastSegNo gets
> reset after processing an out-of-order .ready file.

Also, this is necessary when lastTLI gets reset after switching to a new
timeline.

> + pg_atomic_flag dirScan;

> I personally don't think it's necessary to use an atomic here. A
> spinlock or LWLock would probably work just fine, as contention seems
> unlikely. If we use a lock, we also don't have to worry about memory
> barriers.

History file should be archived as soon as it gets created. The atomic flag
here will make sure that there is no reordering of read/write instructions
while
accessing the flag in shared memory. Archiver needs to read this flag at
the
beginning of each cycle. Write to atomic flag is synchronized and it
provides
a lockless read. I think an atomic flag here is an efficient choice unless
I am
missing something.

Please find the attached patch v7.

Thanks,
Dipesh