sumdb/tlog: set the hash of the empty tree according to RFC 6962

Updates FiloSottile/sunlight#14

Change-Id: I712ea53fd3a17b66ec310d8f48de44416d0054cc
Reviewed-on: https://go-review.googlesource.com/c/mod/+/590715
Reviewed-by: Russ Cox <[email protected]>
Auto-Submit: Filippo Valsorda <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Joedian Reid <[email protected]>

sumdb/client.go

@@ -142,6 +142,14 @@ func (c *Client) initWork() {
 	c.verifiers = note.VerifierList(verifier)
 	c.name = verifier.Name()
 
+	if c.latest.N == 0 {
+		c.latest.Hash, err = tlog.TreeHash(0, nil)
+		if err != nil {
+			c.initErr = err
+			return
+		}
+	}
+
 	data, err := c.ops.ReadConfig(c.name + "/latest")
 	if err != nil {
 		c.initErr = err

sumdb/tlog/tlog.go

@@ -234,14 +234,22 @@ func (f HashReaderFunc) ReadHashes(indexes []int64) ([]Hash, error) {
 	return f(indexes)
 }
 
+// emptyHash is the hash of the empty tree, per RFC 6962, Section 2.1.
+// It is the hash of the empty string.
+var emptyHash = Hash{
+	0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14,
+	0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24,
+	0x27, 0xae, 0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c,
+	0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55,
+}
+
 // TreeHash computes the hash for the root of the tree with n records,
 // using the HashReader to obtain previously stored hashes
 // (those returned by StoredHashes during the writes of those n records).
 // TreeHash makes a single call to ReadHash requesting at most 1 + log₂ n hashes.
-// The tree of size zero is defined to have an all-zero Hash.
 func TreeHash(n int64, r HashReader) (Hash, error) {
 	if n == 0 {
-		return Hash{}, nil
+		return emptyHash, nil
 	}
 	indexes := subTreeIndex(0, n, nil)
 	hashes, err := r.ReadHashes(indexes)

sumdb/tlog/tlog_test.go

@@ -6,6 +6,7 @@ package tlog
 
 import (
 	"bytes"
+	"crypto/sha256"
 	"fmt"
 	"testing"
 )
@@ -267,3 +268,13 @@ func TestTilePath(t *testing.T) {
 		}
 	}
 }
+
+func TestEmptyTree(t *testing.T) {
+	h, err := TreeHash(0, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if h != sha256.Sum256(nil) {
+		t.Fatalf("TreeHash(0) = %x, want SHA-256('')", h)
+	}
+}