Horton

Re: Make query cancellation keys longer

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Jelte Fennema-Nio <postgres@jeltef.nl>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2024-06-05T15:25:48Z
Lists: pgsql-hackers 
On Fri, Mar 8, 2024 at 5:20 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> The nice thing about relying on the message length was that we could
> just redefine the CancelRequest message to have a variable length
> secret, and old CancelRequest with 4-byte secret was compatible with the
> new definition too. But it doesn't matter much, so added an explicit
> length field.

I think I liked your original idea better than this one.

> One consequence of this patch that I didn't mention earlier is that it
> makes libpq incompatible with server version 9.2 and below, because the
> minor version negotiation was introduced in version 9.3. We could teach
> libpq to disconnect and reconnect with minor protocol version 3.0, if
> connecting with 3.1 fails, but IMHO it's better to not complicate this
> and accept the break in backwards-compatibility. 9.3 was released in
> 2013. We dropped pg_dump support for versions older than 9.2 a few years
> ago, this raises the bar for pg_dump to 9.3 as well.

I think we shouldn't underestimate the impact of bumping the minor
protocol version. Minor version negotiation is probably not supported
by all drivers and Jelte says that it's not supported by any poolers,
so for anybody but direct libpq users, there will be some breakage.
Now, on the one hand, as Jelte has said, there's little value in
having a protocol version if we're too afraid to make use of it. But
on the other hand, is this problem serious enough to justify the
breakage we'll cause? I'm not sure. It seems pretty silly to be using
a 32-bit value for this in 2024, but I'm sure some people aren't going
to like it, and they may not all have noticed this thread. On the
third hand, if we do this, it may help to unblock a bunch of other
pending patches that also want to do protocol-related things.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

  1. Add timingsafe_bcmp(), for constant-time memory comparison

  2. Add missing declarations to pg_config.h.in

  3. docs: Add a new section and a table listing protocol versions

  4. Make cancel request keys longer

  5. libpq: Add min/max_protocol_version connection options

  6. libpq: Handle NegotiateProtocolVersion message differently

  7. docs: Update phrase on message lengths in the protocol

  8. libpq: Trace all NegotiateProtocolVersion fields

  9. libpq: Add PQfullProtocolVersion to exports.txt

  10. Move cancel key generation to after forking the backend