Block for link address resolution

Previously, if address resolution for UDP or Ping sockets required sending
packets using Write in Transport layer, Resolve would return ErrWouldBlock
and Write would return ErrNoLinkAddress. Meanwhile startAddressResolution
would run in background. Further calls to Write using same address would also
return ErrNoLinkAddress until resolution has been completed successfully.

Since Write is not allowed to block and System Calls need to be
interruptible in System Call layer, the caller to Write is responsible for
blocking upon return of ErrWouldBlock.

Now, when startAddressResolution is called a notification channel for
the completion of the address resolution is returned.
The channel will traverse up to the calling function of Write as well as
ErrNoLinkAddress. Once address resolution is complete (success or not) the
channel is closed. The caller would call Write again to send packets and
check if address resolution was compeleted successfully or not.

Fixes google/gvisor#5

Change-Id: Idafaf31982bee1915ca084da39ae7bd468cebd93
PiperOrigin-RevId: 214962200

2 files changed, 29 insertions, 3 deletions

diff --git a/pkg/dhcp/client.go b/pkg/dhcp/client.go
index cf8472c5f..92c634a14 100644
--- a/pkg/dhcp/client.go
+++ b/pkg/dhcp/client.go
@@ -195,10 +195,23 @@ func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) (cfg
 	wopts := tcpip.WriteOptions{
 		To: serverAddr,
 	}
-	if _, err := ep.Write(tcpip.SlicePayload(h), wopts); err != nil {
+	var resCh <-chan struct{}
+	if _, resCh, err = ep.Write(tcpip.SlicePayload(h), wopts); err != nil && resCh == nil {
 		return Config{}, fmt.Errorf("dhcp discovery write: %v", err)
 	}
 
+	if resCh != nil {
+		select {
+		case <-resCh:
+		case <-ctx.Done():
+			return Config{}, fmt.Errorf("dhcp client address resolution: %v", tcpip.ErrAborted)
+		}
+
+		if _, _, err := ep.Write(tcpip.SlicePayload(h), wopts); err != nil {
+			return Config{}, fmt.Errorf("dhcp discovery write: %v", err)
+		}
+	}
+
 	we, ch := waiter.NewChannelEntry(nil)
 	wq.EventRegister(&we, waiter.EventIn)
 	defer wq.EventUnregister(&we)
@@ -289,7 +302,7 @@ func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) (cfg
 		reqOpts = append(reqOpts, option{optClientID, clientID})
 	}
 	h.setOptions(reqOpts)
-	if _, err := ep.Write(tcpip.SlicePayload(h), wopts); err != nil {
+	if _, _, err := ep.Write(tcpip.SlicePayload(h), wopts); err != nil {
 		return Config{}, fmt.Errorf("dhcp discovery write: %v", err)
 	}
 
diff --git a/pkg/dhcp/server.go b/pkg/dhcp/server.go
index 003e272b2..26700bdbc 100644
--- a/pkg/dhcp/server.go
+++ b/pkg/dhcp/server.go
@@ -95,9 +95,22 @@ func (c *epConn) Read() (buffer.View, tcpip.FullAddress, error) {
 }
 
 func (c *epConn) Write(b []byte, addr *tcpip.FullAddress) error {
-	if _, err := c.ep.Write(tcpip.SlicePayload(b), tcpip.WriteOptions{To: addr}); err != nil {
+	_, resCh, err := c.ep.Write(tcpip.SlicePayload(b), tcpip.WriteOptions{To: addr})
+	if err != nil && resCh == nil {
 		return fmt.Errorf("write: %v", err)
 	}
+
+	if resCh != nil {
+		select {
+		case <-resCh:
+		case <-c.ctx.Done():
+			return fmt.Errorf("dhcp server address resolution: %v", tcpip.ErrAborted)
+		}
+
+		if _, _, err := c.ep.Write(tcpip.SlicePayload(b), tcpip.WriteOptions{To: addr}); err != nil {
+			return fmt.Errorf("write: %v", err)
+		}
+	}
 	return nil
 }