Allow self.alt host for self-requests

[itowlson]

This is a proposal to address spinframework/spin-js-sdk#298.

The issue there is that JS guests use the inbuilt fetch function to make HTTP requests. If fetch sees a URL without a host, it helpfully prepends the host it thinks it's running on before letting Spin see it. So for a self-request such as fetch("/back"), Spin sees http://localhost:3000/back or whatever. Which is not on the allow list so gets the banhammer.

This PR reluctantly compromises with fetch by allowing self-requests via the pseudo-host self.alt. So if you do fetch("http://self.alt/back"), Spin will treat it as a self-request: it will validate permission using the self-request permission and route it as a self request.

The developer experience is, admittedly, not very lovely. But it hopefully gives JS folks an escape hatch.

Notes for reviewers:

• Self-request permission is still given via allowed_outbound_hosts = ["http://self"], not http://self.alt. Should we allow self.alt as another way of expressing the same permission?
• I implemented this only in wasi-http because that's what JS goes through. Should I implement it in spin-http too? It looked faffier there because of the way the types line up, but certainly do-able.

[karthik2804]

@itowlson this overall looks good, an alternate suggestion that came up during the original PR adding this feature also seems to indicate that this will benefit go.
#1710 (comment)

[karthik2804]

I am okay with adding this to just the wasi-http.

[radu-matei]

Self-request permission is still given via allowed_outbound_hosts = ["http://self"], not http://self.alt/. Should we allow self.alt as another way of expressing the same permission?

Not a blocker; but allowing self.alt since it's what you use here makes sense to me.

I implemented this only in wasi-http because that's what JS goes through. Should I implement it in spin-http too? It looked faffier there because of the way the types line up, but certainly do-able.

This only being in WASI HTTP makes perfect sense to me.

LGTM-ing since this is a pretty big blocker for the JS SDK.

Thanks!

[lann]

nit: Commit message says spin.alt 🙂

[lann]

Self-request permission is still given via allowed_outbound_hosts = ["http://self"], not http://self.alt/. Should we allow self.alt as another way of expressing the same permission?

I would say yes, to reduce confusion.

[itowlson]

Added bonus host. Note that both self and self.alt will be accepted for either relative or self.alt requests.

Fixed commit message - thanks for the spot @lann!

[lann]

If we want to get this through quickly that's fine with me, but we really should have test coverage for this.

[itowlson]

If we want to get this through quickly that's fine with me, but we really should have test coverage for this.

Yep, agree. Note we can't run tests at the moment because the runner got lost in the repo move, but worth having for when the lights come back on...

[itowlson]

Added a test which Works On My Machine even if CI is packing a sad right now.