add finish "slop"

We try to "finish" with `KValue + slop` peers but only wait for KValue. This
means we don't have to wait for timeouts.

query.go

@@ -22,6 +22,10 @@ import (
 
 var maxQueryConcurrency = AlphaValue
 
+// We "finish" queries with KValue + finishSlop peers but only wait for KValue
+// peers. This helps us account for unreachable peers.
+var finishSlop = 4
+
 type dhtQuery struct {
 	dht         *IpfsDHT
 	key         string      // the key we're querying for
@@ -192,9 +196,6 @@ func (r *dhtQueryRecurseResult) Finish(ctx context.Context) ([]peer.ID, error) {
 }
 
 func (r *dhtQueryRecurseResult) FinishWith(ctx context.Context, fn func(context.Context, peer.ID) error) ([]peer.ID, error) {
-	ctx, cancel := context.WithCancel(ctx)
-	defer cancel()
-
 	// Get a sorted list of peers to query.
 	failed := make(map[peer.ID]bool, len(r.failed))
 	for _, p := range r.failed {
@@ -218,9 +219,10 @@ func (r *dhtQueryRecurseResult) FinishWith(ctx context.Context, fn func(context.
 	closest = kb.SortClosestPeers(closest, kb.ConvertKey(r.query.key))
 
 	// Query them.
+	sloppyK := KValue + finishSlop
 	bucket := make([]peer.ID, 0, KValue)
 	workQ := make(chan peer.ID)
-	resultQ := make(chan peer.ID, KValue)
+	resultQ := make(chan peer.ID, sloppyK)
 
 	newQuery := fn != nil
 	if !newQuery {
@@ -231,8 +233,13 @@ func (r *dhtQueryRecurseResult) FinishWith(ctx context.Context, fn func(context.
 	}
 
 	var wg sync.WaitGroup
-	wg.Add(KValue)
-	for i := 0; i < KValue; i++ {
+	defer wg.Wait()
+
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	wg.Add(sloppyK)
+	for i := 0; i < sloppyK; i++ {
 		go func() {
 			defer wg.Done()
 			for p := range workQ {
@@ -243,6 +250,7 @@ func (r *dhtQueryRecurseResult) FinishWith(ctx context.Context, fn func(context.
 			}
 		}()
 	}
+
 	go func() {
 		wg.Wait()
 		close(resultQ)
@@ -269,11 +277,17 @@ func (r *dhtQueryRecurseResult) FinishWith(ctx context.Context, fn func(context.
 			bucket = append(bucket, successPeer)
 		}
 	}
+
 	close(workQ)
 
-	for p := range resultQ {
+	for len(bucket) < KValue {
+		p, ok := <-resultQ
+		if !ok {
+			break
+		}
 		bucket = append(bucket, p)
 	}
+
 	return bucket, ctx.Err()
 }