Re: PQcancel may hang in the recv call

On Thu, May 19, 2016 at 10:37 AM, Peter Juhasz <pjuhasz(at)uhusystems(dot)com>
wrote:

> Hi all,
>
> this is somewhat involved so please bear with me.
>
> We've found a situation where canceling a query may cause the client to
> hang, possibly indefinitely. This can happen if the network connection
> fails in a specific way.
>
> The reason for this lies in the way the PQcancel function (which
> eventually gets called from the higher level interface's cancel
> function) is implemented. It works by opening a second connection to
> the postmaster (on the same host/port as the existing connection),
> send()-ing a cancellation message via the newly opened connection, then
> calling recv() to receive an indication that the message was processed.
>
> However, if the network fails in a way that the connection appears to
> have been established but subsequent packages are dropped silently,
> this recv() call will block.
>
> My questions:
>
> Is this known?
> Is this a bug?
> What can be done to fix or work around it, apart from applying a
> timeout wrapper the cancel operation as well?
>
>
​It does sound familiar. Providing the version number(s) on which you
encountered this behavior would be helpful. Or HEAD if you have or are
testing against current code.

David J.