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-To: Alfred Perlstein <bright@wintelcom.net>
-cc: pgsql-hackers@postgreSQL.org
-Subject: Re: pg_dump possible fix, need testers. (was: Re: [HACKERS] pg_dump disaster)
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-Comments: In-reply-to Alfred Perlstein <bright@wintelcom.net>
- message dated "Sun, 23 Jan 2000 02:23:41 -0800"
-Date: Sun, 23 Jan 2000 12:55:36 -0500
-Message-ID: <6208.948650136@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
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-
->> 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
-
-************
-
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- Wed, 24 Jan 2001 10:33:42 -0800 (PST)
-Date: Wed, 24 Jan 2001 10:33:42 -0800
-From: Alfred Perlstein <bright@wintelcom.net>
-To: Tom Lane <tgl@sss.pgh.pa.us>
-Cc: Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
-Subject: Re: [HACKERS] Libpq async issues
-Message-ID: <20010124103342.B26076@fw.wintelcom.net>
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-
-* Tom Lane <tgl@sss.pgh.pa.us> [010124 10:27] wrote:
-> Alfred Perlstein <bright@wintelcom.net> writes:
-> > * Bruce Momjian <pgman@candle.pha.pa.us> [010124 07:58] wrote:
-> >> I have added this email to TODO.detail and a mention in the TODO list.
->
-> > The bug mentioned here is long gone,
->
-> Au contraire, the misdesign is still there. The nonblock-mode code
-> will *never* be reliable under stress until something is done about
-> that, and that means fairly extensive code and API changes.
-
-The "bug" is the one mentioned in the first paragraph of the email
-where I broke _blocking_ connections for a short period.
-
-I still need to fix async connections for myself (and of course
-contribute it back), but I just haven't had the time. If anyone
-else wants it fixed earlier they can wait for me to do it, do it
-themself, contract me to do it or hope someone else comes along
-to fix it.
-
-I'm thinking that I'll do what you said and have seperate paths
-for writing/reading to the socket and API's to do so that give
-the user the option of a boundry, basically:
-
- buffer this, but don't allow me to write until it's flushed
-
-which would allow for larger than 8k COPY rows to go into the
-backend.
-
---
--Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
-"I have the heart of a child; I keep it in a jar on my desk."
-
projects / postgresql / commitdiff