Per-request timeout

[steve-chavez]

Problem

Currently we support setting a statement_timeout per role and per function, but we don't have a way to support it per table or view.

Solution

Send the timeout through an HTTP header.

Timeout: Second-2

The above is the Timeout header from WebDAV (ref), but it has WebDAV specific semantics so I don't think we can reuse it.

A saner option would be a Prefer header:

Prefer: handling=strict, timeout=2

Some drawbacks I can see is that this can be used to force failure by end users if the statement_timeout is set to a value too low (this in turn can pollute logs). For this maybe we can only allow seconds, instead of milliseconds. So only 1 second as minimum.

This timeout should also have a threshold, it cannot be too high. This threshold could be the role timeout or the function timeout.

[mkleczek]

I like this idea.

Besides handling timeout for relation queries, it also somewhat addresses the problem that setting timeout on functions makes them non-inlinable.

[steve-chavez]

it also somewhat addresses the problem that setting timeout on functions makes them non-inlinable.

True, that's relevant for Table-Valued functions + filters but not for scalar functions. We should add a note to https://docs.postgrest.org/en/v13/references/transactions.html#hoisted-function-settings mentioning that drawback.

[steve-chavez]

Looks ready for implementation. It's also possible that some cases won't need this functionality, so we should revisit #2987 afterwards.