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110 lines
3.1 KiB
Python
110 lines
3.1 KiB
Python
"""
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A script that replaces an old file with a new one, only if the contents
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actually changed. If not, the new file is simply deleted.
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This avoids wholesale rebuilds when a code (re)generation phase does not
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actually change the in-tree generated code.
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"""
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from __future__ import annotations
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import contextlib
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import os
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import os.path
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import sys
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TYPE_CHECKING = False
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if TYPE_CHECKING:
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import typing
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from collections.abc import Iterator
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from io import TextIOWrapper
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_Outcome: typing.TypeAlias = typing.Literal['created', 'updated', 'same']
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@contextlib.contextmanager
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def updating_file_with_tmpfile(
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filename: str,
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tmpfile: str | None = None,
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) -> Iterator[tuple[TextIOWrapper, TextIOWrapper]]:
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"""A context manager for updating a file via a temp file.
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The context manager provides two open files: the source file open
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for reading, and the temp file, open for writing.
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Upon exiting: both files are closed, and the source file is replaced
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with the temp file.
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"""
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# XXX Optionally use tempfile.TemporaryFile?
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if not tmpfile:
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tmpfile = filename + '.tmp'
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elif os.path.isdir(tmpfile):
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tmpfile = os.path.join(tmpfile, filename + '.tmp')
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with open(filename, 'rb') as infile:
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line = infile.readline()
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if line.endswith(b'\r\n'):
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newline = "\r\n"
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elif line.endswith(b'\r'):
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newline = "\r"
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elif line.endswith(b'\n'):
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newline = "\n"
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else:
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raise ValueError(f"unknown end of line: {filename}: {line!a}")
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with open(tmpfile, 'w', newline=newline) as outfile:
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with open(filename) as infile:
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yield infile, outfile
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update_file_with_tmpfile(filename, tmpfile)
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def update_file_with_tmpfile(
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filename: str,
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tmpfile: str,
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*,
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create: bool = False,
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) -> _Outcome:
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try:
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targetfile = open(filename, 'rb')
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except FileNotFoundError:
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if not create:
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raise # re-raise
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outcome: _Outcome = 'created'
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os.replace(tmpfile, filename)
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else:
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with targetfile:
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old_contents = targetfile.read()
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with open(tmpfile, 'rb') as f:
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new_contents = f.read()
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# Now compare!
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if old_contents != new_contents:
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outcome = 'updated'
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os.replace(tmpfile, filename)
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else:
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outcome = 'same'
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os.unlink(tmpfile)
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return outcome
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if __name__ == '__main__':
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument('--create', action='store_true')
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parser.add_argument('--exitcode', action='store_true')
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parser.add_argument('filename', help='path to be updated')
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parser.add_argument('tmpfile', help='path with new contents')
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args = parser.parse_args()
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kwargs = vars(args)
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setexitcode = kwargs.pop('exitcode')
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outcome = update_file_with_tmpfile(**kwargs)
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if setexitcode:
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if outcome == 'same':
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sys.exit(0)
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elif outcome == 'updated':
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sys.exit(1)
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elif outcome == 'created':
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sys.exit(2)
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else:
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raise NotImplementedError
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