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

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Alfred Perlstein <bright(at)wintelcom(dot)net>, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: pg_dump possible fix, need testers. (was: Re: pg_dump disaster)
Date: 2000-09-30 02:29:49
Message-ID: 200009300229.WAA02718@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Can someone remind me of where we left this?


> >> Um, I didn't have any trouble at all reproducing Patrick's complaint.
> >> pg_dump any moderately large table (I used tenk1 from the regress
> >> database) and try to load the script with psql.  Kaboom.
> 
> > This is after or before my latest patch?
> 
> Before.  I haven't updated since yesterday...
> 
> > I can't seem to reproduce this problem,
> 
> Odd.  Maybe there is something different about the kernel's timing of
> message sending on your platform.  I see it very easily on HPUX 10.20,
> and Patrick sees it very easily on whatever he's using (netbsd I think).
> You might try varying the situation a little, say
> 	psql mydb <dumpfile
> 	psql -f dumpfile mydb
> 	psql mydb
> 		\i dumpfile
> and the same with -h localhost (to get a TCP/IP connection instead of
> Unix domain).  At the moment (pre-patch) I see failures with the
> first two of these, but not with the \i method.  -h doesn't seem to
> matter for me, but it might for you.
> 
> > Telling me something is wrong without giving suggestions on how
> > to fix it, nor direct pointers to where it fails doesn't help me
> > one bit.  You're not offering constructive critism, you're not
> > even offering valid critism, you're just waving your finger at
> > "problems" that you say exist but don't pin down to anything specific.
> 
> I have been explaining it as clearly as I could.  Let's try it
> one more time.
> 
> > I spent hours looking over what I did to pqFlush and pqPutnBytes
> > because of what you said earlier when all the bug seems to have
> > come down to is that I missed that the socket is set to non-blocking
> > in all cases now.
> 
> 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

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