Go: extract and expose struct tags, interface method IDs

[smowton]

This enables us to distinguish all database types in QL. Previously structs with the same field names and types but differing tags, and interface types with matching method names and at least one non-exported method but declared in differing packages, were impossible or only sometimes possible to distinguish in QL. With this change these types can be
distinguished, as well as permitting queries to examine struct field tags, e.g. to read JSON field name associations.

This is a pre-requisite to (some approaches to) dealing with Go 1.23's more direct exposure of type aliases, since it enables us to distinguish all types that are distinct in the database in QL, and therefore implement up-to-aliasing type matching, known in the Go spec as identical types.

[owen-mc]

Longer review to follow.

[mbg]

Broadly looks good! Thank you for improving this and moving it out into it's own PR. I just have a few suggestions in addition to @owen-mc's comments, which also make sense.

Also, to sanity check: in the PR description you discuss that part of the motivation here is to be able to distinguish types better. That makes sense and I found the relevant part of the Go specification for this in https://go.dev/ref/spec#Type_identity. For structs:

Two struct types are identical if they have the same sequence of fields, and if corresponding fields have the same names, and identical types, and identical tags. Non-exported field names from different packages are always different.

For interfaces:

Two interface types are identical if they define the same type set.

Looking over the tests here, I can see that the tests exercise the new functionality and that seems to behave as expected. Do the tests cover the new ability to decide (in)equality that you are hoping for? Could you comment on how the tests cover that?

[owen-mc]

Tests failing:

The following files need to be reformatted using gofmt or have compilation errors:
./ql/test/library-tests/semmle/go/Types/pkg2/tst.go
Error: make: *** [Makefile:15: check-formatting] Error 1
./ql/test/library-tests/semmle/go/Types/struct_tags.go
Error: Process completed with exit code 2.

[smowton]

Retargeted this against main because we're currently not expecting to need this for rc/3.15 if we go for a simpler alias-erasing approach in the interim

[owen-mc]

Good work spotting these problems and fixing them. A few small suggestions for improvement.

Also, shouldn't the label for struct types include the tag of each field? Since differing tags make it a different struct type? Ideally this would have a test as well. This could be done as a follow-up, but it also fits in pretty naturally with this PR.

[smowton]

Note: at #17341 I added a stats update since there are new db tables and they ought to have associated stats. This evidently caused join-order problems since DCA flipped from just fine to catastrophic, so join order fixery (or simply accepting missing stats -- I note C# recently totally removed them and simply manually hacked their join orders where necessary) will be necessary before this can be merged.

[smowton]

@mbg @owen-mc all comments applied.

I have also taken the liberty of resurrecting #9386 and including it here since we have a dbscheme change afoot anyway.

[smowton]

Oh, except, @mbg no there is no direct test of distinguishing two types using these functions -- there isn't a direct identical-type predicate in this PR to test, and @owen-mc getTypeLabel already adds struct-type tags into the label, hence how before this PR two structs that differed only in their tags would be different QL entities, but there would be no QL predicate that could tell them apart.

[owen-mc]

A few optional suggestions and one change I feel more strongly about.

[owen-mc]

[Optional] I think this example would be a bit clearer without the exported method.

Suggested change

	* For example, `interface { Exported() int; notExported() int }` declared in two
	* different packages defines two distinct types, but they appear identical according to
	* `getMethodType`. If the packages were named `a` and `b`, `getMethodType` would yield
	* `notExported -> int` for both, whereas this method would yield `a.notExported -> int`
	* and `b.notExported -> int` respectively.
	* For example, `interface { notExported() int }` declared in two different packages
	* defines two distinct types, but they appear identical according to `getMethodType`.
	* If the packages were named `a` and `b`, `getMethodType` would yield
	* `notExported -> int` for both, whereas this method would yield `a.notExported -> int`
	* and `b.notExported -> int` respectively.

[smowton]

I've added a mention of Exported instead.

[smowton]

(I'll hold off on merging this for now since there are still moderate performance issues brought about by the stats update)

[smowton]

DCA is now pretty good after the latest wave. Considering all the join-order tweaks needed based on the new stats, I'll do a QA run to get a larger performance sample.

[smowton]

QA results were broadly very strong -- analysis time reductions were much more common than increases -- but I've debugged a few of the notably somewhat-slower projects and made one last DCA run to retest the usual suite + those projects showing a slowdown on QA.

[mbg]

Happy with this for when you're happy with the performance results