net: implement cancellable lookup on Plan 9

This change implements cancellable lookup
on Plan 9. The query function has been modified
to return when the ctx.Done channel is closed.

Fixes #25361.

Change-Id: I544b779ceec8d69975bc7363045849c21cbfd59e
Reviewed-on: https://go-review.googlesource.com/112981
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/lookup_plan9.go

@@ -11,34 +11,58 @@ import (
 	"os"
 )
 
-func query(ctx context.Context, filename, query string, bufSize int) (res []string, err error) {
-	file, err := os.OpenFile(filename, os.O_RDWR, 0)
-	if err != nil {
-		return
-	}
-	defer file.Close()
+func query(ctx context.Context, filename, query string, bufSize int) (addrs []string, err error) {
+	queryAddrs := func() (addrs []string, err error) {
+		file, err := os.OpenFile(filename, os.O_RDWR, 0)
+		if err != nil {
+			return nil, err
+		}
+		defer file.Close()
 
-	_, err = file.Seek(0, io.SeekStart)
-	if err != nil {
-		return
-	}
-	_, err = file.WriteString(query)
-	if err != nil {
-		return
+		_, err = file.Seek(0, io.SeekStart)
+		if err != nil {
+			return nil, err
+		}
+		_, err = file.WriteString(query)
+		if err != nil {
+			return nil, err
+		}
+		_, err = file.Seek(0, io.SeekStart)
+		if err != nil {
+			return nil, err
+		}
+		buf := make([]byte, bufSize)
+		for {
+			n, _ := file.Read(buf)
+			if n <= 0 {
+				break
+			}
+			addrs = append(addrs, string(buf[:n]))
+		}
+		return addrs, nil
 	}
-	_, err = file.Seek(0, io.SeekStart)
-	if err != nil {
-		return
+
+	type ret struct {
+		addrs []string
+		err   error
 	}
-	buf := make([]byte, bufSize)
-	for {
-		n, _ := file.Read(buf)
-		if n <= 0 {
-			break
+
+	ch := make(chan ret, 1)
+	go func() {
+		addrs, err := queryAddrs()
+		ch <- ret{addrs: addrs, err: err}
+	}()
+
+	select {
+	case r := <-ch:
+		return r.addrs, r.err
+	case <-ctx.Done():
+		return nil, &DNSError{
+			Name:      query,
+			Err:       ctx.Err().Error(),
+			IsTimeout: ctx.Err() == context.DeadlineExceeded,
 		}
-		res = append(res, string(buf[:n]))
 	}
-	return
 }
 
 func queryCS(ctx context.Context, net, host, service string) (res []string, err error) {