s3: implement backend set command to update running config

backend/s3/s3.go

@@ -4504,6 +4504,26 @@ supplied.
 It may return "Enabled", "Suspended" or "Unversioned". Note that once versioning
 has been enabled the status can't be set back to "Unversioned".
 `,
+}, {
+	Name:  "set",
+	Short: "Set command for updating the config parameters.",
+	Long: `This set command can be used to update the config parameters
+for a running s3 backend.
+
+Usage Examples:
+
+    rclone backend set s3: [-o opt_name=opt_value] [-o opt_name2=opt_value2]
+    rclone rc backend/command command=set fs=s3: [-o opt_name=opt_value] [-o opt_name2=opt_value2]
+    rclone rc backend/command command=set fs=s3: -o session_token=X -o access_key_id=X -o secret_access_key=X
+
+The option keys are named as they are in the config file.
+
+This rebuilds the connection to the s3 backend when it is called with
+the new parameters. Only new parameters need be passed as the values
+will default to those currently in use.
+
+It doesn't return anything.
+`,
 }}
 
 // Command the backend to run a named command
@@ -4598,6 +4618,25 @@ func (f *Fs) Command(ctx context.Context, name string, arg []string, opt map[str
 		return nil, f.CleanUpHidden(ctx)
 	case "versioning":
 		return f.setGetVersioning(ctx, arg...)
+	case "set":
+		newOpt := f.opt
+		err := configstruct.Set(configmap.Simple(opt), &newOpt)
+		if err != nil {
+			return nil, fmt.Errorf("reading config: %w", err)
+		}
+		c, ses, err := s3Connection(f.ctx, &newOpt, f.srv)
+		if err != nil {
+			return nil, fmt.Errorf("updating session: %w", err)
+		}
+		f.c = c
+		f.ses = ses
+		f.opt = newOpt
+		keys := []string{}
+		for k := range opt {
+			keys = append(keys, k)
+		}
+		fs.Logf(f, "Updated config values: %s", strings.Join(keys, ", "))
+		return nil, nil
 	default:
 		return nil, fs.ErrorCommandNotFound
 	}