tool/gocross: add a failing cgo test

Something in gocross is breaking depaware when listing the deps of
cross-compiled cgo-using targets. This adds a test.

Updates #15015

Change-Id: Ice6b965fe719359aec25b8f9dc11d612f733b2f0

tool/gocross/cgo_test.go

@@ -0,0 +1,86 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"log"
+	"maps"
+	"os"
+	"runtime"
+	"slices"
+	"testing"
+
+	"golang.org/x/tools/go/packages"
+	_ "tailscale.com/version"
+)
+
+func TestCgo(t *testing.T) {
+	t.Logf("Go %s", runtime.Version())
+	const pkg = "tailscale.com/tool/gocross/testcgoprog"
+	ipaths, err := pkgPaths(pkg)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(ipaths) != 1 || ipaths[0] != pkg {
+		t.Fatalf("ipaths: %q; want just %q", ipaths, pkg)
+	}
+
+	// Pick a goos to force cross-compilation.
+	goos := "linux"
+	if runtime.GOOS == "linux" {
+		goos = "darwin"
+	}
+
+	env := append(os.Environ(), "GOARCH=amd64", "GOOS="+goos, "CGO_ENABLED=1")
+	cfg := &packages.Config{
+		Mode: packages.NeedImports | packages.NeedDeps | packages.NeedFiles | packages.NeedName,
+		Env:  env,
+	}
+
+	pkgs, err := packages.Load(cfg, pkg)
+	if err != nil {
+		log.Fatalf("for GOOS=%v: %v", goos, err)
+	}
+
+	type edge string // "from -> to"
+	edges := map[edge]bool{}
+	saw := map[string]bool{}
+
+	packages.Visit(pkgs, nil, func(p *packages.Package) {
+		t.Logf("pkg: %s", p.PkgPath)
+		for imp := range p.Imports {
+			e := edge(p.PkgPath + " => " + imp)
+			edges[e] = true
+		}
+		if p.PkgPath == pkg {
+			return
+		}
+		saw[p.PkgPath] = true
+	})
+
+	if len(saw) == 0 {
+		t.Error("didn't see visit any other packages from a cross-compiled cgo root")
+	}
+	if len(edges) == 0 {
+		t.Error("didn't find any edges from a cross-compiled cgo root")
+	}
+
+	for _, s := range slices.Sorted(maps.Keys(saw)) {
+		t.Logf("saw: %s", s)
+	}
+	for _, s := range slices.Sorted(maps.Keys(edges)) {
+		t.Logf("edge: %s", s)
+	}
+}
+
+func pkgPaths(pkg ...string) (ipaths []string, err error) {
+	pkgs, err := packages.Load(nil, pkg...)
+	if err != nil {
+		return nil, err
+	}
+	for _, p := range pkgs {
+		ipaths = append(ipaths, p.PkgPath)
+	}
+	return ipaths, nil
+}

tool/gocross/testcgoprog/testcgoprog.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+// /* Force use of cgo */
+import "C"
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func main() {
+	fmt.Println("testcgoprog", runtime.Version())
+}