x/pkgsite: update codec generation for ast changes

[findleyr]

In https://go.dev/issue/76031, we added a new field to ast.BasicLit. Meanwhile, pkgsite encodes the AST in binary.

The new field therefore either breaks pkgsite encoding of package source (see internal/unit.Documentation.Source), or makes our codec generation stale (running go generate ./internal/godoc/... results in a diff adding the encoding/decoding of the new field (here)

Many questions to answer:

• How does changing our encoding version work?
• Can we update the codec without regenerating all documentation (a rather expensive endeavor at this point).
• Should we punt on updating the codec, and add a mechanism in generation that skips this field? The new field is ValueEnd, and if it's 0 it degenerates to the old behavior.
• What do we do in the future if there was a nontrivial change to the AST that warranted a new encoding version?

@ethanalee-work would you mind looking into this? I don't think it's urgent (I'll fix the failing test), but we should at the very least update pkgsite so that go generate ./... is clean. Ideally, we'd also brainstorm here what to actually do if we need to update the encoding. (Apologies if this is already documented somewhere and I missed it).

CC @jba @nicholashusin @adonovan

[ethanalee-work]

Sure, I will assign it to myself.

[gopherbot]

Change https://go.dev/cl/721800 mentions this issue: internal/godoc: fix codec generation test

[jba]

Gosh, I hope I designed the codec to handle non-breaking changes like this... will look tomorrow.

[findleyr]

@jba to be clear I don't know whether you did or not, but didn't have time to check: I wanted to file this issue before I fixed the broken test.

And even if you didn't, we'll figure it out.

[jba]

This is my solution to adding fields: https://github.com/golang/pkgsite/blob/master/internal/godoc/codec/doc.go#L65-L77

Untested in the field (n.p.i). I hope it works...

[findleyr]

@jba nice. Thanks for looking that up.

@ethanalee-work we should probably write a unit test for this specific case and then go generate ./internal/godoc/... and ensure the test passes. I.e. test that some fixed byte slice still decodes to an equivalent BasicLit.

[gopherbot]

Change https://go.dev/cl/727201 mentions this issue: internal/godoc: add unit test for TestDecodeBasicLit