From d5341fd60cb129082573a53c4278b01394eb3a81 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Mon, 13 Apr 2026 16:57:38 +0000
Subject: [PATCH] tailscaleroot: add test that tsgo rev is in Go build cache
 keys

Verify that GODEBUG=gocachehash=1 output from ./tool/go includes the
git revision from go.toolchain.rev, ensuring that bumping the Tailscale
Go fork (without a Go version number change) properly invalidates the
build cache.

The test only runs in CI or when the current Go binary is the Tailscale
toolchain (GOROOT contains /.cache/tsgo/), so open source contributors
using stock Go aren't forced to download tsgo.

Fixes tailscale/corp#36589

Change-Id: Ia98d3a3aa8c7fa67f9a0293066fa02a1997dcb95
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 cache_key_test.go | 57 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 cache_key_test.go

diff --git a/cache_key_test.go b/cache_key_test.go
new file mode 100644
index 000000000..8600bcd71
--- /dev/null
+++ b/cache_key_test.go
@@ -0,0 +1,57 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tailscaleroot
+
+import (
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"tailscale.com/util/cibuild"
+)
+
+// TestTsgoRevInCacheKey verifies that the Tailscale Go toolchain's git
+// revision (from go.toolchain.rev) is blended into Go build cache keys.
+// Without this, bumping the toolchain to a new commit that doesn't change
+// the Go version number would silently reuse stale cached build artifacts.
+//
+// See https://github.com/tailscale/tailscale/issues/36589.
+func TestTsgoRevInCacheKey(t *testing.T) {
+	goRoot := goEnv(t, "GOROOT")
+	isTsgo := strings.Contains(goRoot, "/.cache/tsgo/")
+	if !cibuild.On() && !isTsgo {
+		t.Skip("skipping; not in CI and not using the Tailscale Go toolchain")
+	}
+
+	rev := strings.TrimSpace(GoToolchainRev)
+	if rev == "" {
+		t.Fatal("go.toolchain.rev is empty")
+	}
+
+	// Build the small stdlib "errors" package with GODEBUG=gocachehash=1,
+	// which causes cmd/go to log its cache key computations to stderr.
+	cmd := exec.Command("go", "build", "errors")
+	cmd.Env = append(os.Environ(), "GODEBUG=gocachehash=1")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("go build errors failed: %v\n%s", err, out)
+	}
+
+	// The cache key output should contain the toolchain rev alongside the
+	// Go version, e.g.:
+	//   HASH[moduleIndex]: "go1.26.2 dfe2a5fd8ee2e68b08ce5ff259269f50ecadf2f4"
+	if !strings.Contains(string(out), rev) {
+		t.Errorf("go.toolchain.rev %q not found in GODEBUG=gocachehash=1 output:\n%s", rev, out)
+	}
+}
+
+func goEnv(t *testing.T, key string) string {
+	t.Helper()
+	out, err := exec.Command("go", "env", key).Output()
+	if err != nil {
+		t.Fatalf("go env %s: %v", key, err)
+	}
+	return strings.TrimSpace(string(out))
+}