Re: Make query cancellation keys longer

This is a preliminary review. I'll look at this more closely soon.

On Thu, 29 Feb 2024 at 22:26, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> If the client requests the "_pq_.extended_query_cancel" protocol
> feature, the server will generate a longer 256-bit cancellation key.

Huge +1 for this general idea. This is a problem I ran into with
PgBouncer, and had to make some concessions when fitting the
information I wanted into the available bits. Also from a security
perspective I don't think the current amount of bits have stood the
test of time.

+ ADD_STARTUP_OPTION("_pq_.extended_query_cancel", "");

Since this parameter doesn't actually take a value (just empty
string). I think this behaviour makes more sense as a minor protocol
version bump instead of a parameter.

+ if (strcmp(conn->workBuffer.data, "_pq_.extended_query_cancel") == 0)
+ {
+ /* that's ok */
+ continue;
+ }

Please see this thread[1], which in the first few patches makes
pqGetNegotiateProtocolVersion3 actually usable for extending the
protocol. I started that, because very proposed protocol change that's
proposed on the list has similar changes to
pqGetNegotiateProtocolVersion3 and I think we shouldn't make these
changes ad-hoc hacked into the current code, but actually do them once
in a way that makes sense for all protocol changes.

> Documentation is still missing

I think at least protocol message type documentation would be very
helpful in reviewing, because that is really a core part of this
change. Based on the current code I think it should have a few
changes:

+ int cancel_key_len = 5 + msgLength - (conn->inCursor - conn->inStart);
+
+ conn->be_cancel_key = malloc(cancel_key_len);
+ if (conn->be_cancel_key == NULL)

This is using the message length to determine the length of the cancel
key in BackendKey. That is not something we generally do in the
protocol. It's even documented: "Notice that although each message
includes a byte count at the beginning, the message format is defined
so that the message end can be found without reference to the byte
count." So I think the patch should be changed to include the length
of the cancel key explicitly in the message.

[1]: https://www.postgresql.org/message-id/flat/CAGECzQSr2%3DJPJHNN06E_jTF2%2B0E60K%3DhotyBw5wY%3Dq9Wvmt7DQ%40mail.gmail.com#359e4222eb161da37124be1a384f8d92