net: remove redundant cgoLookupCNAME return parameter

`completed bool` just means `err == nil` after CL 446179.

While here, add a test for cgoLookupCNAME.

Co-authored-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Change-Id: I6081a089fde3cb27af4fbde6f04301fc3755272c
Reviewed-on: https://go-review.googlesource.com/c/go/+/698615
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/net/cgo_stub.go

@@ -31,7 +31,7 @@ func cgoLookupIP(ctx context.Context, network, name string) (addrs []IPAddr, err
 	panic("cgo stub: cgo not available")
 }
 
-func cgoLookupCNAME(ctx context.Context, name string) (cname string, err error, completed bool) {
+func cgoLookupCNAME(ctx context.Context, name string) (cname string, err error) {
 	panic("cgo stub: cgo not available")
 }
 

src/net/cgo_unix.go

@@ -297,16 +297,16 @@ func cgoSockaddr(ip IP, zone string) (*_C_struct_sockaddr, _C_socklen_t) {
 	return nil, 0
 }
 
-func cgoLookupCNAME(ctx context.Context, name string) (cname string, err error, completed bool) {
+func cgoLookupCNAME(ctx context.Context, name string) (cname string, err error) {
 	resources, err := resSearch(ctx, name, int(dnsmessage.TypeCNAME), int(dnsmessage.ClassINET))
 	if err != nil {
-		return
+		return "", err
 	}
 	cname, err = parseCNAMEFromResources(resources)
 	if err != nil {
-		return "", err, false
+		return "", err
 	}
-	return cname, nil, true
+	return cname, nil
 }
 
 // resSearch will make a call to the 'res_nsearch' routine in the C library

src/net/cgo_unix_test.go

@@ -8,6 +8,7 @@ package net
 
 import (
 	"context"
+	"internal/testenv"
 	"testing"
 )
 
@@ -67,3 +68,12 @@ func TestCgoLookupPTRWithCancel(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestCgoLookupCNAME(t *testing.T) {
+	mustHaveExternalNetwork(t)
+	testenv.SkipFlakyNet(t)
+	defer dnsWaitGroup.Wait()
+	if _, err := cgoLookupCNAME(t.Context(), "www.iana.org."); err != nil {
+		t.Error(err)
+	}
+}

src/net/lookup_unix.go

@@ -87,8 +87,8 @@ func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int
 func (r *Resolver) lookupCNAME(ctx context.Context, name string) (string, error) {
 	order, conf := systemConf().hostLookupOrder(r, name)
 	if order == hostLookupCgo {
-		if cname, err, ok := cgoLookupCNAME(ctx, name); ok {
+		if cname, err := cgoLookupCNAME(ctx, name); err == nil {
-			return cname, err
+			return cname, nil
 		}
 	}
 	return r.goLookupCNAME(ctx, name, order, conf)