86 lines
3.0 KiB
Python
86 lines
3.0 KiB
Python
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"""Capture every volume from one atomic filesystem snapshot.
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Copying a live tree file by file cannot produce a point in time: a database can
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write between two files and leave a control file and its write-ahead log
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disagreeing, which no recovery can repair. A snapshot freezes the whole subject
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at once, so a database reads it as a crash and replays its log - a case it is
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built for. That also removes the reason to stop containers at all.
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The snapshot kind is stated by the caller rather than probed, because falling
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back to a live copy when a probe is inconclusive would hand out backups that
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look consistent and are not.
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"""
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from __future__ import annotations
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import os
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from collections.abc import Callable, Iterator
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from contextlib import contextmanager
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from .shell import execute_shell_command
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KINDS = ("btrfs", "zfs")
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class SnapshotError(RuntimeError):
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"""A snapshot could not be created, resolved or removed."""
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def _resolver(subject: str, root: str) -> Callable[[str], str]:
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def resolve(path: str) -> str:
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relative = os.path.relpath(os.path.abspath(path), os.path.abspath(subject))
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if relative.startswith(".."):
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raise SnapshotError(f"{path} lies outside the snapshot subject {subject}")
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return os.path.join(root, relative) if relative != "." else root
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return resolve
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def _btrfs(subject: str, name: str, run: Callable[[str], list[str]]) -> tuple[str, str]:
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target = os.path.join(os.path.dirname(os.path.abspath(subject)), f".{name}")
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run(f"btrfs subvolume snapshot -r {subject} {target}")
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return target, f"btrfs subvolume delete {target}"
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def _zfs(subject: str, name: str, run: Callable[[str], list[str]]) -> tuple[str, str]:
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output = run(f"zfs list -H -o name {subject}")
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dataset = (output[0] if output else "").strip()
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if not dataset:
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raise SnapshotError(f"no zfs dataset is mounted at {subject}")
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run(f"zfs snapshot {dataset}@{name}")
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root = os.path.join(subject, ".zfs", "snapshot", name)
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return root, f"zfs destroy {dataset}@{name}"
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_CREATE = {"btrfs": _btrfs, "zfs": _zfs}
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@contextmanager
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def volume_snapshot(
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kind: str,
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subject: str,
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tag: str,
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run: Callable[[str], list[str]] = execute_shell_command,
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) -> Iterator[Callable[[str], str]]:
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"""Yield a resolver mapping a path under ``subject`` into a snapshot of it.
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Args:
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kind: ``btrfs`` or ``zfs``; the caller states it, nothing is probed.
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subject: the btrfs subvolume or zfs dataset mountpoint holding the
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volumes, e.g. ``/var/lib/docker``.
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tag: unique suffix for the snapshot name, e.g. the backup timestamp.
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run: shell runner, injected so the mechanics are testable.
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Raises:
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SnapshotError: the kind is unknown, or the snapshot cannot be created.
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"""
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create = _CREATE.get(kind)
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if create is None:
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raise SnapshotError(f"unknown snapshot kind {kind!r}; expected one of {KINDS}")
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root, remove = create(subject, f"baudolo-{tag}", run)
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try:
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yield _resolver(subject, root)
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finally:
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run(remove)
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