core: write test showing that TD is not stored properly at genesis

The ToBlock method applies a default value for an empty
difficulty value. This default is not carried over through the Commit
method because the TotalDifficulty database write writes the
original difficulty value (nil) instead of the defaulty value
present on the genesis Block.

Date: 2021-10-22 08:25:32-07:00
Signed-off-by: meows <b5c6@protonmail.com>

core/genesis_test.go

@@ -209,3 +209,33 @@ func TestGenesisHashes(t *testing.T) {
 		}
 	}
 }
+
+func TestGenesis_Commit(t *testing.T) {
+	genesis := &Genesis{
+		BaseFee: big.NewInt(params.InitialBaseFee),
+		Config:  params.TestChainConfig,
+		// difficulty is nil
+	}
+
+	db := rawdb.NewMemoryDatabase()
+	genesisBlock, err := genesis.Commit(db)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if genesis.Difficulty != nil {
+		t.Fatalf("assumption wrong")
+	}
+
+	// This value should have been set as default in the ToBlock method.
+	if genesisBlock.Difficulty().Cmp(params.GenesisDifficulty) != 0 {
+		t.Errorf("assumption wrong: want: %d, got: %v", params.GenesisDifficulty, genesisBlock.Difficulty())
+	}
+
+	// Expect the stored total difficulty to be the difficulty of the genesis block.
+	stored := rawdb.ReadTd(db, genesisBlock.Hash(), genesisBlock.NumberU64())
+
+	if stored.Cmp(genesisBlock.Difficulty()) != 0 {
+		t.Errorf("inequal difficulty; stored: %v, genesisBlock: %v", stored, genesisBlock.Difficulty())
+	}
+}