dotnet/maui · error · ValueError

Unsupported compression format: {file_extension}

Error message

Unsupported compression format: {file_extension}

What it means

extract_debs raises ValueError when the data.tar member's extension is not one of .xz, .gz, or .zst. The script only knows how to open those compression modes; any other extension (e.g. .bz2, .lzma, or no extension) is rejected after the archive is already extracted from the .deb.

Source

Thrown at eng/common/cross/install-debs.py:268

        subprocess.run(f"{ar_tool} p {os.path.abspath(deb_file)} {tar_filename} > {tar_file_path}", check=True, shell=True)

        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='fully_trusted')

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

    if os.path.exists(lib_dir):
        if os.path.islink(lib_dir):
            os.remove(lib_dir)
        else:
            os.makedirs(usr_lib_dir, exist_ok=True)

            for item in os.listdir(lib_dir):
                src = os.path.join(lib_dir, item)
                dest = os.path.join(usr_lib_dir, item)

View on GitHub (pinned to f377ff1c5e)

Solutions

Prefer a suite/mirror whose debs use xz or zst (current Debian/Ubuntu default).
If .bz2 support is required, extend the branch to add `mode = 'r:bz2'` for the .bz2 case.
Verify the member name with `ar t file.deb`; if it is data.tar (uncompressed), add an `else: mode = 'r'` branch.
Avoid hand-modified debs with non-standard compression.

Example fix

# before
elif file_extension == ".zst":
    ...
else:
    raise ValueError(f"Unsupported compression format: {file_extension}")

# after
elif file_extension == ".bz2":
    mode = "r:bz2"
elif file_extension == ".zst":
    ...
else:
    raise ValueError(f"Unsupported compression format: {file_extension}")

Defensive patterns

Strategy: validation

Validate before calling

ext = os.path.splitext(tar_file_path)[1].lower()
if ext not in ('.xz', '.gz', '.zst'):
    raise ValueError(f"Unsupported compression format: {ext}")

Type guard

def is_supported_compression(ext): return ext.lower() in ('.xz', '.gz', '.zst')

Try / catch

try:
    extract_debs([deb], ...)
except ValueError as e:
    print(e); re_fetch_from_xz_mirror(deb)

Prevention

Prefer suites whose debs use xz or zst.
Extend the compression branch if you must support bz2/lzma.
Inspect member names with `ar t` before extraction.

When it happens

Trigger: A .deb whose data member is data.tar.bz2 or another unsupported compression, or a tar_file_path whose extension parsing yields an unexpected value (e.g. data.tar with no compression suffix).

Common situations: Older Debian archives using bzip2, experimental lzma-compressed debs, or a renamed/mangled member name that confuses splitext.

Related errors

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/1ff4f0339af6848a. Report an issue: GitHub.