Re: query cancel issues in contrib/dblink

From: Itagaki Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
To: pgsql-hackers(at)postgresql(dot)org, Merlin Moncure <mmoncure(at)gmail(dot)com>
Subject: Re: query cancel issues in contrib/dblink
Date: 2009-06-29 03:42:16
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Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:

> Takahiro<itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> wrote:
> > contrib/dblink seems to have no treatments for query cancels.
> > (1) Users need to wait for completion of remote query.
> > (2) PGresult objects will be memory leak.

Here is a patch to fix the issues. I hope the fixes will be ported
to older versions if possible.

(1) is fixed by using non-blocking APIs in libpq. I think we should
always use non-blocking APIs even if the dblink function itself is
a blocking-function.

(2) is fixed by RegisterXactCallback(AtEOXact_dblink). However, there
might be any better solutions -- for example, ResourceOwner framework.

> > For (1), asynchronous libpq functions should be used instead of blocking
> > ones, and wait for the remote query using a loop with CHECK_FOR_INTERRUPTS().
> How would you structure this loop exactly?

Please check execute_query() and wait_for_result() in the patch.

ITAGAKI Takahiro
NTT Open Source Software Center

Attachment Content-Type Size
dblink.patch application/octet-stream 6.3 KB

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