go test parallel subtests data race: t.Parallel fix

TL;DR

Parallel subtests race when they read a shared loop variable or write shared state. The classic bug: the loop variable is reused across iterations, so every parallel subtest sees the last value. Fix it by copying the loop variable into a new local inside the loop body before calling t.Parallel(), and give each subtest its own data instead of a shared map or struct. Always develop table tests with go test -race on.

The query

go test parallel subtests data race: t.Parallel fix

Use this when

• go test -race flags a race inside a table-driven test.
• Parallel subtests see wrong or flapping input values.

Not for

• Data races in non-test production code.
• Slow tests; this is about correctness, not speed.

Steps

1. Run go test -race -run TestName ./... to reproduce. Expected output: the race detector prints the racing goroutines and stack traces.
2. Find the shared access: usually the loop variable or a shared map/struct written by subtests. Expected output: the report points at the shared value.
3. Copy the loop variable into a fresh local at the top of the loop body (tc := tc) before the subtest closure. Expected output: each subtest closes over its own copy.
4. Move any shared mutable state into per-subtest locals or guard it with a mutex. Expected output: no cross-subtest writes remain.
5. Re-run with -race several times (-count=5). Expected output: clean runs, no race reports.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_VsqEuze1gOlJ6FfX24RIhg