commaai/openpilot · error · ValueError

Unsupported compression type: {compression}

Error message

Unsupported compression type: {compression}

What it means

make_decompressor() only accepts 'bz2' and 'zst'. decompress_file() passes either the caller-supplied compression argument or the value sniffed from the 4-byte header (compression_type returns None for unknown magic). A None or unknown value reaching make_decompressor raises this ValueError, meaning the caller forced a bad compression name or the file magic was unrecognized and None propagated.

Source

Thrown at openpilot/tools/lib/file_downloader.py:66

  if compression:
    url_without_query = f"decompressed-{compression}:{url_without_query}"
  return os.path.join(Paths.download_cache_root(), hashlib.sha256(url_without_query.encode()).hexdigest())


def compression_type(data):
  if data.startswith(b'BZh'):
    return 'bz2'
  if data.startswith(b'\x28\xb5\x2f\xfd'):
    return 'zst'
  return None


def make_decompressor(compression):
  if compression == 'bz2':
    return bz2.BZ2Decompressor()
  if compression == 'zst':
    return zstd.ZstdDecompressor().decompressobj()
  raise ValueError(f"Unsupported compression type: {compression}")


def decompress_file(source, destination, compression=None):
  with open(source, 'rb') as src, open(destination, 'wb') as dst:
    header = src.read(4)
    compression = compression or compression_type(header)
    decompressor = make_decompressor(compression)
    dst.write(decompressor.decompress(header))
    while data := src.read(1024 * 1024):
      dst.write(decompressor.decompress(data))
  if not decompressor.eof:
    raise EOFError(f"Compressed {compression} file ended before the end-of-stream marker")


def materialize_cached_file(source, url, compression):
  local_path = cache_file_path(url, compression)
  if os.path.exists(local_path):
    return local_path

View on GitHub (pinned to 516ec1e682)

Solutions

1. If you know the true format, pass it explicitly: decompress_file(src, dst, compression='bz2') - but only bz2/zst are supported
2. For gzip/xz, decompress outside this helper (gzip module / lzma module) - it deliberately supports only bz2 and zstd
3. Check the first 4 bytes of the file (xxd | head) to see whether the magic matches what you claimed

Example fix

# before
decompress_file(src, dst, compression='gzip')

# after (gzip is unsupported here - use the stdlib)
import gzip
with gzip.open(src, 'rb') as f_in, open(dst, 'wb') as f_out:
    shutil.copyfileobj(f_in, f_out)

comp = compression if compression else compression_type(open(source, 'rb').read(4))
assert comp in ('bz2', 'zst'), f"unsupported/undetected compression {comp!r}; this helper handles only bz2 and zst"

def is_supported_compression(name: str | None) -> bool:
    """True when name is one of the decompressor's supported formats."""
    return name in ('bz2', 'zst')

try:
    decompress_file(src, dst, compression=comp)
except ValueError as e:
    if 'Unsupported compression' in str(e):
        raise SystemExit(f'{comp} not supported; use gzip/lzma modules for other formats')
    raise

When it happens

Trigger: Calling decompress_file(..., compression='gzip') or 'xz'; passing compression=None on a file whose first 4 bytes are neither BZh ('BZh') nor the zstd magic 0x28B52FFD; corrupt header bytes.

Common situations: Server starts serving gzip/plain data while the client assumed bz2; truncated download losing the magic bytes; hardcoded compression name drifting from actual format.

Related errors

• invalid config backup: {backup}
• No camera file for segment {seg_idx}
• No valid camera paths
• Function body is empty
• No reference series found. Set an input timeseries or return

AI-assisted analysis of commaai/openpilot@516ec1e682 (2026-08-15). Data as JSON: /api/errors/4f53d589bd9aac78. Report an issue: GitHub.