net: use baked-in port numbers as fallback if cgo port lookup fails

Fixes TestLookupPort_Minimal on android.

Fixes #18213

Change-Id: I1b65e790525d339a4cb7f17afe7e3a02c4587302
Reviewed-on: https://go-review.googlesource.com/34014Reviewed-by: Russ Cox <rsc@golang.org>

src/net/lookup_unix.go

@@ -76,13 +76,15 @@ func (r *Resolver) lookupIP(ctx context.Context, host string) (addrs []IPAddr, e
 }
 
 func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int, error) {
-	// TODO: use the context if there ever becomes a need. Related
-	// is issue 15321. But port lookup generally just involves
-	// local files, and the os package has no context support. The
-	// files might be on a remote filesystem, though. This should
-	// probably race goroutines if ctx != context.Background().
 	if !r.PreferGo && systemConf().canUseCgo() {
 		if port, err, ok := cgoLookupPort(ctx, network, service); ok {
+			if err != nil {
+				// Issue 18213: if cgo fails, first check to see whether we
+				// have the answer baked-in to the net package.
+				if port, err := goLookupPort(network, service); err == nil {
+					return port, nil
+				}
+			}
 			return port, err
 		}
 	}

src/net/port_unix.go

@@ -10,12 +10,11 @@ package net
 
 import "sync"
 
-var servicesError error
 var onceReadServices sync.Once
 
 func readServices() {
-	var file *file
-	if file, servicesError = open("/etc/services"); servicesError != nil {
+	file, err := open("/etc/services")
+	if err != nil {
 		return
 	}
 	for line, ok := file.readLine(); ok; line, ok = file.readLine() {