[BUG]: ddtrace/tracer/internal: compile failure on GOARCH=386 (SpanAttributes size assertion)

[puni9869]

Tracer Version(s)

v2.9.0

Go Version(s)

1.26.1

Bug Report

Description

Since v2.9.0, dd-trace-go/v2 fails to compile when targeting 32-bit architectures (GOARCH=386). The compile-time size assertion in ddtrace/tracer/internal/span_attributes.go hardcodes 56 bytes, which is only correct on 64-bit platforms.

Error

# github.com/DataDog/dd-trace-go/v2/ddtrace/tracer/internal
../../go/pkg/mod/github.com/!data!dog/dd-trace-go/v2@v2.9.1/ddtrace/tracer/internal/span_attributes.go:55:19: invalid argument: index 28 out of bounds [0:1]
../../go/pkg/mod/github.com/!data!dog/dd-trace-go/v2@v2.9.1/ddtrace/tracer/internal/span_attributes.go:56:19: unsafe.Sizeof(SpanAttributes{}) - 56 (constant -28 of type uintptr) overflows uintptr

Suggested Fix

Replace the hardcoded constant with something architecture-aware:

// Compile-time layout check: ensure no unexpected padding exists.
const expectedSize = unsafe.Offsetof(SpanAttributes{}.vals) + unsafe.Sizeof([numAttrs]string{})
var _ = [1]byte{}[expectedSize-unsafe.Sizeof(SpanAttributes{})]
var _ = [1]byte{}[unsafe.Sizeof(SpanAttributes{})-expectedSize]

Or simply remove the assertion since the struct layout doesn't have correctness implications that depend on an exact byte count.

Environment

• Go: 1.26.1
• dd-trace-go: v2.9.1
• GOOS: windows
• GOARCH: 386

Notes

The [README states](https://github.com/DataDog/dd-trace-go#go-support-policy): "This library only officially supports first class ports of Go." windows/386 and linux/386 are both first-class Go ports. The v2.9.0 release notes (PR #4538) do not mention dropping 32-bit support.

Reproduction Code

Reproduction

GOOS=windows GOARCH=386 CGO_ENABLED=0 go build ./...

Any project that imports dd-trace-go/v2 (directly or transitively) will fail to compile for 32-bit targets.

Error Logs

Root Cause

In span_attributes.go lines 55-56:

var _ = [1]byte{}[56-unsafe.Sizeof(SpanAttributes{})]
var _ = [1]byte{}[unsafe.Sizeof(SpanAttributes{})-56]

The struct layout comment says:

1B setMask + 1B readOnly + 6B padding + [3]string (48B) = 56B

On 64-bit, string = 16 bytes → [3]string = 48B → total = 56B ✓
On 32-bit, string = 8 bytes → [3]string = 24B → total = 28B ✗

The assertion should either:

• Use a platform-aware expected size (e.g., unsafe.Sizeof(SpanAttributes{}) compared against a computed value), or
• Be guarded with a build constraint (//go:build !386 && !arm)

Go Env Output

GOOS=windows GOARCH=386 CGO_ENABLED=0 go build ./...