strengthen priority tree code and add tests

The priority tree codes has been strengthened to handle some new
corner cases.

- Handle setting the parent to itself.
- Handle exclusive reprioritization to the root stream.
- Handle reprioritization to stream dependent of stream being
reprioritized.

In addition, split out the adjustment function so that it can be
tested independently of the serverConn and add tests.

priority_test.go

@@ -0,0 +1,121 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+// See https://code.google.com/p/go/source/browse/CONTRIBUTORS
+// Licensed under the same terms as Go itself:
+// https://code.google.com/p/go/source/browse/LICENSE
+
+package http2
+
+import (
+	"testing"
+)
+
+func TestPriority(t *testing.T) {
+	// A -> B
+	// move A's parent to B
+	streams := make(map[uint32]*stream)
+	a := &stream{
+		parent: nil,
+		weight: 16,
+	}
+	streams[1] = a
+	b := &stream{
+		parent: a,
+		weight: 16,
+	}
+	streams[2] = b
+	adjustStreamPriority(streams, 1, PriorityParam{
+		Weight:    20,
+		StreamDep: 2,
+	})
+	if a.parent != b {
+		t.Errorf("Expected A's parent to be B")
+	}
+	if a.weight != 20 {
+		t.Errorf("Expected A's weight to be 20; got %d", a.weight)
+	}
+	if b.parent != nil {
+		t.Errorf("Expected B to have no parent")
+	}
+	if b.weight != 16 {
+		t.Errorf("Expected B's weight to be 16; got %d", b.weight)
+	}
+}
+
+func TestPriorityExclusiveZero(t *testing.T) {
+	// A B and C are all children of the 0 stream.
+	// Exclusive reprioritization to any of the streams
+	// should bring the rest of the streams under the
+	// reprioritized stream
+	streams := make(map[uint32]*stream)
+	a := &stream{
+		parent: nil,
+		weight: 16,
+	}
+	streams[1] = a
+	b := &stream{
+		parent: nil,
+		weight: 16,
+	}
+	streams[2] = b
+	c := &stream{
+		parent: nil,
+		weight: 16,
+	}
+	streams[3] = c
+	adjustStreamPriority(streams, 3, PriorityParam{
+		Weight:    20,
+		StreamDep: 0,
+		Exclusive: true,
+	})
+	if a.parent != c {
+		t.Errorf("Expected A's parent to be C")
+	}
+	if a.weight != 16 {
+		t.Errorf("Expected A's weight to be 16; got %d", a.weight)
+	}
+	if b.parent != c {
+		t.Errorf("Expected B's parent to be C")
+	}
+	if b.weight != 16 {
+		t.Errorf("Expected B's weight to be 16; got %d", b.weight)
+	}
+	if c.parent != nil {
+		t.Errorf("Expected C to have no parent")
+	}
+	if c.weight != 20 {
+		t.Errorf("Expected C's weight to be 20; got %d", b.weight)
+	}
+}
+
+func TestPriorityOwnParent(t *testing.T) {
+	streams := make(map[uint32]*stream)
+	a := &stream{
+		parent: nil,
+		weight: 16,
+	}
+	streams[1] = a
+	b := &stream{
+		parent: a,
+		weight: 16,
+	}
+	streams[2] = b
+	adjustStreamPriority(streams, 1, PriorityParam{
+		Weight:    20,
+		StreamDep: 1,
+	})
+	if a.parent != nil {
+		t.Errorf("Expected A's parent to be nil")
+	}
+	if a.weight != 20 {
+		t.Errorf("Expected A's weight to be 20; got %d", a.weight)
+	}
+	if b.parent != a {
+		t.Errorf("Expected B's parent to be A")
+	}
+	if b.weight != 16 {
+		t.Errorf("Expected B's weight to be 16; got %d", b.weight)
+	}
+
+}

server.go

@@ -1213,7 +1213,7 @@ func (sc *serverConn) processHeaders(f *HeadersFrame) error {
 
 	sc.streams[id] = st
 	if f.HasPriority() {
-		sc.adjustStreamPriority(st.id, f.Priority)
+		adjustStreamPriority(sc.streams, st.id, f.Priority)
 	}
 	sc.curOpenStreams++
 	sc.req = requestParam{
@@ -1276,13 +1276,12 @@ func (sc *serverConn) processHeaderBlockFragment(st *stream, frag []byte, end bo
 }
 
 func (sc *serverConn) processPriority(f *PriorityFrame) error {
-	sc.adjustStreamPriority(f.StreamID, f.PriorityParam)
+	adjustStreamPriority(sc.streams, f.StreamID, f.PriorityParam)
 	return nil
 }
 
-func (sc *serverConn) adjustStreamPriority(streamID uint32, priority PriorityParam) {
-	// TODO: untested
-	st, ok := sc.streams[streamID]
+func adjustStreamPriority(streams map[uint32]*stream, streamID uint32, priority PriorityParam) {
+	st, ok := streams[streamID]
 	if !ok {
 		// TODO: not quite correct (this streamID might
 		// already exist in the dep tree, but be closed), but
@@ -1290,10 +1289,27 @@ func (sc *serverConn) adjustStreamPriority(streamID uint32, priority PriorityPar
 		return
 	}
 	st.weight = priority.Weight
-	st.parent = sc.streams[priority.StreamDep] // might be nil
-	if priority.Exclusive && st.parent != nil {
-		for _, openStream := range sc.streams {
-			if openStream.parent == st.parent {
+	parent := streams[priority.StreamDep] // might be nil
+	if parent == st {
+		// if client tries to set this stream to be the parent of itself
+		// ignore and keep going
+		return
+	}
+
+	// section 5.3.3: If a stream is made dependent on one of its
+	// own dependencies, the formerly dependent stream is first
+	// moved to be dependent on the reprioritized stream's previous
+	// parent. The moved dependency retains its weight.
+	for piter := parent; piter != nil; piter = piter.parent {
+		if piter == st {
+			parent.parent = st.parent
+			break
+		}
+	}
+	st.parent = parent
+	if priority.Exclusive && (st.parent != nil || priority.StreamDep == 0) {
+		for _, openStream := range streams {
+			if openStream != st && openStream.parent == st.parent {
 				openStream.parent = st
 			}
 		}