cmd/utils, node: switch to Pebble as the default db if none exists (#27136)

* cmd/utils, node: switch to Pebble as the default db if none exists

* node: fall back to LevelDB on platforms not supporting Pebble

* core/rawdb, node: default to Pebble at the node level

* cmd/geth: fix some tests explicitly using leveldb

* ethdb/pebble: allow double closes, makes tests simpler

cmd/geth/dao_test.go

@@ -121,7 +121,7 @@ func testDAOForkBlockNewChain(t *testing.T, test int, genesis string, expectBloc
 	}
 	// Retrieve the DAO config flag from the database
 	path := filepath.Join(datadir, "geth", "chaindata")
-	db, err := rawdb.NewLevelDBDatabase(path, 0, 0, "", false)
+	db, err := rawdb.NewPebbleDBDatabase(path, 0, 0, "", false)
 	if err != nil {
 		t.Fatalf("test %d: failed to open test database: %v", test, err)
 	}

cmd/geth/genesis_test.go

@@ -146,8 +146,8 @@ func TestCustomBackend(t *testing.T) {
 		return nil
 	}
 	for i, tt := range []backendTest{
-		{ // When not specified, it should default to leveldb
-			execArgs:   []string{"--db.engine", "leveldb"},
+		{ // When not specified, it should default to pebble
+			execArgs:   []string{"--db.engine", "pebble"},
 			execExpect: "0x0000000000001338",
 		},
 		{ // Explicit leveldb

cmd/utils/flags.go

@@ -101,8 +101,8 @@ var (
 	}
 	DBEngineFlag = &cli.StringFlag{
 		Name:     "db.engine",
-		Usage:    "Backing database implementation to use ('leveldb' or 'pebble')",
-		Value:    "leveldb",
+		Usage:    "Backing database implementation to use ('pebble' or 'leveldb')",
+		Value:    node.DefaultConfig.DBEngine,
 		Category: flags.EthCategory,
 	}
 	AncientFlag = &flags.DirectoryFlag{