Allow timeout and cancellation via context.Context

Introduce context.Context to enable cancellation in addition to the
existing timeout functionality.  To retain compatability, Timeout is
still available but will only be used if Context is not set.

We use x/net/context.Context for the moment instead of context.Context
in order to avoid creating a requirement on Go 1.7

client.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/miekg/dns"
+	"golang.org/x/net/context"
 	"golang.org/x/net/ipv4"
 	"golang.org/x/net/ipv6"
 )
@@ -39,7 +40,8 @@ func (s *ServiceEntry) complete() bool {
 type QueryParam struct {
 	Service             string               // Service to lookup
 	Domain              string               // Lookup domain, default "local"
-	Timeout             time.Duration        // Lookup timeout, default 1 second
+	Context             context.Context      // Context
+	Timeout             time.Duration        // Lookup timeout, default 1 second. Ignored if Context is provided
 	Interface           *net.Interface       // Multicast interface to use
 	Entries             chan<- *ServiceEntry // Entries Channel
 	WantUnicastResponse bool                 // Unicast response desired, as per 5.4 in RFC
@@ -79,9 +81,16 @@ func Query(params *QueryParam) error {
 	if params.Domain == "" {
 		params.Domain = "local"
 	}
+
+	if params.Context == nil {
 		if params.Timeout == 0 {
 			params.Timeout = time.Second
 		}
+		params.Context, _ = context.WithTimeout(context.Background(), params.Timeout)
+		if err != nil {
+			return err
+		}
+	}
 
 	// Run the query
 	return client.query(params)
@@ -316,9 +325,6 @@ func (c *client) query(params *QueryParam) error {
 	// Map the in-progress responses
 	inprogress := make(map[string]*ServiceEntry)
 
-	// Listen until we reach the timeout
-	finish := time.After(params.Timeout)
-
 	for {
 		select {
 		case resp := <-msgCh:
@@ -335,7 +341,7 @@ func (c *client) query(params *QueryParam) error {
 				inp.sent = true
 				select {
 				case params.Entries <- inp:
-				case <-finish:
+				case <-params.Context.Done():
 					return nil
 				}
 			} else {
@@ -347,7 +353,7 @@ func (c *client) query(params *QueryParam) error {
 					log.Printf("[ERR] mdns: Failed to query instance %s: %v", inp.Name, err)
 				}
 			}
-		case <-finish:
+		case <-params.Context.Done():
 			return nil
 		}
 	}