Re: [HACKERS] Re: protocol version negotiation (Re: LibpqPGRES_COPY_BOTH - version compatibility)

On Thu, Oct 19, 2017 at 1:35 AM, Badrul Chowdhury <bachow@microsoft.com> wrote:
> The new functionality is for sending 64bit ints. I think 32bits is sufficient for the information we want to pass
aroundin the protocol negotiation phase, so I left this part unchanged.
 

No, it isn't.  That commit didn't add any new functionality, but it
changed the preferred interfaces for assembling protocol messages.
Your patch was still using the old ones.

Attached is an updated patch.  I've made a few modifications:

- I wrote documentation.  For future reference, that's really the job
of the patch submitter.

- I changed the message type byte for the new message from 'Y' to 'v'.
'Y' is apparently used by logical replication as a "type message", but
'v' is not currently used for anything.  It's also somewhat mnemonic.

- I deleted the minimum protocol version from the new message.  I know
there were a few votes for including it, but I think it's probably
useless.  The client should always request the newest version it can
support; if that's not new enough for the server, then we're dead
anyway and we might as well just handle that via ERROR. Moreover, it
seems questionable whether we'd ever deprecate 3.0 support in the
server anyway, or if we do, it'll probably be because 4.0 has been
stable for a decade or so.  Desupporting 3.0 while continuing to
support 3.x,x>0 seems like a remote outcome (and, like I say, if it
does happen, ERROR is a good-enough response).  If there's some use
case for having a client request an older protocol version than the
newest one it can support, then this change could be revisited, or we
can just handle that by retrying the whole connection attempt.

- I changed the test for whether to send NegotiateProtocolVersion to
send it only when the client requests a version too new for the
server.  I think that if the client requests a version older than what
the server could support, the server should just silently use the
older version.  That's arguable.  You could argue that the message
should be sent anyway (at least to post-3.0 clients) as a way of
reporting what happened with _pq_.<whatever> parameters, but I have
two counter-arguments.  Number one, maybe clients just shouldn't send
_pq_.<whatever> parameters that the protocol version they're using
doesn't support, or if they do, be prepared for them to have no
effect.  Number two, this is exactly the sort of thing that we can
change in future minor protocol versions if we wish.  For example, we
could define protocol version 3.1 or 3.43 or whatever as always
sending a NegotiateProtocolVersion message.  There's no need for the
code to make 3.0 compatible with future versions to decide what
choices those future versions might make.

- Made the prefix matching check for "_pq_." rather than "_pq_".  I
think we're imagining extensions like _pq_.magic_fairy_dust, not
_pq_magic_fairy_dust.

- I got rid of the optional_parameters thing in Port and just passed a
list of unrecognized parameters around directly.  Once some parameters
are recognized, e.g. _pq_.magic_fairy_dust = 1, we'll probably have
dedicated fields in the Port for them (i.e. int magic_fairy_dust)
rather than digging them out of some list.  Moreover, with your
design, we'd end up having to make NegotiateServerProtocol exclude
things that are actually recognized, which would be annoying.

- I got rid of the pq_flush().  There's no need for this because we're
always going to send another protocol message (ErrorResponse or
AuthenticationSomething) afterwards.

- I renamed NegotiateServerProtocol to SendNegotiateProtocolVersion
and moved it to what I think is a more sensible place in the file.

- I removed the error check for 2.x, x != 0.  I may have advocated for
this before, but on reflection, I don't see much advantage in
rejecting things that work today.

- I fixed the above-mentioned failure to use the new interface for
assembling the NegotiateProtocolVersion message.

- I did some work on the comments.

Barring objections, I plan to commit this and back-patch it all the
way.  Of course, this is not a bug fix, but Tom previously proposed
back-patching it and I think that's a good idea, because if we don't,
it will be a very long time before servers with this code become
commonplace in the wild.  Back-patching doesn't completely fix that
problem because not everybody applies upgrades and some people may be
running EOL versions, but it will still help.

Also attached is a patch I used for testing purposes, some version of
which we might eventually use when we actually introduce version 3.1
of the protocol.  It bumps the protocol version that libpq uses from
3.0 to 3.1 without changing what the server thinks the latest protocol
version is, so the server always replies with a
NegotiateProtocolVersion message.  It also teaches libpq to ignore the
NegotiateProtocolVersion message.  With that patch applied, make
check-world passes, which seems to show that the server-side changes
are not totally broken.  More testing would be great, of course.

Some thoughts on the future:

- libpq should grow an option to force a specific protocol version.
Andres already proposed one to force 2.0, but now we probably want to
generalize that to also allow forcing a particular minor version.
That seems useful for testing, if nothing else, and might let us add
TAP tests that this stuff works as intended.

- Possibly we should commit the portion of the testing patch which
ignores NegotiateProtocolVersion to v11, maybe also adding a
connection status function that lets users inquire about whether a
NegotiateProtocolVersion message was received and, if so, what
parameters it reported as unrecognized and what minor version it
reported the server as speaking.   The existing PQprotocolVersion
interface looks inadequate, as it seems to return only the major
version.

- On further reflection, I think the reconnect functionality you
proposed previously is probably a good idea.  It won't be necessary
with servers that have been patched to send NegotiateProtocolVersion,
but there may be quite a few that haven't for a long time to come, and
although an automated reconnect is a little annoying, it's a lot less
annoying than an outright connection failure.  So that part of your
patch should probably be resubmitted when and if we bump the version
to 3.1.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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