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Re: Streaming replication and non-blocking I/O

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Streaming replication and non-blocking I/O
Date: 2010-01-14 10:04:24
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Fujii Masao wrote:
> On Wed, Jan 13, 2010 at 7:27 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>> the frontend always puts the
>> connection to non-blocking mode, while the backend uses blocking mode.
> Really? By default (i.e., without the expressly setting by using
> PQsetnonblocking()), the connection is set to blocking mode even
> in frontend. Am I missing something?

That's right. The underlying socket is always put to non-blocking mode
in libpq. PQsetnonblocking() only affects whether libpq commands wait
and retry if the output buffer is full.

>> At least with SSL, I think it's possible for pq_wait() to return false
>> positives, if the SSL layer decides to renegotiate the connection
>> causing data to flow in the other direction in the underlying TCP
>> connection. A false positive would lead cause walsender to block
>> indefinitely on the pq_getbyte() call.
> Sorry. I could not understand that issue scenario. Could you explain
> it in more detail?

1. Walsender calls pq_wait() which calls select(), waiting for timeout,
or data to become available for reading in the underlying socket.

2. Client issues an SSL renegotiation by sending a message to the server

3. Server receives the message, and select() returns indicating that
data has arrived

4. Walsender calls HandleEndOfRep() which calls pq_getbyte().
pq_readbyte() calls SSL_read(), which receives the renegotiation message
and handles it. No application data has arrived, however, so SSL_read()
blocks for some to arrive. It never does.

I don't understand enough of SSL to know if renegotiation can actually
happen like that, but the man page of SSL_read() suggests so. But a
similar thing can happen if an SSL record is broken into two TCP
packets. select() returns immediately as the first packet arrives, but
SSL_read() will block until the 2nd packet arrives.

  Heikki Linnakangas

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