net: move resolver forcing functions to one file

The forceGoDNS/forceCgoDNS functions currently return nil
or an empty func on plan9, but they don't have to because we
support go resolver there (Dial field required).

src/net/lookup_test.go

@@ -1509,22 +1509,6 @@ func TestLookupPortIPNetworkString(t *testing.T) {
 	})
 }
 
-func allResolvers(t *testing.T, f func(t *testing.T)) {
-	t.Run("default resolver", f)
-	t.Run("forced go resolver", func(t *testing.T) {
-		if fixup := forceGoDNS(); fixup != nil {
-			defer fixup()
-			f(t)
-		}
-	})
-	t.Run("forced cgo resolver", func(t *testing.T) {
-		if fixup := forceCgoDNS(); fixup != nil {
-			defer fixup()
-			f(t)
-		}
-	})
-}
-
 func TestLookupNoSuchHost(t *testing.T) {
 	mustHaveExternalNetwork(t)
 

src/net/main_conf_test.go

@@ -2,11 +2,28 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !plan9
-
 package net
 
-import "testing"
+import (
+	"context"
+	"runtime"
+	"testing"
+)
+
+func allResolvers(t *testing.T, f func(t *testing.T)) {
+	t.Run("default resolver", f)
+	t.Run("forced go resolver", func(t *testing.T) {
+		// On plan9 the forceGoDNS might not force the go resolver, currently
+		// it is only forced when the Resolver.Dial field is populated.
+		// See conf.go mustUseGoResolver.
+		defer forceGoDNS()()
+		f(t)
+	})
+	t.Run("forced cgo resolver", func(t *testing.T) {
+		defer forceCgoDNS()()
+		f(t)
+	})
+}
 
 // forceGoDNS forces the resolver configuration to use the pure Go resolver
 // and returns a fixup function to restore the old settings.
@@ -25,7 +42,6 @@ func forceGoDNS() func() {
 
 // forceCgoDNS forces the resolver configuration to use the cgo resolver
 // and returns a fixup function to restore the old settings.
-// (On non-Unix systems forceCgoDNS returns nil.)
 func forceCgoDNS() func() {
 	c := systemConf()
 	oldGo := c.netGo
@@ -48,12 +64,34 @@ func TestForceCgoDNS(t *testing.T) {
 	if order != hostLookupCgo {
 		t.Fatalf("hostLookupOrder returned: %v, want cgo", order)
 	}
+	order, _ = systemConf().addrLookupOrder(nil, "192.0.2.1")
+	if order != hostLookupCgo {
+		t.Fatalf("addrLookupOrder returned: %v, want cgo", order)
+	}
+	if systemConf().mustUseGoResolver(nil) {
+		t.Fatal("mustUseGoResolver = true, want false")
+	}
 }
 
 func TestForceGoDNS(t *testing.T) {
+	var resolver *Resolver
+	if runtime.GOOS == "plan9" {
+		resolver = &Resolver{
+			Dial: func(_ context.Context, _, _ string) (Conn, error) {
+				panic("unreachable")
+			},
+		}
+	}
 	defer forceGoDNS()()
-	order, _ := systemConf().hostLookupOrder(nil, "go.dev")
+	order, _ := systemConf().hostLookupOrder(resolver, "go.dev")
 	if order == hostLookupCgo {
 		t.Fatalf("hostLookupOrder returned: %v, want go resolver order", order)
 	}
+	order, _ = systemConf().addrLookupOrder(resolver, "192.0.2.1")
+	if order == hostLookupCgo {
+		t.Fatalf("addrLookupOrder returned: %v, want go resolver order", order)
+	}
+	if !systemConf().mustUseGoResolver(resolver) {
+		t.Fatal("mustUseGoResolver = false, want true")
+	}
 }

src/net/main_noconf_test.go

@@ -1,22 +0,0 @@
-// Copyright 2015 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build plan9
-
-package net
-
-import "runtime"
-
-// See main_conf_test.go for what these (don't) do.
-func forceGoDNS() func() {
-	switch runtime.GOOS {
-	case "plan9":
-		return func() {}
-	default:
-		return nil
-	}
-}
-
-// See main_conf_test.go for what these (don't) do.
-func forceCgoDNS() func() { return nil }