util/sha256x: make Hash.Sum non-escaping (#5338)

Since Hash is a concrete type, we can make it such that
Sum never escapes the input.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/sha256x/sha256.go

@@ -45,10 +45,19 @@ func (h *Hash) Sum(b []byte) []byte {
 		h.h.Write(h.x[:h.nx])
 		h.nx = 0
 	}
-	return h.h.Sum(b)
+
+	// Unfortunately hash.Hash.Sum always causes the input to escape since
+	// escape analysis cannot prove anything past an interface method call.
+	// Assuming h already escapes, we call Sum with h.x first,
+	// and then the copy the result to b.
+	sum := h.h.Sum(h.x[:0])
+	return append(b, sum...)
 }
 
 func (h *Hash) Reset() {
+	if h.h == nil {
+		h.h = sha256.New()
+	}
 	h.h.Reset()
 	h.nx = 0
 }

util/sha256x/sha256_test.go

@@ -64,6 +64,7 @@ func hashSuite(h hasher) {
 		h.HashBytes(bytes[:(i+1)*13])
 	}
 }
+
 func Test(t *testing.T) {
 	c := qt.New(t)
 	h1 := New()
@@ -73,6 +74,16 @@ func Test(t *testing.T) {
 	c.Assert(h1.Sum(nil), qt.DeepEquals, h2.Sum(nil))
 }
 
+func TestSumAllocations(t *testing.T) {
+	c := qt.New(t)
+	h := New()
+	n := testing.AllocsPerRun(100, func() {
+		var a [sha256.Size]byte
+		h.Sum(a[:0])
+	})
+	c.Assert(n, qt.Equals, 0.0)
+}
+
 func Fuzz(f *testing.F) {
 	f.Fuzz(func(t *testing.T, seed int64) {
 		c := qt.New(t)