rc: Add GetFsNamedFileOK to get an fs which could also be a file

fs/rc/cache.go

@@ -5,27 +5,71 @@ package rc
 import (
 	"context"
 	"errors"
+	"fmt"
 
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/cache"
 	"github.com/rclone/rclone/fs/config/configmap"
+	"github.com/rclone/rclone/fs/filter"
+	"github.com/rclone/rclone/fs/fspath"
 )
 
-// GetFsNamed gets an fs.Fs named fsName either from the cache or creates it afresh
-func GetFsNamed(ctx context.Context, in Params, fsName string) (f fs.Fs, err error) {
-	fsString, err := in.GetString(fsName)
+// getFsName gets an fs name from fsName either from the cache or direct
+func getFsName(in Params, fsName string) (fsString string, err error) {
+	fsString, err = in.GetString(fsName)
 	if err != nil {
 		if !IsErrParamInvalid(err) {
-			return nil, err
+			return fsString, err
 		}
 		fsString, err = getConfigMap(in, fsName)
 		if err != nil {
-			return nil, err
+			return fsString, err
 		}
 	}
+	return fsString, err
+}
+
+// GetFsNamed gets an fs.Fs named fsName either from the cache or creates it afresh
+func GetFsNamed(ctx context.Context, in Params, fsName string) (f fs.Fs, err error) {
+	fsString, err := getFsName(in, fsName)
+	if err != nil {
+		return nil, err
+	}
 	return cache.Get(ctx, fsString)
 }
 
+// GetFsNamedFileOK gets an fs.Fs named fsName either from the cache or creates it afresh
+//
+// If the fs.Fs points to a single file then it returns a new ctx with
+// filters applied to make the listings return only that file.
+func GetFsNamedFileOK(ctx context.Context, in Params, fsName string) (newCtx context.Context, f fs.Fs, err error) {
+	fsString, err := getFsName(in, fsName)
+	if err != nil {
+		return ctx, nil, err
+	}
+	f, err = cache.Get(ctx, fsString)
+	if err == nil {
+		return ctx, f, nil
+	} else if !errors.Is(err, fs.ErrorIsFile) {
+		return ctx, nil, err
+	}
+	// f points to the directory above the file so find the remote name
+	_, fileName, err := fspath.Split(fsString)
+	if err != nil {
+		return ctx, f, err
+	}
+	ctx, fi := filter.AddConfig(ctx)
+	if !fi.InActive() {
+		return ctx, f, fmt.Errorf("can't limit to single files when using filters: %q", fileName)
+	}
+	// Limit transfers to this file
+	err = fi.AddFile(fileName)
+	if err != nil {
+		return ctx, f, fmt.Errorf("failed to limit to single file: %w", err)
+	}
+	return ctx, f, nil
+}
+
 // getConfigMap gets the config as a map from in and converts it to a
 // config string
 //