THRIFT-5806: Count zero set fields on a nil union rather than dereferencing it

[slachiewicz]

JIRA: THRIFT-5806
Client: go

CountSetFields read the union's fields straight off the receiver, so a struct holding a nil union field panicked the moment it was serialized:

panic: runtime error: invalid memory address or nil pointer dereference

Returning 0 for a nil receiver lets the union's own arity check speak instead:

*Descendant error writing struct: *Descendant write union: exactly one field must be set (0 set)

Scope

This PR is only the nil guard: no generated write path changes, no wire bytes change.

Omitting an unset default-requiredness pointer field from the output, the way Python, Java, Node.js and C# omit it, is a wire change rather than a panic fix and is THRIFT-6204, #3827, which is stacked on this PR. The measurements of what that guard reaches are in that PR.

Tests

StructWithUnsetUnion in UnionDefaultValueTest.thrift plus TestNilUnion and TestStructWithUnsetUnion.

Verified: with the master generator TestNilUnion fails with panic: runtime error: invalid memory address or nil pointer dereference; with this change go test ./tests/ -count=1 in lib/go/test passes.

This change was created with AI assistance (Claude Opus 5).

[Jens-G]

Code review

Found 1 issue:

1. The commit has no AI attribution trailer, although the PR description says the change was created with AI assistance (AGENTS.md says "Always label AI-assisted commits and PRs. Use one or both of: Co-Authored-By: <AI tool name and version> / Generated-by: <AI tool name and version>"). The PR text names no tool or version either. The commits of Cap every build workflow job at 60 minutes #3816 and THRIFT-6200: Use the declaring package for arguments in the Go -remote stub #3817 carry such a trailer.

thrift/AGENTS.md

Lines 60 to 74 in 3eeb97b

	- **Always** label AI-assisted commits and PRs. Use one or both of:
	```
	Co-Authored-By: <AI tool name and version>
	Generated-by: <AI tool name and version>
	```
	Example:
	```
	THRIFT-9999: Fix connection timeout handling in Go client
	Client: go
	Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
	```
	- Apply this label even when AI only generated a portion of the change.
	- **Never** put tool-internal links into a commit message, PR description, JIRA comment or e-mail.

🤖 Generated with Claude Code

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[slachiewicz]

Rebased on master and added the Co-Authored-By: Claude Opus 5 trailer to the commit; the PR description now names the tool as well. Patch content is unchanged (8517fd8).

This comment was created with AI assistance.