ipns record selection via sequence numbers

This commit adds a sequence number to the IpnsEntry protobuf
that is used to determine which among a set of entries for the same key
is the 'most correct'.

GetValues has been added to the routing interface to retrieve a set of
records from the dht, for the caller to select from.

GetValue (singular) will call GetValues, select the 'best' record, and
then update that record to peers we received outdated records from.
This will help keep the dht consistent.

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

routing/dht/routing.go

@@ -1,6 +1,7 @@
 package dht
 
 import (
+	"bytes"
 	"sync"
 
 	context "github.com/ipfs/go-ipfs/Godeps/_workspace/src/golang.org/x/net/context"
@@ -76,16 +77,71 @@ func (dht *IpfsDHT) PutValue(ctx context.Context, key key.Key, value []byte) err
 }
 
 // GetValue searches for the value corresponding to given Key.
-// If the search does not succeed, a multiaddr string of a closer peer is
-// returned along with util.ErrSearchIncomplete
 func (dht *IpfsDHT) GetValue(ctx context.Context, key key.Key) ([]byte, error) {
+	vals, err := dht.GetValues(ctx, key, 3)
+	if err != nil {
+		return nil, err
+	}
+
+	var recs [][]byte
+	for _, v := range vals {
+		recs = append(recs, v.Val)
+	}
+
+	i, err := dht.Selector.BestRecord(key, recs)
+	if err != nil {
+		return nil, err
+	}
+
+	best := recs[i]
+	log.Debugf("GetValue %v %v", key, best)
+	if best == nil {
+		log.Errorf("GetValue yielded correct record with nil value.")
+		return nil, routing.ErrNotFound
+	}
+
+	fixupRec, err := record.MakePutRecord(dht.peerstore.PrivKey(dht.self), key, best, true)
+	if err != nil {
+		// probably shouldnt actually 'error' here as we have found a value we like,
+		// but this call failing probably isnt something we want to ignore
+		return nil, err
+	}
+
+	for _, v := range vals {
+		// if someone sent us a different 'less-valid' record, lets correct them
+		if !bytes.Equal(v.Val, best) {
+			go func(v routing.RecvdVal) {
+				err := dht.putValueToPeer(ctx, v.From, key, fixupRec)
+				if err != nil {
+					log.Error("Error correcting DHT entry: ", err)
+				}
+			}(v)
+		}
+	}
+
+	return best, nil
+}
+
+func (dht *IpfsDHT) GetValues(ctx context.Context, key key.Key, nvals int) ([]routing.RecvdVal, error) {
+	var vals []routing.RecvdVal
+	var valslock sync.Mutex
+
 	// If we have it local, dont bother doing an RPC!
-	val, err := dht.getLocal(key)
+	lrec, err := dht.getLocal(key)
 	if err == nil {
+		// TODO: this is tricky, we dont always want to trust our own value
+		// what if the authoritative source updated it?
 		log.Debug("have it locally")
-		return val, nil
-	} else {
-		log.Debug("failed to get value locally: %s", err)
+		vals = append(vals, routing.RecvdVal{
+			Val:  lrec.GetValue(),
+			From: dht.self,
+		})
+
+		if nvals <= 1 {
+			return vals, nil
+		}
+	} else if nvals == 0 {
+		return nil, err
 	}
 
 	// get closest peers in the routing table
@@ -104,14 +160,26 @@ func (dht *IpfsDHT) GetValue(ctx context.Context, key key.Key) ([]byte, error) {
 			ID:   p,
 		})
 
-		val, peers, err := dht.getValueOrPeers(ctx, p, key)
+		rec, peers, err := dht.getValueOrPeers(ctx, p, key)
 		if err != nil {
 			return nil, err
 		}
 
-		res := &dhtQueryResult{value: val, closerPeers: peers}
-		if val != nil {
-			res.success = true
+		res := &dhtQueryResult{closerPeers: peers}
+
+		if rec.GetValue() != nil {
+			rv := routing.RecvdVal{
+				Val:  rec.GetValue(),
+				From: p,
+			}
+			valslock.Lock()
+			vals = append(vals, rv)
+
+			// If weve collected enough records, we're done
+			if len(vals) >= nvals {
+				res.success = true
+			}
+			valslock.Unlock()
 		}
 
 		notif.PublishQueryEvent(parent, &notif.QueryEvent{
@@ -124,17 +192,15 @@ func (dht *IpfsDHT) GetValue(ctx context.Context, key key.Key) ([]byte, error) {
 	})
 
 	// run it!
-	result, err := query.Run(ctx, rtp)
-	if err != nil {
-		return nil, err
+	_, err = query.Run(ctx, rtp)
+	if len(vals) == 0 {
+		if err != nil {
+			return nil, err
+		}
 	}
 
-	log.Debugf("GetValue %v %v", key, result.value)
-	if result.value == nil {
-		return nil, routing.ErrNotFound
-	}
+	return vals, nil
 
-	return result.value, nil
 }
 
 // Value provider layer of indirection.