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, 10 insertions, 6 deletions

diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index cc9b24e23..6c6400c33 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -89,11 +89,15 @@ func (r *Route) Capabilities() LinkEndpointCapabilities {
 // Resolve attempts to resolve the link address if necessary. Returns ErrWouldBlock in
 // case address resolution requires blocking, e.g. wait for ARP reply. Waker is
 // notified when address resolution is complete (success or not).
-func (r *Route) Resolve(waker *sleep.Waker) *tcpip.Error {
+//
+// If address resolution is required, ErrNoLinkAddress and a notification channel is
+// returned for the top level caller to block. Channel is closed once address resolution
+// is complete (success or not).
+func (r *Route) Resolve(waker *sleep.Waker) (<-chan struct{}, *tcpip.Error) {
 	if !r.IsResolutionRequired() {
 		// Nothing to do if there is no cache (which does the resolution on cache miss) or
 		// link address is already known.
-		return nil
+		return nil, nil
 	}
 
 	nextAddr := r.NextHop
@@ -101,16 +105,16 @@ func (r *Route) Resolve(waker *sleep.Waker) *tcpip.Error {
 		// Local link address is already known.
 		if r.RemoteAddress == r.LocalAddress {
 			r.RemoteLinkAddress = r.LocalLinkAddress
-			return nil
+			return nil, nil
 		}
 		nextAddr = r.RemoteAddress
 	}
-	linkAddr, err := r.ref.linkCache.GetLinkAddress(r.ref.nic.ID(), nextAddr, r.LocalAddress, r.NetProto, waker)
+	linkAddr, ch, err := r.ref.linkCache.GetLinkAddress(r.ref.nic.ID(), nextAddr, r.LocalAddress, r.NetProto, waker)
 	if err != nil {
-		return err
+		return ch, err
 	}
 	r.RemoteLinkAddress = linkAddr
-	return nil
+	return nil, nil
 }
 
 // RemoveWaker removes a waker that has been added in Resolve().