Re: [oauth] Stabilize the libpq-oauth ABI (and allow alternative implementations?)

Hello!

The patches look good, I don't have comments for them directly.

I also tried out a simple client-server plugin pair with them, both
built as external plugins, and it works.

I have some practical findings based on that:

1. If I provide an incorrect library path to psql, it suggests that I
should install the libpq-oauth package. It would be better to tell the
user that PGOAUTHMODULE is invalid in this case. For example:

PGOAUTHMODULE=/not/existing/path/to/oauth42_flow.so  bin/psql
"oauth_issuer=... oauth_client_id=..."
psql: error: connection to server at "127.0.0.1", port 5432 failed: no
OAuth flows are available (try installing the libpq-oauth package)

2. Even if I use a custom library (which doesn't default to the usual
get the discovery json - do a device flow  - etc flow), it tries to
construct/validate the well known discovery url. Why does it do that,
if it doesn't try to use the URL directly? The only validation done
inside libpq is now to validate that the URL matches the server URL.
The server doesn't do this validation at all, it accepts any string I
provide and starts up. If we want this URL validation, shouldn't it
happen in the server, and in the client side oauth plugin? (Not doing
it on the server but enforcing it on the client seems like a strange
choice, as typos/misconfigurations will need server restarts)

3. Is it really still OAuth, and not a generic pluggable SASLBEARER
authentication? Yes, I still have to provide the "oauth_issuer" and
"oauth_client_id" parameters, but I don't have to do anything with it.
I can implement any client side authentication I want in a libpq
plugin, as long as I am able to verify it on the server side by
sending a single token using SASL. That token doesn't have to be an
OAuth token, because I can change both the creation and validation
part. So why don't you call this SASLBEARER, with the default provided
implementation being OAuthBearer, that seems to be a better fitting
name for it? (this was already the case for custom applications with
PQsetAuthDataHook, and because of that I already wanted to ask this
question, but now I can also misuse it in psql/other command line
tools/existing applications/...)

4. Have you thought about parameter passing, if my custom plugin needs
extra configuration? Especially related to the above question, but
even in the scope of OIDC. For example there's my next 5th question.
Or if used by a webpage and it has to pass a callback URL to the
provider.

5. I was about to submit a separate patch we got some requests about:
a way to supply an OAuth token directly to psql (PGOAUTHTOKEN=...
bin/psql ...), by implementing PQsetAuthDataHook in psql/other command
line tools. (Multiple users asked for this) I realized that I could
possibly implement that using this new plugin API, and keep it outside
of the core, but from a user experience perspective simply using an
environment variable without custom plugins could be better. What
direction would you prefer with that? (and this is related to the
previous question with again the additional parameter - the custom
user provided token)

6. Still about PQsetAuthDataHook, this seems to have a limited use
case: if I am in the control of the application, writing it,
PQsetAuthDataHook seems to be a better choice. These plugins are
useful if I have to modify an existing generic application, to use a
different authentication method, and that seems to be useful,
especially if I treat it as a generic SASLBEARER API. But this also
goes back to the security questions I raised in another thread: could
an application say that it doesn't want to use plugins? Could it be
configured in another way, other than environment variables? (if an
application has a config file for example, it seems to be more
practical to make this plugin part of that config file, instead of
relying on a completely different environment variable)