Implement backblaze-b2 gateway support (#5002)

Fixes https://github.com/minio/minio/issues/4072

cmd/gateway-main.go

@@ -125,6 +125,31 @@ EXAMPLES:
 
 `
 
+const b2GatewayTemplate = `NAME:
+  {{.HelpName}} - {{.Usage}}
+
+USAGE:
+  {{.HelpName}} {{if .VisibleFlags}}[FLAGS]{{end}}
+{{if .VisibleFlags}}
+FLAGS:
+  {{range .VisibleFlags}}{{.}}
+  {{end}}{{end}}
+ENVIRONMENT VARIABLES:
+  ACCESS:
+     MINIO_ACCESS_KEY: B2 account id.
+     MINIO_SECRET_KEY: B2 application key.
+
+  BROWSER:
+     MINIO_BROWSER: To disable web browser access, set this value to "off".
+
+EXAMPLES:
+  1. Start minio gateway server for B2 backend.
+      $ export MINIO_ACCESS_KEY=accountID
+      $ export MINIO_SECRET_KEY=applicationKey
+      $ {{.HelpName}}
+
+`
+
 var (
 	azureBackendCmd = cli.Command{
 		Name:               "azure",
@@ -143,6 +168,7 @@ var (
 		Flags:              append(serverFlags, globalFlags...),
 		HideHelpCommand:    true,
 	}
+
 	gcsBackendCmd = cli.Command{
 		Name:               "gcs",
 		Usage:              "Google Cloud Storage.",
@@ -152,12 +178,21 @@ var (
 		HideHelpCommand:    true,
 	}
 
+	b2BackendCmd = cli.Command{
+		Name:               "b2",
+		Usage:              "Backblaze B2.",
+		Action:             b2GatewayMain,
+		CustomHelpTemplate: b2GatewayTemplate,
+		Flags:              append(serverFlags, globalFlags...),
+		HideHelpCommand:    true,
+	}
+
 	gatewayCmd = cli.Command{
 		Name:            "gateway",
 		Usage:           "Start object storage gateway.",
 		Flags:           append(serverFlags, globalFlags...),
 		HideHelpCommand: true,
-		Subcommands:     []cli.Command{azureBackendCmd, s3BackendCmd, gcsBackendCmd},
+		Subcommands:     []cli.Command{azureBackendCmd, s3BackendCmd, gcsBackendCmd, b2BackendCmd},
 	}
 )
 
@@ -168,6 +203,7 @@ const (
 	azureBackend gatewayBackend = "azure"
 	s3Backend    gatewayBackend = "s3"
 	gcsBackend   gatewayBackend = "gcs"
+	b2Backend    gatewayBackend = "b2"
 	// Add more backends here.
 )
 
@@ -177,6 +213,7 @@ const (
 // - Azure Blob Storage.
 // - AWS S3.
 // - Google Cloud Storage.
+// - Backblaze B2.
 // - Add your favorite backend here.
 func newGatewayLayer(backendType gatewayBackend, arg string) (GatewayLayer, error) {
 	switch backendType {
@@ -189,6 +226,11 @@ func newGatewayLayer(backendType gatewayBackend, arg string) (GatewayLayer, erro
 		// will be removed when gcs is ready for production use.
 		log.Println(colorYellow("\n               *** Warning: Not Ready for Production ***"))
 		return newGCSGateway(arg)
+	case b2Backend:
+		// FIXME: The following print command is temporary and
+		// will be removed when B2 is ready for production use.
+		log.Println(colorYellow("\n               *** Warning: Not Ready for Production ***"))
+		return newB2Gateway()
 	}
 
 	return nil, fmt.Errorf("Unrecognized backend type %s", backendType)
@@ -285,6 +327,17 @@ func gcsGatewayMain(ctx *cli.Context) {
 	gatewayMain(ctx, gcsBackend)
 }
 
+func b2GatewayMain(ctx *cli.Context) {
+	if ctx.Args().Present() && ctx.Args().First() == "help" {
+		cli.ShowCommandHelpAndExit(ctx, "b2", 1)
+	}
+
+	// Validate gateway arguments.
+	fatalIf(validateGatewayArguments(ctx.GlobalString("address"), ctx.Args().First()), "Invalid argument")
+
+	gatewayMain(ctx, b2Backend)
+}
+
 // Handler for 'minio gateway'.
 func gatewayMain(ctx *cli.Context, backendType gatewayBackend) {
 	// Get quiet flag from command line argument.
@@ -393,6 +446,8 @@ func gatewayMain(ctx *cli.Context, backendType gatewayBackend) {
 			mode = globalMinioModeGatewayGCS
 		case s3Backend:
 			mode = globalMinioModeGatewayS3
+		case b2Backend:
+			mode = globalMinioModeGatewayB2
 		}
 
 		// Check update mode.