fix: improve context deadline handling

1. Continue to best-effort provide, but still return an error when we fail to
send provider records to the _best_ peers.
2. Continue returning the best peer's we've found in GetClosestPeers, but also
return an error to indicate that we didn't find the closest ones.

And fix the hang test.

ext_test.go

@@ -21,7 +21,9 @@ import (
 	mocknet "github.com/libp2p/go-libp2p/p2p/net/mock"
 )
 
-func TestHang(t *testing.T) {
+// Test that one hung request to a peer doesn't prevent another request
+// using that same peer from obeying its context.
+func TestHungRequest(t *testing.T) {
 	ctx := context.Background()
 	mn, err := mocknet.FullMeshConnected(ctx, 2)
 	if err != nil {
@@ -44,22 +46,27 @@ func TestHang(t *testing.T) {
 	ctx1, cancel1 := context.WithTimeout(ctx, 1*time.Second)
 	defer cancel1()
 
-	peers, err := d.GetClosestPeers(ctx1, testCaseCids[0].KeyString())
-	if err != nil {
-		t.Fatal(err)
-	}
+	done := make(chan error, 1)
+	go func() {
+		_, err := d.GetClosestPeers(ctx1, testCaseCids[0].KeyString())
+		done <- err
+	}()
 
 	time.Sleep(100 * time.Millisecond)
 	ctx2, cancel2 := context.WithTimeout(ctx, 100*time.Millisecond)
 	defer cancel2()
-	_ = d.Provide(ctx2, testCaseCids[0], true)
-	if ctx2.Err() != context.DeadlineExceeded {
+	err = d.Provide(ctx2, testCaseCids[0], true)
+	if err != context.DeadlineExceeded {
 		t.Errorf("expected to fail with deadline exceeded, got: %s", ctx2.Err())
 	}
 	select {
-	case <-peers:
-		t.Error("GetClosestPeers should not have returned yet")
+	case <-done:
+		t.Errorf("GetClosestPeers should not have returned yet")
 	default:
+		err = <-done
+		if err != context.DeadlineExceeded {
+			t.Errorf("expected the deadline to be exceeded, got %s", err)
+		}
 	}
 
 }

lookup.go

@@ -68,6 +68,9 @@ func multihashLoggableKey(mh multihash.Multihash) logging.LoggableMap {
 
 // Kademlia 'node lookup' operation. Returns a channel of the K closest peers
 // to the given key
+//
+// If the context is canceled, this function will return the context error along
+// with the closest K peers it has found so far.
 func (dht *IpfsDHT) GetClosestPeers(ctx context.Context, key string) (<-chan peer.ID, error) {
 	//TODO: I can break the interface! return []peer.ID
 	e := logger.EventBegin(ctx, "getClosestPeers", loggableKey(key))
@@ -121,5 +124,5 @@ func (dht *IpfsDHT) GetClosestPeers(ctx context.Context, key string) (<-chan pee
 		dht.routingTable.ResetCplRefreshedAtForID(kadID, time.Now())
 	}
 
-	return out, nil
+	return out, ctx.Err()
 }

routing.go

@@ -461,7 +461,20 @@ func (dht *IpfsDHT) Provide(ctx context.Context, key cid.Cid, brdcst bool) (err
 		defer cancel()
 	}
 
+	var exceededDeadline bool
 	peers, err := dht.GetClosestPeers(closerCtx, string(keyMH))
+	switch err {
+	case context.DeadlineExceeded:
+		// If the _inner_ deadline has been exceeded but the _outer_
+		// context is still fine, provide the value to the closest peers
+		// we managed to find, even if they're not the _actual_ closest peers.
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+		exceededDeadline = true
+	case nil:
+	default:
+	}
 	if err != nil {
 		return err
 	}
@@ -484,7 +497,10 @@ func (dht *IpfsDHT) Provide(ctx context.Context, key cid.Cid, brdcst bool) (err
 		}(p)
 	}
 	wg.Wait()
-	return nil
+	if exceededDeadline {
+		return context.DeadlineExceeded
+	}
+	return ctx.Err()
 }
 func (dht *IpfsDHT) makeProvRecord(key []byte) (*pb.Message, error) {
 	pi := peer.AddrInfo{