http: don't list parent directory when pointing at a single file

When an HTTP URL points to a single file, listing the parent
directory is unnecessary and may fail entirely on servers that
disable directory listings but still serve HEAD/GET on the file.

Remember the file name in the Fs and short-circuit List to return
just that one object.

See: https://forum.rclone.org/t/how-to-combine-on-the-fly-http-archive-remote-to-list-crc32s-in-a-http-hosted-zip/53761

backend/http/http.go

@@ -156,6 +156,7 @@ type Fs struct {
 	endpoint    *url.URL
 	endpointURL string // endpoint as a string
 	httpClient  *http.Client
+	fileName    string // set if we are pointing to a file
 }
 
 // Object is a remote object that has been stat'd (so it exists, but is not necessarily open for reading)
@@ -297,6 +298,7 @@ func (f *Fs) httpConnection(ctx context.Context, opt *Options) (isFile bool, err
 
 	if isFile {
 		// Correct root if definitely pointing to a file
+		f.fileName = path.Base(f.root)
 		f.root = path.Dir(f.root)
 		if f.root == "." || f.root == "/" {
 			f.root = ""
@@ -564,6 +566,17 @@ func (f *Fs) readDir(ctx context.Context, dir string) (names []string, err error
 // This should return ErrDirNotFound if the directory isn't
 // found.
 func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err error) {
+	// pointed at a single file: only that file is visible
+	if f.fileName != "" {
+		if dir != "" {
+			return nil, fs.ErrorDirNotFound
+		}
+		obj, err := f.NewObject(ctx, f.fileName)
+		if err != nil {
+			return nil, err
+		}
+		return fs.DirEntries{obj}, nil
+	}
 	if !strings.HasSuffix(dir, "/") && dir != "" {
 		dir += "/"
 	}

backend/http/http_internal_test.go

@@ -331,7 +331,19 @@ func TestIsAFileRoot(t *testing.T) {
 	f, err := NewFs(context.Background(), remoteName, "one%.txt", m)
 	assert.Equal(t, err, fs.ErrorIsFile)
 
-	testListRoot(t, f, false)
+	entries, err := f.List(context.Background(), "")
+	require.NoError(t, err)
+
+	require.Equal(t, 1, len(entries))
+
+	e := entries[0]
+	assert.Equal(t, "one%.txt", e.Remote())
+	assert.Equal(t, int64(5+lineEndSize), e.Size())
+	_, ok := e.(*Object)
+	assert.True(t, ok)
+
+	_, err = f.List(context.Background(), "anysub")
+	assert.Equal(t, fs.ErrorDirNotFound, err)
 }
 
 func TestIsAFileSubDir(t *testing.T) {