Re: pg_dump possible fix, need testers. (was: Re: pg_dump disaster)

* Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> [000929 19:32] wrote:
> Can someone remind me of where we left this?

[snip]

Sorry for the really long follow-up, but this was posted a long time
ago and I wanted to refresh everyone's memory as to the problem.

The problem is that when doing a COPY from stdin using libpq and
my non-blocking query settings there is a big problem, namely that
the size of the query is limited to the size of the output buffer.

The reason is that I couldn't figure any way to determine what the
application was doing, basically, if i'm sending a query I want to
expand the output buffer to accomidate the new query, however if
I'm doing a COPY out, I would like to be informed if the backend
is lagging behind.

The concept is that someone doing queries needs to be retrieving
results from the database backend between queries, however someone
doing a COPY doesn't until they issue the terminating \\. line.

I'm pretty sure I know what to do now, it's pretty simple actually,
I can examine the state of the connection, if it's in PGASYNC_COPY_IN
then I don't grow the buffer, I inform the application that the
data will block, if it's no PGASYNC_COPY_IN I allow the buffer to grow
protecting the application from blocking.

Tom, I'd like to know what do you think before running off to implement
this.

thanks,
-Alfred

> > Letting the socket mode default to blocking will hide the problems from
> > existing clients that don't care about non-block mode. But people who
> > try to actually use the nonblock mode are going to see the same kinds of
> > problems that psql is exhibiting.
> >
> > > The old sequence of events that happened was as follows:
> >
> > > user sends data almost filling the output buffer...
> > > user sends another line of text overflowing the buffer...
> > > pqFlush is invoked blocking the user until the output pipe clears...
> > > and repeat.
> >
> > Right.
> >
> > > The nonblocking code allows sends to fail so the user can abort
> > > sending stuff to the backend in order to process other work:
> >
> > > user sends data almost filling the output buffer...
> > > user sends another line of text that may overflow the buffer...
> > > pqFlush is invoked,
> > > if the pipe can't be cleared an error is returned allowing the user to
> > > retry the send later.
> > > if the flush succeeds then more data is queued and success is returned
> >
> > But you haven't thought through the mechanics of the "error is returned
> > allowing the user to retry" code path clearly enough. Let's take
> > pqPutBytes for an example. If it returns EOF, is that a hard error or
> > does it just mean that the application needs to wait a while? The
> > application *must* distinguish these cases, or it will do the wrong
> > thing: for example, if it mistakes a hard error for "wait a while",
> > then it will wait forever without making any progress or producing
> > an error report.
> >
> > You need to provide a different return convention that indicates
> > what happened, say
> > EOF (-1) => hard error (same as old code)
> > 0 => OK
> > 1 => no data was queued due to risk of blocking
> > And you need to guarantee that the application knows what the state is
> > when the can't-do-it-yet return is made; note that I specified "no data
> > was queued" above. If pqPutBytes might queue some of the data before
> > returning 1, the application is in trouble again. While you apparently
> > foresaw that in recoding pqPutBytes, your code doesn't actually work.
> > There is the minor code bug that you fail to update "avail" after the
> > first pqFlush call, and the much more fundamental problem that you
> > cannot guarantee to have queued all or none of the data. Think about
> > what happens if the passed nbytes is larger than the output buffer size.
> > You may pass the first pqFlush successfully, then get into the loop and
> > get a won't-block return from pqFlush in the loop. What then?
> > You can't simply refuse to support the case nbytes > bufsize at all,
> > because that will cause application failures as well (too long query
> > sends it into an infinite loop trying to queue data, most likely).
> >
> > A possible answer is to specify that a return of +N means "N bytes
> > remain unqueued due to risk of blocking" (after having queued as much
> > as you could). This would put the onus on the caller to update his
> > pointers/counts properly; propagating that into all the internal uses
> > of pqPutBytes would be no fun. (Of course, so far you haven't updated
> > *any* of the internal callers to behave reasonably in case of a
> > won't-block return; PQfn is just one example.)
> >
> > Another possible answer is to preserve pqPutBytes' old API, "queue or
> > bust", by the expedient of enlarging the output buffer to hold whatever
> > we can't send immediately. This is probably more attractive, even
> > though a long query might suck up a lot of space that won't get
> > reclaimed as long as the connection lives. If you don't do this then
> > you are going to have to make a lot of ugly changes in the internal
> > callers to deal with won't-block returns. Actually, a bulk COPY IN
> > would probably be the worst case --- the app could easily load data into
> > the buffer far faster than it could be sent. It might be best to extend
> > PQputline to have a three-way return and add code there to limit the
> > growth of the output buffer, while allowing all internal callers to
> > assume that the buffer is expanded when they need it.
> >
> > pqFlush has the same kind of interface design problem: the same EOF code
> > is returned for either a hard error or can't-flush-yet, but it would be
> > disastrous to treat those cases alike. You must provide a 3-way return
> > code.
> >
> > Furthermore, the same sort of 3-way return code convention will have to
> > propagate out through anything that calls pqFlush (with corresponding
> > documentation updates). pqPutBytes can be made to hide a pqFlush won't-
> > block return by trying to enlarge the output buffer, but in most other
> > places you won't have a choice except to punt it back to the caller.
> >
> > PQendcopy has the same interface design problem. It used to be that
> > (unless you passed a null pointer) PQendcopy would *guarantee* that
> > the connection was no longer in COPY state on return --- by resetting
> > it, if necessary. So the return code was mainly informative; the
> > application didn't have to do anything different if PQendcopy reported
> > failure. But now, a nonblocking application does need to pay attention
> > to whether PQendcopy completed or not --- and you haven't provided a way
> > for it to tell. If 1 is returned, the connection might still be in
> > COPY state, or it might not (PQendcopy might have reset it). If the
> > application doesn't distinguish these cases then it will fail.
> >
> > I also think that you want to take a hard look at the automatic "reset"
> > behavior upon COPY failure, since a PQreset call will block the
> > application until it finishes. Really, what is needed to close down a
> > COPY safely in nonblock mode is a pair of entry points along the line of
> > "PQendcopyStart" and "PQendcopyPoll", with API conventions similar to
> > PQresetStart/PQresetPoll. This gives you the ability to do the reset
> > (if one is necessary) without blocking the application. PQendcopy
> > itself will only be useful to blocking applications.
> >
> > > I'm sorry if they don't work for some situations other than COPY IN,
> > > but it's functionality that I needed and I expect to be expanded on
> > > by myself and others that take interest in nonblocking operation.
> >
> > I don't think that the nonblock code is anywhere near production quality
> > at this point. It may work for you, if you don't stress it too hard and
> > never have a communications failure; but I don't want to see us ship it
> > as part of Postgres unless these issues get addressed.
> >
> > regards, tom lane
> >
> > ************
> >
>
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
> + If your life is a hard drive, | 830 Blythe Avenue
> + Christ can be your backup. | Drexel Hill, Pennsylvania 19026

--
-Alfred Perlstein - [bright(at)wintelcom(dot)net|alfred(at)freebsd(dot)org]
"I have the heart of a child; I keep it in a jar on my desk."