scripts/xz_auto.py

# Copyright 2020 The ChromiumOS Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""Run xz from PATH with a thread for each core in the system."""

import functools
import getopt
import os
import subprocess
import sys

from chromite.lib import commandline
from chromite.lib import osutils


PIXZ_DISABLE_VAR = "FOR_TEST_XZ_AUTO_NO_PIXZ"
XZ_DISABLE_VAR = "FOR_TEST_XZ_AUTO_NO_XZ_DECOMPRESSION"


@functools.lru_cache
def HasPixz():
    """Returns path to pixz if it's on PATH or None otherwise."""
    return PIXZ_DISABLE_VAR not in os.environ and osutils.Which("pixz")


def ParsePixzArgs(argv):
    """Determines flags to pass to pixz, per argv.

    Returns:
        A tuple containing:
        - A raw list of flags to pass to pixz.
        - An optional input file.
        - An optional output file (only exists if the input file is present).
    """
    # Glancing at docs, the following opts are supported. -i and -o are ignored,
    # since we assert in `main` that they're not present, but include parsing
    # for them anyway.
    flags, args = getopt.gnu_getopt(
        args=argv,
        shortopts="dlxi:o:0123456789p:tkch",
    )
    if not args:
        file_to_compress = None
        target = None
    elif len(args) == 1:
        file_to_compress = args[0]
        target = None
    else:
        file_to_compress = args[0]
        target = args[1]

    raw_flag_list = []
    for key, val in flags:
        raw_flag_list.append(key)
        if val:
            raw_flag_list.append(val)

    return raw_flag_list, file_to_compress, target


def Execvp(argv) -> None:
    """Execs the given argv."""
    os.execvp(argv[0], argv)


def ExecCompressCommand(stdout, argv) -> None:
    """Execs compression command."""
    # It appears that in order for pixz to do parallel decompression,
    # compression needs to be done with pixz. xz itself is only capable of
    # parallel compression.
    if not HasPixz():
        cmd = ["xz"]

        if stdout:
            cmd.append("-zc")
        else:
            cmd.append("-z")
        cmd += argv
        Execvp(cmd)

    cmd = ["pixz"]
    raw_flag_list, compressed_file_name, output_file = ParsePixzArgs(argv)

    # Pixz treats tarballs specially: if it detects that a tarball has been
    # passed to it, it'll also write a small index in the output file that
    # makes operations like listing the tar faster. If this tar autodetection
    # is enabled and pixz is asked to compress an empty file, it breaks. In
    # addition, these indices have no apparent impact on decompression
    # parallelism, so they're not super useful to us. Disable the feature
    # wholesale.
    if "-t" not in raw_flag_list:
        raw_flag_list.append("-t")

    autodelete_input_file = False
    if not compressed_file_name:
        assert not output_file
        compressed_file_name = "/dev/stdin"
        output_file = "/dev/stdout"
    elif stdout:
        output_file = "/dev/stdout"
    elif not output_file:
        # Pixz defaults to a `.pxz` suffix (or `.tpxz` if it's compressing a
        # tar file). We need the suffix to be consistent, so force it here.
        output_file = f"{compressed_file_name}.xz"
        autodelete_input_file = True

    cmd += raw_flag_list
    cmd.append(compressed_file_name)
    if output_file:
        cmd.append(output_file)

    if not autodelete_input_file:
        Execvp(cmd)

    return_code = subprocess.call(cmd)
    if not return_code:
        os.unlink(compressed_file_name)
    sys.exit(return_code)


def ExecXzDecompressCommand(stdout, argv) -> None:
    """Executes `xz` with the given params."""
    cmd = ["xz"]
    if stdout:
        cmd.append("-dc")
    else:
        cmd.append("-d")
    cmd += argv
    Execvp(cmd)


def ExecDecompressCommand(stdout, argv) -> None:
    """Execs decompression command."""
    if not HasPixz():
        ExecXzDecompressCommand(stdout, argv)

    cmd = ["pixz", "-d"]
    raw_flag_list, compressed_file_name, output_file_name = ParsePixzArgs(argv)
    cmd += raw_flag_list

    assert compressed_file_name or not output_file_name
    if not compressed_file_name:
        Execvp(cmd)

    # HACK: When passed a file, pixz will jump around it and try to find the
    # file's index. If the file we originally compressed was empty and we
    # requested no index, pixz will error out because lzma will report no
    # entries to it, and pixz doesn't handle that well.
    #
    # Since we need to support files with indices and without, we can't pass
    # `-t`. If we do, that causes pixz to error out occasionally on tar files
    # with indices. :(
    #
    # In any case, at the time I checked, empty xz files are 32 bytes, so just
    # opt to use xz for anything under 4KB. pixz archives are xz-compatible
    # anyway.
    if (
        XZ_DISABLE_VAR not in os.environ
        and os.path.isfile(compressed_file_name)
        and os.path.getsize(compressed_file_name) <= 4 * 1024
    ):
        ExecXzDecompressCommand(stdout, argv)

    cmd.append(compressed_file_name)

    # Explicitly tell pixz the file is the input, so it will dump the output
    # to stdout, instead of automatically choosing an output name.
    if stdout:
        output_file_name = "/dev/stdout"

    if output_file_name:
        cmd.append(output_file_name)

    Execvp(cmd)


def GetParser():
    """Return a command line parser."""
    parser = commandline.ArgumentParser(description=__doc__)
    parser.add_argument(
        "-d",
        "--decompress",
        "--uncompress",
        help="Decompress rather than compress.",
        action="store_true",
    )
    parser.add_argument(
        "-c",
        dest="stdout",
        action="store_true",
        help="Write to standard output and don't delete input files.",
    )
    return parser


def main(argv) -> None:
    parser = GetParser()
    known_args, argv = parser.parse_known_args()
    if "-i" in argv or "-o" in argv:
        parser.error("It is invalid to use -i or -o with xz_auto")

    if known_args.decompress:
        ExecDecompressCommand(known_args.stdout, argv)
    else:
        ExecCompressCommand(known_args.stdout, argv)