cmd/go: gccCompilerID cache validation always fails due to a trailing NUL field #81553

Go version

go version go1.27.1 darwin/arm64 (also present on tip)

Output of go env in your module/workspace:

CC='cc'
CGO_ENABLED='1'
GOARCH='arm64'
GOOS='darwin'
GOVERSION='go1.27.1'

What did you do?

Read the compiler ID cache validation in gccCompilerID (src/cmd/go/internal/work/exec.go). The cache entry is written as pairs of filename \x00 statinfo \x00 followed by the compiler ID, and validated on read like this:

stats := strings.Split(string(data[:len(data)-len(id)]), "\x00")
if len(stats)%2 != 0 {
	goto Miss
}

What did you see happen?

The payload itself ends with a NUL byte, so the split always produces a trailing empty field and an odd number of fields, and validation always takes goto Miss. The cached compiler ID is never reused: every go process that needs it re-runs the C compiler via gccToolID, which is the expense this cache was added to avoid (CL 392454, Go 1.19).

The loop below the parity check also advances by one over the name/stat pairs (i++ rather than i += 2), so an entry recording two files would fail validation even with the parity check fixed.

What did you expect to see?

A cache hit: repeated go invocations should validate the entry with os.Stat and reuse the stored compiler ID without re-running the compiler.