src/explicit_learning/atomic_io.py

"""Atomic, interruption-safe file writers.

Every stage output is written to a temporary file in the destination directory,
``fsync``-ed, then atomically renamed over the target with :func:`os.replace`.
An interrupted write therefore cannot corrupt an existing valid artifact: the
old file is untouched until the rename succeeds. JSONL batches are flushed and
``fsync``-ed once per logical batch, leaving a valid line boundary on crash
without a durability syscall for every row.
"""

from __future__ import annotations

import json
import os
import tempfile
from collections.abc import Iterable, Iterator
from pathlib import Path
from typing import Any

from .hashing import canonical_json


class AtomicWriteError(RuntimeError):
    """Raised when an atomic write cannot be completed."""


def _fsync_dir(directory: Path) -> None:
    """Best-effort fsync of the directory holding renamed files."""
    if not directory.exists():
        return
    fd = os.open(str(directory), os.O_RDONLY)
    try:
        os.fsync(fd)
    except OSError:
        # Some filesystems (e.g. network mounts) do not support directory fsync.
        # The rename is still atomic; we only lose the durability guarantee.
        pass
    finally:
        os.close(fd)


def atomic_write_bytes(
    path: str | Path,
    data: bytes,
    *,
    fsync_dir: bool = True,
) -> None:
    """Atomically write ``data`` to ``path`` via temp + fsync + rename."""
    target = Path(path)
    target.parent.mkdir(parents=True, exist_ok=True)
    with tempfile.NamedTemporaryFile(
        dir=target.parent,
        prefix=f".{target.name}.",
        suffix=".tmp",
        delete=False,
    ) as tmp_handle:
        tmp_path = Path(tmp_handle.name)
        tmp_handle.write(data)
        tmp_handle.flush()
        os.fsync(tmp_handle.fileno())
    try:
        os.replace(tmp_path, target)
        if fsync_dir:
            _fsync_dir(target.parent)
    except BaseException:
        tmp_path.unlink(missing_ok=True)
        raise


def atomic_write_text(
    path: str | Path,
    text: str,
    *,
    encoding: str = "utf-8",
    fsync_dir: bool = True,
) -> None:
    """Atomically write ``text`` to ``path``."""
    atomic_write_bytes(path, text.encode(encoding), fsync_dir=fsync_dir)


def atomic_write_json(
    path: str | Path,
    obj: Any,
    *,
    fsync_dir: bool = True,
) -> None:
    """Atomically write a single JSON object to ``path`` using canonical JSON."""
    atomic_write_text(path, canonical_json(obj) + "\n", fsync_dir=fsync_dir)


def _encode_jsonl_row(row: Any) -> bytes:
    return (canonical_json(row) + "\n").encode("utf-8")


def atomic_write_jsonl(
    path: str | Path,
    rows: Iterable[Any],
    *,
    fsync_dir: bool = True,
) -> None:
    """Atomically write an iterable of rows as canonical JSONL."""
    buffer = bytearray()
    for row in rows:
        buffer += _encode_jsonl_row(row)
    atomic_write_bytes(path, bytes(buffer), fsync_dir=fsync_dir)


class JsonlAppender:
    """Append JSONL records to a file with per-record fsync.

    Appending never rewrites earlier bytes, so an interrupted append leaves the
    existing prefix valid. Each record is written and flushed before the next,
    so a crash loses at most the in-flight record. Call :meth:`close` (or use as
    a context manager) to fsync the directory once at the end.
    """

    def __init__(self, path: str | Path, *, fsync_dir: bool = True) -> None:
        self.path = Path(path)
        self.path.parent.mkdir(parents=True, exist_ok=True)
        self._fsync_dir = fsync_dir
        # "ab" never truncates; creates the file if absent.
        self._handle = self.path.open("ab")

    def append(self, row: Any) -> None:
        self._handle.write(_encode_jsonl_row(row))
        self._handle.flush()
        os.fsync(self._handle.fileno())

    def extend(self, rows: Iterable[Any]) -> None:
        """Append one logical batch and fsync once.

        Reward generation emits many rows together. Fsyncing every individual
        row made training spend most of its wall time in durability syscalls;
        a single batch write still leaves a valid JSONL prefix after a crash and
        checkpoint recovery rolls the batch back as a unit.
        """

        payload = b"".join(_encode_jsonl_row(row) for row in rows)
        if not payload:
            return
        self._handle.write(payload)
        self._handle.flush()
        os.fsync(self._handle.fileno())

    def close(self) -> None:
        try:
            self._handle.close()
        finally:
            if self._fsync_dir:
                _fsync_dir(self.path.parent)

    def __enter__(self) -> JsonlAppender:
        return self

    def __exit__(self, *exc: object) -> None:
        self.close()


def read_jsonl(path: str | Path) -> Iterator[Any]:
    """Yield parsed JSON objects from a JSONL file, skipping blank lines."""
    with Path(path).open("r", encoding="utf-8") as handle:
        for line in handle:
            stripped = line.strip()
            if not stripped:
                continue
            yield json.loads(stripped)


def count_jsonl(path: str | Path) -> int:
    """Count non-blank lines in a JSONL file without parsing each row."""
    count = 0
    with Path(path).open("r", encoding="utf-8") as handle:
        for line in handle:
            if line.strip():
                count += 1
    return count