lib/protocol: Use BlocksHash to compare block lists when available (#6401)

This is an optimization for faster equal checks on block lists.

lib/protocol/protocol_test.go

@@ -867,3 +867,51 @@ func TestDispatcherToCloseDeadlock(t *testing.T) {
 		t.Fatal("timed out before dispatcher loop terminated")
 	}
 }
+
+func TestBlocksEqual(t *testing.T) {
+	blocksOne := []BlockInfo{{Hash: []byte{1, 2, 3, 4}}}
+	blocksTwo := []BlockInfo{{Hash: []byte{5, 6, 7, 8}}}
+	hashOne := []byte{42, 42, 42, 42}
+	hashTwo := []byte{29, 29, 29, 29}
+
+	cases := []struct {
+		b1 []BlockInfo
+		h1 []byte
+		b2 []BlockInfo
+		h2 []byte
+		eq bool
+	}{
+		{blocksOne, hashOne, blocksOne, hashOne, true},  // everything equal
+		{blocksOne, hashOne, blocksTwo, hashTwo, false}, // nothing equal
+		{blocksOne, hashOne, blocksOne, nil, true},      // blocks compared
+		{blocksOne, nil, blocksOne, nil, true},          // blocks compared
+		{blocksOne, nil, blocksTwo, nil, false},         // blocks compared
+		{blocksOne, hashOne, blocksTwo, hashOne, true},  // hashes equal, blocks not looked at
+		{blocksOne, hashOne, blocksOne, hashTwo, false}, // hashes different, blocks not looked at
+		{blocksOne, hashOne, nil, nil, false},           // blocks is different from no blocks
+		{blocksOne, nil, nil, nil, false},               // blocks is different from no blocks
+		{nil, hashOne, nil, nil, true},                  // nil blocks are equal, even of one side has a hash
+	}
+
+	for _, tc := range cases {
+		f1 := FileInfo{Blocks: tc.b1, BlocksHash: tc.h1}
+		f2 := FileInfo{Blocks: tc.b2, BlocksHash: tc.h2}
+
+		if !f1.BlocksEqual(f1) {
+			t.Error("f1 is always equal to itself", f1)
+		}
+		if !f2.BlocksEqual(f2) {
+			t.Error("f2 is always equal to itself", f2)
+		}
+		if res := f1.BlocksEqual(f2); res != tc.eq {
+			t.Log("f1", f1.BlocksHash, f1.Blocks)
+			t.Log("f2", f2.BlocksHash, f2.Blocks)
+			t.Errorf("f1.BlocksEqual(f2) == %v but should be %v", res, tc.eq)
+		}
+		if res := f2.BlocksEqual(f1); res != tc.eq {
+			t.Log("f1", f1.BlocksHash, f1.Blocks)
+			t.Log("f2", f2.BlocksHash, f2.Blocks)
+			t.Errorf("f2.BlocksEqual(f1) == %v but should be %v", res, tc.eq)
+		}
+	}
+}