trie: fixup cachegen overflow at request of nick

trie/hasher.go

@@ -58,6 +58,8 @@ func (h *hasher) hash(n node, db DatabaseWriter, force bool) (node, node, error)
 			return hash, n, nil
 		}
 		if n.canUnload(h.cachegen, h.cachelimit) {
+			// Evict the node from cache. All of its subnodes will have a lower or equal
+			// cache generation number.
 			return hash, hash, nil
 		}
 		if !dirty {

trie/node.go

@@ -64,17 +64,11 @@ type nodeFlag struct {
 
 // canUnload tells whether a node can be unloaded.
 func (n *nodeFlag) canUnload(cachegen, cachelimit uint16) bool {
-	var dist uint16
+	return !n.dirty && cachegen-n.gen >= cachelimit
-	if n.gen > cachegen {
-		dist = n.gen - cachegen
-	} else {
-		dist = cachegen - n.gen
-	}
-	return !n.dirty && dist > cachelimit
 }
 
-func (n *fullNode) canUnload(gen, limit uint16) bool  { return (&n.flags).canUnload(gen, limit) }
+func (n *fullNode) canUnload(gen, limit uint16) bool  { return n.flags.canUnload(gen, limit) }
-func (n *shortNode) canUnload(gen, limit uint16) bool { return (&n.flags).canUnload(gen, limit) }
+func (n *shortNode) canUnload(gen, limit uint16) bool { return n.flags.canUnload(gen, limit) }
 func (n hashNode) canUnload(uint16, uint16) bool      { return false }
 func (n valueNode) canUnload(uint16, uint16) bool     { return false }
 

trie/node_test.go

@@ -0,0 +1,58 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package trie
+
+import "testing"
+
+func TestCanUnload(t *testing.T) {
+	tests := []struct {
+		flag                 nodeFlag
+		cachegen, cachelimit uint16
+		want                 bool
+	}{
+		{
+			flag: nodeFlag{dirty: true, gen: 0},
+			want: false,
+		},
+		{
+			flag:     nodeFlag{dirty: false, gen: 0},
+			cachegen: 0, cachelimit: 0,
+			want: true,
+		},
+		{
+			flag:     nodeFlag{dirty: false, gen: 65534},
+			cachegen: 65535, cachelimit: 1,
+			want: true,
+		},
+		{
+			flag:     nodeFlag{dirty: false, gen: 65534},
+			cachegen: 0, cachelimit: 1,
+			want: true,
+		},
+		{
+			flag:     nodeFlag{dirty: false, gen: 1},
+			cachegen: 65535, cachelimit: 1,
+			want: true,
+		},
+	}
+
+	for _, test := range tests {
+		if got := test.flag.canUnload(test.cachegen, test.cachelimit); got != test.want {
+			t.Errorf("%+v\n   got %t, want %t", test, got, test.want)
+		}
+	}
+}