mirror of https://github.com/postgres/postgres
Bruce Momjian
24 years ago
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From owner-pgsql-hackers@hub.org Sun Jan 23 13:31:03 2000 |
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Sun, 23 Jan 2000 12:55:36 -0500 (EST) |
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To: Alfred Perlstein <bright@wintelcom.net> |
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cc: pgsql-hackers@postgreSQL.org |
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Subject: Re: pg_dump possible fix, need testers. (was: Re: [HACKERS] pg_dump disaster) |
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In-reply-to: <20000123022341.J26520@fw.wintelcom.net> |
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References: <20000122211427.C26520@fw.wintelcom.net> <200001230525.AAA08020@candle.pha.pa.us> <20000122220256.H26520@fw.wintelcom.net> <5120.948606837@sss.pgh.pa.us> <20000123022341.J26520@fw.wintelcom.net> |
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Comments: In-reply-to Alfred Perlstein <bright@wintelcom.net> |
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message dated "Sun, 23 Jan 2000 02:23:41 -0800" |
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Date: Sun, 23 Jan 2000 12:55:36 -0500 |
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Message-ID: <6208.948650136@sss.pgh.pa.us> |
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From: Tom Lane <tgl@sss.pgh.pa.us> |
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Sender: owner-pgsql-hackers@postgreSQL.org |
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Status: ORr |
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|
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>> Um, I didn't have any trouble at all reproducing Patrick's complaint. |
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>> pg_dump any moderately large table (I used tenk1 from the regress |
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>> database) and try to load the script with psql. Kaboom. |
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> This is after or before my latest patch? |
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Before. I haven't updated since yesterday... |
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> I can't seem to reproduce this problem, |
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Odd. Maybe there is something different about the kernel's timing of |
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message sending on your platform. I see it very easily on HPUX 10.20, |
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and Patrick sees it very easily on whatever he's using (netbsd I think). |
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You might try varying the situation a little, say |
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psql mydb <dumpfile |
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psql -f dumpfile mydb |
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psql mydb |
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\i dumpfile |
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and the same with -h localhost (to get a TCP/IP connection instead of |
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Unix domain). At the moment (pre-patch) I see failures with the |
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first two of these, but not with the \i method. -h doesn't seem to |
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matter for me, but it might for you. |
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|
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> Telling me something is wrong without giving suggestions on how |
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> to fix it, nor direct pointers to where it fails doesn't help me |
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> one bit. You're not offering constructive critism, you're not |
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> even offering valid critism, you're just waving your finger at |
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> "problems" that you say exist but don't pin down to anything specific. |
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I have been explaining it as clearly as I could. Let's try it |
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one more time. |
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> I spent hours looking over what I did to pqFlush and pqPutnBytes |
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> because of what you said earlier when all the bug seems to have |
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> come down to is that I missed that the socket is set to non-blocking |
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> in all cases now. |
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|
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Letting the socket mode default to blocking will hide the problems from |
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existing clients that don't care about non-block mode. But people who |
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try to actually use the nonblock mode are going to see the same kinds of |
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problems that psql is exhibiting. |
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|
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> The old sequence of events that happened was as follows: |
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> user sends data almost filling the output buffer... |
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> user sends another line of text overflowing the buffer... |
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> pqFlush is invoked blocking the user until the output pipe clears... |
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> and repeat. |
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Right. |
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> The nonblocking code allows sends to fail so the user can abort |
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> sending stuff to the backend in order to process other work: |
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> user sends data almost filling the output buffer... |
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> user sends another line of text that may overflow the buffer... |
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> pqFlush is invoked, |
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> if the pipe can't be cleared an error is returned allowing the user to |
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> retry the send later. |
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> if the flush succeeds then more data is queued and success is returned |
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|
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But you haven't thought through the mechanics of the "error is returned |
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allowing the user to retry" code path clearly enough. Let's take |
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pqPutBytes for an example. If it returns EOF, is that a hard error or |
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does it just mean that the application needs to wait a while? The |
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application *must* distinguish these cases, or it will do the wrong |
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thing: for example, if it mistakes a hard error for "wait a while", |
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then it will wait forever without making any progress or producing |
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an error report. |
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You need to provide a different return convention that indicates |
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what happened, say |
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EOF (-1) => hard error (same as old code) |
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0 => OK |
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1 => no data was queued due to risk of blocking |
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And you need to guarantee that the application knows what the state is |
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when the can't-do-it-yet return is made; note that I specified "no data |
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was queued" above. If pqPutBytes might queue some of the data before |
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returning 1, the application is in trouble again. While you apparently |
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foresaw that in recoding pqPutBytes, your code doesn't actually work. |
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There is the minor code bug that you fail to update "avail" after the |
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first pqFlush call, and the much more fundamental problem that you |
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cannot guarantee to have queued all or none of the data. Think about |
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what happens if the passed nbytes is larger than the output buffer size. |
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You may pass the first pqFlush successfully, then get into the loop and |
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get a won't-block return from pqFlush in the loop. What then? |
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You can't simply refuse to support the case nbytes > bufsize at all, |
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because that will cause application failures as well (too long query |
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sends it into an infinite loop trying to queue data, most likely). |
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A possible answer is to specify that a return of +N means "N bytes |
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remain unqueued due to risk of blocking" (after having queued as much |
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as you could). This would put the onus on the caller to update his |
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pointers/counts properly; propagating that into all the internal uses |
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of pqPutBytes would be no fun. (Of course, so far you haven't updated |
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*any* of the internal callers to behave reasonably in case of a |
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won't-block return; PQfn is just one example.) |
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|
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Another possible answer is to preserve pqPutBytes' old API, "queue or |
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bust", by the expedient of enlarging the output buffer to hold whatever |
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we can't send immediately. This is probably more attractive, even |
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though a long query might suck up a lot of space that won't get |
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reclaimed as long as the connection lives. If you don't do this then |
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you are going to have to make a lot of ugly changes in the internal |
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callers to deal with won't-block returns. Actually, a bulk COPY IN |
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would probably be the worst case --- the app could easily load data into |
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the buffer far faster than it could be sent. It might be best to extend |
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PQputline to have a three-way return and add code there to limit the |
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growth of the output buffer, while allowing all internal callers to |
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assume that the buffer is expanded when they need it. |
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pqFlush has the same kind of interface design problem: the same EOF code |
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is returned for either a hard error or can't-flush-yet, but it would be |
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disastrous to treat those cases alike. You must provide a 3-way return |
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code. |
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Furthermore, the same sort of 3-way return code convention will have to |
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propagate out through anything that calls pqFlush (with corresponding |
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documentation updates). pqPutBytes can be made to hide a pqFlush won't- |
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block return by trying to enlarge the output buffer, but in most other |
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places you won't have a choice except to punt it back to the caller. |
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|
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PQendcopy has the same interface design problem. It used to be that |
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(unless you passed a null pointer) PQendcopy would *guarantee* that |
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the connection was no longer in COPY state on return --- by resetting |
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it, if necessary. So the return code was mainly informative; the |
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application didn't have to do anything different if PQendcopy reported |
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failure. But now, a nonblocking application does need to pay attention |
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to whether PQendcopy completed or not --- and you haven't provided a way |
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for it to tell. If 1 is returned, the connection might still be in |
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COPY state, or it might not (PQendcopy might have reset it). If the |
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application doesn't distinguish these cases then it will fail. |
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I also think that you want to take a hard look at the automatic "reset" |
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behavior upon COPY failure, since a PQreset call will block the |
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application until it finishes. Really, what is needed to close down a |
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COPY safely in nonblock mode is a pair of entry points along the line of |
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"PQendcopyStart" and "PQendcopyPoll", with API conventions similar to |
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PQresetStart/PQresetPoll. This gives you the ability to do the reset |
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(if one is necessary) without blocking the application. PQendcopy |
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itself will only be useful to blocking applications. |
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> I'm sorry if they don't work for some situations other than COPY IN, |
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> but it's functionality that I needed and I expect to be expanded on |
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> by myself and others that take interest in nonblocking operation. |
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I don't think that the nonblock code is anywhere near production quality |
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at this point. It may work for you, if you don't stress it too hard and |
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never have a communications failure; but I don't want to see us ship it |
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as part of Postgres unless these issues get addressed. |
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regards, tom lane |
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************ |
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|
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From pgsql-hackers-owner+M3768@postgresql.org Wed Jan 24 14:20:02 2001 |
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Wed, 24 Jan 2001 10:33:42 -0800 (PST) |
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Date: Wed, 24 Jan 2001 10:33:42 -0800 |
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From: Alfred Perlstein <bright@wintelcom.net> |
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To: Tom Lane <tgl@sss.pgh.pa.us> |
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Cc: Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org |
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Subject: Re: [HACKERS] Libpq async issues |
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Message-ID: <20010124103342.B26076@fw.wintelcom.net> |
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References: <6208.948650136@sss.pgh.pa.us> <200101241339.IAA11747@candle.pha.pa.us> <20010124084720.T26076@fw.wintelcom.net> <13021.980355551@sss.pgh.pa.us> |
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Status: OR |
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|
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* Tom Lane <tgl@sss.pgh.pa.us> [010124 10:27] wrote: |
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> Alfred Perlstein <bright@wintelcom.net> writes: |
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> > * Bruce Momjian <pgman@candle.pha.pa.us> [010124 07:58] wrote: |
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> >> I have added this email to TODO.detail and a mention in the TODO list. |
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> |
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> > The bug mentioned here is long gone, |
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> |
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> Au contraire, the misdesign is still there. The nonblock-mode code |
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> will *never* be reliable under stress until something is done about |
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> that, and that means fairly extensive code and API changes. |
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The "bug" is the one mentioned in the first paragraph of the email |
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where I broke _blocking_ connections for a short period. |
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I still need to fix async connections for myself (and of course |
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contribute it back), but I just haven't had the time. If anyone |
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else wants it fixed earlier they can wait for me to do it, do it |
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themself, contract me to do it or hope someone else comes along |
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to fix it. |
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I'm thinking that I'll do what you said and have seperate paths |
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for writing/reading to the socket and API's to do so that give |
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the user the option of a boundry, basically: |
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buffer this, but don't allow me to write until it's flushed |
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which would allow for larger than 8k COPY rows to go into the |
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backend. |
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-- |
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-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org] |
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"I have the heart of a child; I keep it in a jar on my desk." |
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