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

1 files changed, 14 insertions, 17 deletions

diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index f2dd98f35..6ed805357 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -258,10 +258,10 @@ func (e *endpoint) prepareForWrite(to *tcpip.FullAddress) (retry bool, err *tcpi
 
 // Write writes data to the endpoint's peer. This method does not block
 // if the data cannot be written.
-func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tcpip.Error) {
+func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, <-chan struct{}, *tcpip.Error) {
 	// MSG_MORE is unimplemented. (This also means that MSG_EOR is a no-op.)
 	if opts.More {
-		return 0, tcpip.ErrInvalidOptionValue
+		return 0, nil, tcpip.ErrInvalidOptionValue
 	}
 
 	to := opts.To
@@ -271,14 +271,14 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 
 	// If we've shutdown with SHUT_WR we are in an invalid state for sending.
 	if e.shutdownFlags&tcpip.ShutdownWrite != 0 {
-		return 0, tcpip.ErrClosedForSend
+		return 0, nil, tcpip.ErrClosedForSend
 	}
 
 	// Prepare for write.
 	for {
 		retry, err := e.prepareForWrite(to)
 		if err != nil {
-			return 0, err
+			return 0, nil, err
 		}
 
 		if !retry {
@@ -303,7 +303,7 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 
 			// Recheck state after lock was re-acquired.
 			if e.state != stateConnected {
-				return 0, tcpip.ErrInvalidEndpointState
+				return 0, nil, tcpip.ErrInvalidEndpointState
 			}
 		}
 	} else {
@@ -312,7 +312,7 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 		nicid := to.NIC
 		if e.bindNICID != 0 {
 			if nicid != 0 && nicid != e.bindNICID {
-				return 0, tcpip.ErrNoRoute
+				return 0, nil, tcpip.ErrNoRoute
 			}
 
 			nicid = e.bindNICID
@@ -322,13 +322,13 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 		to = &toCopy
 		netProto, err := e.checkV4Mapped(to, false)
 		if err != nil {
-			return 0, err
+			return 0, nil, err
 		}
 
 		// Find the enpoint.
 		r, err := e.stack.FindRoute(nicid, e.id.LocalAddress, to.Addr, netProto)
 		if err != nil {
-			return 0, err
+			return 0, nil, err
 		}
 		defer r.Release()
 
@@ -338,23 +338,20 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 
 	if route.IsResolutionRequired() {
 		waker := &sleep.Waker{}
-		if err := route.Resolve(waker); err != nil {
+		if ch, err := route.Resolve(waker); err != nil {
 			if err == tcpip.ErrWouldBlock {
 				// Link address needs to be resolved. Resolution was triggered the background.
 				// Better luck next time.
-				//
-				// TODO: queue up the request and send after link address
-				// is resolved.
 				route.RemoveWaker(waker)
-				return 0, tcpip.ErrNoLinkAddress
+				return 0, ch, tcpip.ErrNoLinkAddress
 			}
-			return 0, err
+			return 0, nil, err
 		}
 	}
 
 	v, err := p.Get(p.Size())
 	if err != nil {
-		return 0, err
+		return 0, nil, err
 	}
 
 	ttl := route.DefaultTTL()
@@ -363,9 +360,9 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 	}
 
 	if err := sendUDP(route, buffer.View(v).ToVectorisedView(), e.id.LocalPort, dstPort, ttl); err != nil {
-		return 0, err
+		return 0, nil, err
 	}
-	return uintptr(len(v)), nil
+	return uintptr(len(v)), nil, nil
 }
 
 // Peek only returns data from a single datagram, so do nothing here.