THRIFT-6204: Omit an unset default-requiredness field from the Go writer

[slachiewicz]

JIRA: THRIFT-6204
Client: go

Stacked on #3822; the first commit is that PR. Review the second one.

Default requiredness means "write if set" (doc/specs/idl.md), and Python, Java, Node.js and C# all omit such a field when it holds no value. The Go writer wrote it unconditionally, so a nil pointer field either panicked on the way out — that half is #3822 — or, when its struct had no members, went onto the wire as an empty struct. Guarding the write with the generated IsSet helper, as optional fields already are, is a two-line change.

This is a wire change, and that is the whole question. Opening it as a draft because that is a maintainer's call, not mine.

What moves

A nil field whose struct has no members is written today and omitted after:

before:  0c 00 01 00 08 00 02 00 00 00 07 00    field 1 present, empty struct
after:         08 00 02 00 00 00 07 00          field 1 absent

A peer that declares that field required accepts the first and rejects the second:

before bytes -> err=<nil>
after bytes  -> err=Required field E is not set

Nothing panicked in that case, so this is working code changing behaviour, not a crash being fixed.

Blast radius

is_pointer_field() is true for every struct, union and exception field whatever its requiredness, and for every cpp.ref field, so the guard reaches wider than unions. Regenerating every IDL under test/, lib/go/test/ and tutorial/ with and without it:

Measure	Count
generated files changed	75 of 650
write sites newly guarded	227
changed files holding service Args/Result structs	59

The last row is the one to weigh: this reaches RPC argument and result encoding, not only user-declared structs.

How I would land it

Three options, and I have no stake in which:

1. As is, in a major, with a release note.
2. Behind a go: generator option defaulting off, so nobody's wire changes without asking.
3. Not at all, with the panic fixed in THRIFT-5806: Count zero set fields on a nil union rather than dereferencing it #3822 and the spec deviation documented.

Say which and I will rework it.

Verified: go test ./tests/ -count=1 in lib/go/test passes after regenerating gopath through the Makefile.am recipe. go build ./gopath/src/... reports only the two pre-existing includestest.Numberz errors that #3817 fixes.

CI

The two red cross-test jobs are inherited from master — d6782a87 turned the Python peer-certificate check on and 178 ip-ssl cases fail identically on every open PR, including a two-line .asf.yaml change. See #3818.

This change was created with AI assistance.