Jean-Pierre MartineauandClaude Fable 5.1
c9f5a71ce7
Apache-2.0 does not require per-file headers, but a file lifted out of the tree carried no marking at all. One comment line, first (or right after a shebang), on every .py under meshterm/, tests/, packaging/ and scripts/, plus the extensionless SPI bridge; tests/test_spdx.py keeps a new file from landing without one. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
56 lines
2.6 KiB
Python
56 lines
2.6 KiB
Python
# SPDX-License-Identifier: Apache-2.0
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"""THE way MeshTerm replaces a file it owns: all of the new contents, or none of them.
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Every store under ``core/`` keeps its state in one file that it rewrites whole. Writing
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in place means a crash — or a laptop lid closing — halfway through leaves a truncated
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file where a contact list used to be, so each write goes to a neighbouring temporary file
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first and lands with a single rename, which the filesystem either does or doesn't.
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The neighbouring file's name carries the writing process's id. That is not fussiness:
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MeshTerm's data directory is shared by every copy on the machine — a checkout, a
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downloaded build, a second instance for a second radio — and a fixed ``.tmp`` name means
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two of them writing at the same moment take turns clobbering one scratch file, then each
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rename whatever is in it over the real one. The rename is atomic; the thing being renamed
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was not. Per-process names make the two writes independent again, and the last rename
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simply wins, which is a lost update rather than a corrupted file.
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``MESHTERM_HOME`` is the way to avoid sharing the directory in the first place — see
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:func:`meshterm.core.config.default_config_dir`.
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"""
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from __future__ import annotations
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import os
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import stat
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from pathlib import Path
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__all__ = ["write_atomically"]
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def write_atomically(path: Path, text: str, *, owner_only: bool = False) -> None:
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"""Replace ``path`` with ``text`` in one step, creating parent directories as needed.
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Args:
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path: The file to replace. Its directory is created if it does not exist.
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text: The complete new contents, written as UTF-8.
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owner_only: Restrict the file to the owner (``0600``) before it is put in place,
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for anything holding a credential. Applied to the temporary file rather than
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the destination, so there is no moment where the real file exists with wider
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permissions. Best-effort: not every platform or filesystem honours it.
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"""
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path.parent.mkdir(parents=True, exist_ok=True)
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tmp = path.with_suffix(f"{path.suffix}.{os.getpid()}.tmp")
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try:
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tmp.write_text(text, encoding="utf-8")
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if owner_only:
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try:
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os.chmod(tmp, stat.S_IRUSR | stat.S_IWUSR)
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except OSError: # pragma: no cover - platform-dependent
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pass
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tmp.replace(path)
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except BaseException:
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# A failed write leaves no litter behind. `missing_ok` because the failure may
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# well have been the write that would have created it.
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tmp.unlink(missing_ok=True)
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raise
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