What causes "expected *os.File, got %T" in AlistGo/alist?

Change calculateSHA256 to accept io.Reader (SHA-256 needs only sequential reads) and drop the *os.File requirement entirely

AlistGo/alist · error

expected *os.File, got %T

Error message

expected *os.File, got %T

What it means

PutResult calls file.CacheFullInTempFile(), which returns model.File — an interface, not a concrete *os.File. Per internal/stream/stream.go, the returned value is either the stream's pre-set *os.File tmp file, OR the stream's original Reader when that Reader already implements model.File (e.g. a driver-supplied seekable file or an in-memory File implementation). The unconditional type assertion tempFile.(*os.File) fails for any non-*os.File implementation.

Source

Thrown at drivers/mediafire/driver.go:348

	return nil
}

func (d *Mediafire) Put(ctx context.Context, dstDir model.Obj, file model.FileStreamer, up driver.UpdateProgress) error {
	_, err := d.PutResult(ctx, dstDir, file, up)
	return err
}

func (d *Mediafire) PutResult(ctx context.Context, dstDir model.Obj, file model.FileStreamer, up driver.UpdateProgress) (model.Obj, error) {

	tempFile, err := file.CacheFullInTempFile()
	if err != nil {
		return nil, err
	}
	defer tempFile.Close()

	osFile, ok := tempFile.(*os.File)
	if !ok {
		return nil, fmt.Errorf("expected *os.File, got %T", tempFile)
	}

	fileHash, err := d.calculateSHA256(osFile)
	if err != nil {
		return nil, err
	}

	checkResp, err := d.uploadCheck(ctx, file.GetName(), file.GetSize(), fileHash, dstDir.GetID())
	if err != nil {
		return nil, err
	}

	if checkResp.Response.ResumableUpload.AllUnitsReady == "yes" {
		up(100.0)
	}

	if checkResp.Response.HashExists == "yes" && checkResp.Response.InAccount == "yes" {
		up(100.0)

View on GitHub (pinned to 843d9dc814)

Solutions

Change calculateSHA256 to accept io.Reader (SHA-256 needs only sequential reads) and drop the *os.File requirement entirely
Alternatively use the model.File interface and only Rewind/Seek when it exposes it
If an *os.File is truly required (e.g. for upload resumption), wrap non-*os.File values by copying to a temp file first

Example fix

// before: hard type assertion on an interface return
osFile, ok := tempFile.(*os.File)
if !ok {
    return nil, fmt.Errorf("expected *os.File, got %T", tempFile)
}
fileHash, err := d.calculateSHA256(osFile)

// after: hash any reader, no assertion needed
if seeker, ok := tempFile.(io.Seeker); ok {
    _, _ = seeker.Seek(0, io.SeekStart)
}
fileHash, err := d.calculateSHA256(tempFile) // signature: (r io.Reader) (string, error)

Defensive patterns

Strategy: type-guard

Type guard

// accept any model.File; only seek when possible
func toHashReader(f model.File) io.Reader {
    if s, ok := f.(io.Seeker); ok {
        _, _ = s.Seek(0, io.SeekStart)
    }
    return f
}

Try / catch

Replace the panic-prone unconditional assertion with the two-value form and a graceful path: if osFile, ok := tempFile.(*os.File); ok { use it } else { rewind via io.Seeker and hash the reader directly } — the hash never needed *os.File.

Prevention

Never hard-assert concrete types on interface returns from framework APIs (CacheFullInTempFile returns model.File)
Write helpers like calculateSHA256 against io.Reader, the narrowest interface you need
Test uploads with mock FileStreamers that return non-*os.File model.File implementations

When it happens

Trigger: Uploading via a FileStreamer whose Reader is a custom model.File (in-memory buffer, another driver's file handle, or a SeekableStream wrapping a non-temp reader) — CacheFullInTempFile returns it as-is without copying to an *os.File, and the assertion panics path fails.

Common situations: Server-side copy between storages, or upload pipelines where the stream originates from another driver rather than an HTTP multipart upload; also unit tests with mock FileStreamers.

Related errors

AI-assisted analysis of AlistGo/alist@843d9dc814 (2026-08-15). Data as JSON: /api/errors/be8f4b1e3ce43328. Report an issue: GitHub.