Fix race in patricia tree

This commit fixes a major issue added in 703b087.

There are still wins on allocation though.

benchmark                      old ns/op     new ns/op     delta
BenchmarkCMuxConnHTTP1-4       783           836           +6.77%
BenchmarkCMuxConnHTTP2-4       895           806           -9.94%
BenchmarkCMuxConnHTTP1n2-4     1000          1026          +2.60%
BenchmarkCMuxConnHTTP2n1-4     916           961           +4.91%

benchmark                      old allocs     new allocs     delta
BenchmarkCMuxConnHTTP1-4       3              4              +33.33%
BenchmarkCMuxConnHTTP2-4       4              4              +0.00%
BenchmarkCMuxConnHTTP1n2-4     4              5              +25.00%
BenchmarkCMuxConnHTTP2n1-4     4              5              +25.00%

benchmark                      old bytes     new bytes     delta
BenchmarkCMuxConnHTTP1-4       272           280           +2.94%
BenchmarkCMuxConnHTTP2-4       304           304           +0.00%
BenchmarkCMuxConnHTTP1n2-4     304           312           +2.63%
BenchmarkCMuxConnHTTP2n1-4     304           312           +2.63%

patricia.go

@@ -22,8 +22,8 @@ import (
 // patriciaTree is a simple patricia tree that handles []byte instead of string
 // and cannot be changed after instantiation.
 type patriciaTree struct {
-	root *ptNode
-	buf  []byte // preallocated buffer to read data while matching
+	root     *ptNode
+	maxDepth int // max depth of the tree.
 }
 
 func newPatriciaTree(bs ...[]byte) *patriciaTree {
@@ -34,8 +34,8 @@ func newPatriciaTree(bs ...[]byte) *patriciaTree {
 		}
 	}
 	return &patriciaTree{
-		root: newNode(bs),
-		buf:  make([]byte, max+1),
+		root:     newNode(bs),
+		maxDepth: max + 1,
 	}
 }
 
@@ -48,13 +48,15 @@ func newPatriciaTreeString(strs ...string) *patriciaTree {
 }
 
 func (t *patriciaTree) matchPrefix(r io.Reader) bool {
-	n, _ := io.ReadFull(r, t.buf)
-	return t.root.match(t.buf[:n], true)
+	buf := make([]byte, t.maxDepth)
+	n, _ := io.ReadFull(r, buf)
+	return t.root.match(buf[:n], true)
 }
 
 func (t *patriciaTree) match(r io.Reader) bool {
-	n, _ := io.ReadFull(r, t.buf)
-	return t.root.match(t.buf[:n], false)
+	buf := make([]byte, t.maxDepth)
+	n, _ := io.ReadFull(r, buf)
+	return t.root.match(buf[:n], false)
 }
 
 type ptNode struct {