p2p: Move check for getheaders locators earlier in the call

This moves the check that ensures that a reply to a getheaders connects
to the existing locators ealier in the call stack for processing a
headers response.

This ensures the wallet disconnects from misbehaving peers before
processing block hashes.

p2p/peering.go

@@ -114,9 +114,10 @@ type RemotePeer struct {
 
 	// headers message management.  Headers can either be fetched synchronously
 	// or used to push block notifications with sendheaders.
-	requestedHeaders   chan<- *wire.MsgHeaders // non-nil result chan when synchronous getheaders in process
-	sendheaders        bool                    // whether a sendheaders message was sent
-	requestedHeadersMu sync.Mutex
+	requestedHeaders    chan<- *wire.MsgHeaders // non-nil result chan when synchronous getheaders in process
+	requestedHeadersLoc []*chainhash.Hash       // non-nil when requested headers with getheaders
+	sendheaders         bool                    // whether a sendheaders message was sent
+	requestedHeadersMu  sync.Mutex
 
 	// init state message management.
 	requestedInitState   chan<- *wire.MsgInitState // non-nil result chan when synchronous getinitstate in process
@@ -1097,7 +1098,7 @@ func (rp *RemotePeer) receivedCFilterV2(ctx context.Context, msg *wire.MsgCFilte
 	}
 }
 
-func (rp *RemotePeer) addRequestedHeaders(c chan<- *wire.MsgHeaders) (sendheaders, newRequest bool) {
+func (rp *RemotePeer) addRequestedHeaders(c chan<- *wire.MsgHeaders, loc []*chainhash.Hash) (sendheaders, newRequest bool) {
 	rp.requestedHeadersMu.Lock()
 	if rp.sendheaders {
 		rp.requestedHeadersMu.Unlock()
@@ -1108,13 +1109,15 @@ func (rp *RemotePeer) addRequestedHeaders(c chan<- *wire.MsgHeaders) (sendheader
 		return false, false
 	}
 	rp.requestedHeaders = c
+	rp.requestedHeadersLoc = loc
 	rp.requestedHeadersMu.Unlock()
 	return false, true
 }
 
 func (rp *RemotePeer) deleteRequestedHeaders() {
 	rp.requestedHeadersMu.Lock()
 	rp.requestedHeaders = nil
+	rp.requestedHeadersLoc = nil
 	rp.requestedHeadersMu.Unlock()
 }
 
@@ -1146,6 +1149,29 @@ func (rp *RemotePeer) receivedHeaders(ctx context.Context, msg *wire.MsgHeaders)
 		return
 	}
 
+	// The parent of the first header (if there is one) MUST be one of the
+	// block locators we used to request headers from the peer when this
+	// is a response to a getheaders request.
+	if len(msg.Headers) > 0 && rp.requestedHeadersLoc != nil {
+		wantParent := msg.Headers[0].PrevBlock
+		contains := false
+		for _, loc := range rp.requestedHeadersLoc {
+			if *loc == wantParent {
+				contains = true
+				break
+			}
+		}
+		if !contains {
+			op := errors.Opf(opf, rp.raddr)
+			err := errors.E(op, errors.Protocol,
+				"peer sent headers that do not connect "+
+					"to block locators")
+			rp.Disconnect(err)
+			rp.requestedHeadersMu.Unlock()
+			return
+		}
+	}
+
 	// Sanity check the headers connect to each other in sequence.
 	var prevHash chainhash.Hash
 	var prevHeight uint32
@@ -1186,6 +1212,7 @@ func (rp *RemotePeer) receivedHeaders(ctx context.Context, msg *wire.MsgHeaders)
 	// Headers as a response to getheaders.
 	c := rp.requestedHeaders
 	rp.requestedHeaders = nil
+	rp.requestedHeadersLoc = nil
 	rp.requestedHeadersMu.Unlock()
 	select {
 	case <-ctx.Done():
@@ -1988,7 +2015,7 @@ func (rp *RemotePeer) Headers(ctx context.Context, blockLocators []*chainhash.Ha
 		HashStop:           *hashStop,
 	}
 	c := make(chan *wire.MsgHeaders, 1)
-	sendheaders, newRequest := rp.addRequestedHeaders(c)
+	sendheaders, newRequest := rp.addRequestedHeaders(c, blockLocators)
 	if sendheaders {
 		op := errors.Opf(opf, rp.raddr)
 		return nil, errors.E(op, errors.Invalid, "synchronous getheaders after sendheaders is unsupported")
@@ -2020,28 +2047,6 @@ func (rp *RemotePeer) Headers(ctx context.Context, blockLocators []*chainhash.Ha
 		case m := <-c:
 			stalled.Stop()
 
-			// The parent of the first header (if there is one) MUST
-			// be one of the block locators we used to request
-			// headers from the peer.
-			if len(m.Headers) > 0 {
-				wantParent := m.Headers[0].PrevBlock
-				contains := false
-				for _, loc := range blockLocators {
-					if *loc == wantParent {
-						contains = true
-						break
-					}
-				}
-				if !contains {
-					op := errors.Opf(opf, rp.raddr)
-					err := errors.E(op, errors.Protocol,
-						"peer sent headers that do not connect "+
-							"to block locators")
-					rp.Disconnect(err)
-					return nil, err
-				}
-			}
-
 			return m.Headers, nil
 		}
 	}