dotnet/aspnetcore · error · ValueError

Unsupported compression format: {file_extension}

Error message

Unsupported compression format: {file_extension}

What it means

extract_deb_file chooses a tarfile open mode based on the data.tar extension: .xz -> r:xz, .gz -> r:gz, .zst -> decompress via zstandard then r. Any other extension raises ValueError. This guards against a .deb using a compression the rootfs builder cannot decompress. Modern Debian is moving to .zst (zstd), which IS handled via the zstandard library, but more exotic formats (bz2, lz, lzma, or a future format) are not.

Source

Thrown at eng/common/cross/install-debs.py:332

            subprocess.run([ar_tool, "p", os.path.abspath(deb_file), tar_filename], check=True, stdout=outfile, stderr=subprocess.PIPE)

        file_extension = os.path.splitext(tar_file_path)[1].lower()

        if file_extension == ".xz":
            mode = "r:xz"
        elif file_extension == ".gz":
            mode = "r:gz"
        elif file_extension == ".zst":
            # zstd is not supported by standard library yet
            decompressed_tar_path = tar_file_path.replace(".zst", "")
            with open(tar_file_path, "rb") as zst_file, open(decompressed_tar_path, "wb") as decompressed_file:
                dctx = zstandard.ZstdDecompressor()
                dctx.copy_stream(zst_file, decompressed_file)

            tar_file_path = decompressed_tar_path
            mode = "r"
        else:
            raise ValueError(f"Unsupported compression format: {file_extension}")

        with tarfile.open(tar_file_path, mode) as tar:
            tar.extractall(path=extract_dir, filter=_rootfs_extraction_filter)

def _rootfs_extraction_filter(member, dest_path):
    """Tarfile extraction filter based on the 'data' filter that additionally
    rewrites absolute-target symlinks/hardlinks into rootfs-relative paths.
    """
    if (member.issym() or member.islnk()) and os.path.isabs(member.linkname):
        link_dir = os.path.dirname(member.name)
        new_linkname = os.path.relpath(member.linkname.lstrip('/'),
                                       start=link_dir or '.')
        member = member.replace(linkname=new_linkname, deep=False)
    return tarfile.data_filter(member, dest_path)

def finalize_setup(rootfsdir):
    lib_dir = os.path.join(rootfsdir, 'lib')
    usr_lib_dir = os.path.join(rootfsdir, 'usr', 'lib')

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Identify the actual extension: 'ar t <deb>' shows the data.tar member name; check its suffix.
2. Switch to a suite/mirror whose .debs use .xz or .zst (the supported set).
3. If you genuinely need .bz2/.lzma support, extend the switch to add 'r:bz2'/'r:lzma' modes — but prefer using a mainstream mirror.
4. Verify the .deb is from an official Debian/Ubuntu repository and not a hand-built package with a non-standard data.tar compression.

Example fix

# before — suite ships .lzma data.tar
python3 install-debs.py --suite old-distro ...
# ValueError: Unsupported compression format: .lzma

# after — use a suite with .xz/.zst debs
python3 install-debs.py --suite bookworm ...

# or extend support (if you must)
# in extract_deb_file:
#   elif file_extension == '.bz2':
#       mode = 'r:bz2'
#   elif file_extension == '.lzma':
#       mode = 'r:xz'

# Pre-check the data.tar compression before extraction
import os
SUPPORTED = {'.xz', '.gz', '.zst'}
def supported_data_tar_compression(deb_file: str, ar: str = 'ar') -> bool:
    r = subprocess.run([ar, 't', os.path.abspath(deb_file)], capture_output=True)
    for line in r.stdout.decode().splitlines():
        if line.startswith('data.tar'):
            return os.path.splitext(line.lower())[1] in SUPPORTED
    return False

SUPPORTED = {'.xz', '.gz', '.zst'}
def is_supported_compression(ext: str) -> bool:
    return ext.lower() in SUPPORTED

try:
    extract_deb_file(deb_file, tmp_dir, extract_dir, ar_tool)
except ValueError as e:
    if 'Unsupported compression format' in str(e):
        ext = str(e).split(':')[-1].strip()
        print(f'.deb uses {ext}; extend extract_deb_file or switch to a .xz/.zst suite')
        raise
    raise

When it happens

Trigger: The data.tar member inside the .deb has an extension other than .xz/.gz/.zst (e.g., .bz2, .lzma, .tar with no compression, or a doubly-suffixed name the os.path.splitext does not reduce correctly). The switch at install-debs.py:316-332 falls through to the else branch and raises.

Common situations: Older Ubuntu .debs that used .lzma; a custom/patched .deb with bz2 data.tar; a filename like 'data.tar.foo' from a non-standard archive tool; a misconfigured os.path.splitext on a double extension producing an unexpected suffix.

Related errors

• Could not find 'data.tar.*' in {deb_file}.
• SHA256 mismatch for {url}: expected {checksum}, got {sha256}
• Failed to download {url}, Status Code: {response.status}
• Failed to download {url} after {max_retries} attempts.
• SHA256 mismatch for {path}: expected {packages_sha}, got {sh

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/07c6e7e1c089d508. Report an issue: GitHub.