internal/restic: Don't allocate in Tree.Insert

name         old time/op    new time/op    delta
BuildTree-8    34.6µs ± 4%     7.0µs ± 3%  -79.68%  (p=0.000 n=18+19)

name         old alloc/op   new alloc/op   delta
BuildTree-8    34.0kB ± 0%     0.9kB ± 0%  -97.37%  (p=0.000 n=20+20)

name         old allocs/op  new allocs/op  delta
BuildTree-8       108 ± 0%         1 ± 0%  -99.07%  (p=0.000 n=20+20)

internal/restic/tree.go

@@ -14,10 +14,10 @@ type Tree struct {
 	Nodes []*Node `json:"nodes"`
 }
 
-// NewTree creates a new tree object.
-func NewTree() *Tree {
+// NewTree creates a new tree object with the given initial capacity.
+func NewTree(capacity int) *Tree {
 	return &Tree{
-		Nodes: []*Node{},
+		Nodes: make([]*Node, 0, capacity),
 	}
 }
 
@@ -51,8 +51,8 @@ func (t *Tree) Insert(node *Node) error {
 		return errors.Errorf("node %q already present", node.Name)
 	}
 
-	// https://code.google.com/p/go-wiki/wiki/SliceTricks
-	t.Nodes = append(t.Nodes, &Node{})
+	// https://github.com/golang/go/wiki/SliceTricks
+	t.Nodes = append(t.Nodes, nil)
 	copy(t.Nodes[pos+1:], t.Nodes[pos:])
 	t.Nodes[pos] = node
 

internal/restic/tree_test.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strconv"
 	"testing"
 
 	"github.com/restic/restic/internal/repository"
@@ -98,7 +99,7 @@ func TestLoadTree(t *testing.T) {
 	defer cleanup()
 
 	// save tree
-	tree := restic.NewTree()
+	tree := restic.NewTree(0)
 	id, err := repo.SaveTree(context.TODO(), tree)
 	rtest.OK(t, err)
 
@@ -113,3 +114,24 @@ func TestLoadTree(t *testing.T) {
 		"trees are not equal: want %v, got %v",
 		tree, tree2)
 }
+
+func BenchmarkBuildTree(b *testing.B) {
+	const size = 100 // Directories of this size are not uncommon.
+
+	nodes := make([]restic.Node, size)
+	for i := range nodes {
+		// Archiver.SaveTree inputs in sorted order, so do that here too.
+		nodes[i].Name = strconv.Itoa(i)
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		t := restic.NewTree(size)
+
+		for i := range nodes {
+			_ = t.Insert(&nodes[i])
+		}
+	}
+}