cmd/geth: fixes db unavailability for chain commands (ethereum#21415)

* chaincmd should make config nodes instead of full nodes

* add documentation for using makeConfigNode instead of makeFullNode;

* add documentation to functions

* code style

cmd/geth/chaincmd.go

@@ -239,8 +239,7 @@ func initGenesis(ctx *cli.Context) error {
 	if err := json.NewDecoder(file).Decode(genesis); err != nil {
 		utils.Fatalf("invalid genesis file: %v", err)
 	}
-
-	// Open an initialise both full and light databases
+	// Open and initialise both full and light databases
 	stack, _ := makeConfigNode(ctx)
 	defer stack.Close()
 
@@ -409,7 +408,8 @@ func importPreimages(ctx *cli.Context) error {
 	if len(ctx.Args()) < 1 {
 		utils.Fatalf("This command requires an argument.")
 	}
-	stack, _ := makeFullNode(ctx)
+
+	stack, _ := makeConfigNode(ctx)
 	defer stack.Close()
 
 	db := utils.MakeChainDatabase(ctx, stack)
@@ -427,7 +427,8 @@ func exportPreimages(ctx *cli.Context) error {
 	if len(ctx.Args()) < 1 {
 		utils.Fatalf("This command requires an argument.")
 	}
-	stack, _ := makeFullNode(ctx)
+
+	stack, _ := makeConfigNode(ctx)
 	defer stack.Close()
 
 	db := utils.MakeChainDatabase(ctx, stack)
@@ -449,7 +450,7 @@ func copyDb(ctx *cli.Context) error {
 		utils.Fatalf("Source ancient chain directory path argument missing")
 	}
 	// Initialize a new chain for the running node to sync into
-	stack, _ := makeFullNode(ctx)
+	stack, _ := makeConfigNode(ctx)
 	defer stack.Close()
 
 	chain, chainDb := utils.MakeChain(ctx, stack, false)
@@ -557,7 +558,7 @@ func confirmAndRemoveDB(database string, kind string) {
 }
 
 func dump(ctx *cli.Context) error {
-	stack, _ := makeFullNode(ctx)
+	stack, _ := makeConfigNode(ctx)
 	defer stack.Close()
 
 	chain, chainDb := utils.MakeChain(ctx, stack, true)

cmd/geth/config.go

@@ -105,6 +105,7 @@ func defaultNodeConfig() node.Config {
 	return cfg
 }
 
+// makeConfigNode loads geth configuration and creates a blank node instance.
 func makeConfigNode(ctx *cli.Context) (*node.Node, gethConfig) {
 	// Load defaults.
 	cfg := gethConfig{
@@ -145,6 +146,7 @@ func enableWhisper(ctx *cli.Context) bool {
 	return false
 }
 
+// makeFullNode loads geth configuration and creates the Ethereum backend.
 func makeFullNode(ctx *cli.Context) (*node.Node, ethapi.Backend) {
 	stack, cfg := makeConfigNode(ctx)