"""Lock-safe inspection of SQLite database files.

POSIX advisory locks are cancelled **process-wide** by ``close()`` on *any* fd for that file, so a
bare ``open(db_path, "rb") ... close()`` on a live database drops every lock SQLite holds from this
process (a VACUUM's EXCLUSIVE lock, an in-flight BEGIN IMMEDIATE's RESERVED lock). This module
tracks live connections so raw reads happen only when none exist.
"""

from __future__ import annotations

import contextlib
import logging
import os
import sqlite3
import threading
from pathlib import Path
from typing import Optional

logger = logging.getLogger(__name__)

# Guards BOTH the registry and the lifecycle syscalls it describes. Reentrant
# because connect_tracked -> _canonical_db_path -> ... stays on one thread.
_live_lock = threading.RLock()
# canonical path -> number of live connections opened by this process
_live_connections: dict[str, int] = {}


class UntrackableConnectionError(RuntimeError):
    """A connection to a probe-able database could not be tracked. Raised rather than returning an
    untracked connection: on these paths tracking is part of the correctness contract."""


class LiveConnectionError(RuntimeError):
    """A raw file operation was attempted on a database with live connections."""


def _key(path: Path | str) -> str:
    """Canonicalise a *filesystem* path for use as a registry key."""
    try:
        return str(Path(path).resolve())
    except OSError:
        return str(path)


def _canonical_db_path(conn: sqlite3.Connection) -> Optional[str]:
    """The on-disk path of ``main`` as SQLite reports it (immune to ``file:`` URIs, relative paths,
    symlinks). ``None`` for in-memory/unnamed databases, which cannot be byte-probed."""
    try:
        row = conn.execute("PRAGMA database_list").fetchone()
    except sqlite3.Error:
        return None
    if not row or len(row) < 3 or not row[2]:
        return None
    return _key(row[2])


def track_connection(path: Path | str) -> None:
    """Record that this process holds a connection to *path* (prefer :func:`connect_tracked`; this
    is for callers managing their own connection objects, and for tests)."""
    with _live_lock:
        _track_key(_key(path))


def _track_key(key: str, delta: int = 1) -> None:
    """Adjust the live count for an already-canonical key (caller holds ``_live_lock``)."""
    remaining = _live_connections.get(key, 0) + delta
    if remaining > 0:
        _live_connections[key] = remaining
    else:
        _live_connections.pop(key, None)


def untrack_connection(path: Path | str) -> None:
    """Record that one connection to *path* has been closed."""
    with _live_lock:
        _track_key(_key(path), -1)


def _live_main_key(key: str) -> Optional[str]:
    """The tracked main-database key that makes *key* live, or ``None`` (caller holds ``_live_lock``).

    SQLite locks the main file and its WAL sidecars; a raw ``close()`` of any of those
    inodes cancels this process's POSIX locks, but the registry is keyed by the main path."""
    if key in _live_connections:
        return key
    for suffix in ("-wal", "-shm"):
        if key.endswith(suffix) and key[:-len(suffix)] in _live_connections:
            return key[:-len(suffix)]
    return None


def has_live_connection(path: Path | str) -> bool:
    """Whether this process holds a connection to *path* (or to the database it is a sidecar of).

    Point-in-time answer: a raw open/close right after it returns ``False`` can still race a
    new connection. Hold :func:`offline_file_access` across the I/O whenever possible."""
    with _live_lock:
        return _live_main_key(_key(path)) is not None


class _TrackingMixin:
    """Untrack-on-close behaviour, mixable into any Connection subclass.

    The real ``close()`` and the unregister happen together under ``_live_lock`` so a concurrent
    probe can never observe "no live connection" while this descriptor is still open. Unregister
    runs only after ``close()`` succeeds: untracking before a failing close (e.g. cross-thread
    ProgrammingError) would leave the FD open while the byte-probe guard thinks nothing is live.
    """

    _hermes_tracked_path: str | None = None

    def close(self) -> None:  # type: ignore[misc]
        with _live_lock:
            path = getattr(self, "_hermes_tracked_path", None)
            # Close first; untrack only once the descriptor is actually gone. Untracking before a failing
            # close (e.g. cross-thread ProgrammingError) leaves the FD open while the byte-probe guard
            # thinks nothing is live — see #75629.
            super().close()  # type: ignore[misc]
            if path is not None:
                self._hermes_tracked_path = None
                untrack_connection(path)


class TrackedConnection(_TrackingMixin, sqlite3.Connection):
    """A ``sqlite3.Connection`` that untracks its path exactly once on close (callers close in many
    places, some via ``contextlib.closing``, so counting closes by hand is unreliable)."""


_tracked_factory_cache: dict[type, type] = {}


def _tracking_factory(factory: type) -> type:
    """Return *factory* augmented with untrack-on-close. Callers legitimately pass their own
    ``Connection`` subclasses (tests simulate FTS5-less or pragma-failing runtimes); leaving them
    untracked would quietly unguard the database, so the tracking ``close()`` is mixed in."""
    if factory is sqlite3.Connection:
        return TrackedConnection
    if issubclass(factory, _TrackingMixin):
        return factory
    cached = _tracked_factory_cache.get(factory)
    if cached is None:
        cached = type(f"Tracked{factory.__name__}", (_TrackingMixin, factory), {})
        _tracked_factory_cache[factory] = cached
    return cached


def connect_tracked(
    path: Path | str, *, tracking_path: Path | str | None = None, connect_fn=None, **kwargs,
) -> sqlite3.Connection:
    """``sqlite3.connect`` that registers the connection for the lifetime of the fd (released on
    ``close()``). Use for any database that might be byte-probed (``state.db``, ``kanban.db``).
    Open and registration happen together under ``_live_lock`` so a concurrent
    :func:`read_header_bytes_preopen` cannot slip between them and cancel this connection's locks."""
    opener = connect_fn if connect_fn is not None else sqlite3.connect
    kwargs["factory"] = _tracking_factory(kwargs.get("factory", sqlite3.Connection))

    with _live_lock:
        conn = opener(str(path), **kwargs)
        try:
            resolved = _key(tracking_path) if tracking_path is not None else _canonical_db_path(conn)
            if resolved is None:
                return conn  # in-memory / unnamed: nothing on disk to byte-probe
            if not isinstance(conn, _TrackingMixin):
                # The opener substituted its own factory and discarded ours (test doubles
                # simulating FTS5-less runtimes do this). Retag the instance's class so close()
                # still releases the registry entry rather than silently losing probe safety.
                conn = _retrofit_tracking(conn, resolved)
            conn._hermes_tracked_path = resolved
            _track_key(resolved)
            return conn
        except Exception:
            try:
                # Close via sqlite3 directly: the tracking entry was either never made or is
                # being unwound here.
                sqlite3.Connection.close(conn)
            except Exception:
                pass
            raise


def _retrofit_tracking(conn: sqlite3.Connection, resolved: str) -> sqlite3.Connection:
    """Give an already-open connection untrack-on-close semantics by swapping ``__class__`` for
    one mixing in the tracking ``close()`` (used when an opener ignored the factory we asked for)."""
    cls = type(conn)
    try:
        conn.__class__ = _tracking_factory(cls)  # type: ignore[assignment]
        return conn
    except TypeError as exc:
        raise UntrackableConnectionError(
            f"connection to {resolved} uses factory {cls.__name__}, which "
            "cannot release its tracking entry on close; byte-probe safety "
            "for this database would be silently lost"
        ) from exc


def page_count_bytes(conn: sqlite3.Connection) -> Optional[int]:
    """Logical database size in bytes (``page_count * page_size``, the header field at offset 28)
    read via PRAGMA over *conn* so no new fd is opened. ``None`` when the pragmas cannot be read."""
    try:
        page_count = conn.execute("PRAGMA page_count").fetchone()[0]
        page_size = conn.execute("PRAGMA page_size").fetchone()[0]
        return int(page_count) * int(page_size)
    except (sqlite3.Error, TypeError, IndexError, ValueError) as exc:
        logger.debug("page_count/page_size unavailable: %s", exc)
        return None


def file_length_matches_header(conn: sqlite3.Connection) -> Optional[bool]:
    """Whether the file on disk is at least as long as the header claims ("torn extend" check),
    without opening the file (PRAGMA over *conn* + ``stat()``). Advisory in WAL mode: a freshly
    committed page may still live in ``-wal`` so the main file legitimately lags."""
    path_str = _canonical_db_path(conn)
    if path_str is None:
        return None
    logical = page_count_bytes(conn)
    if not logical:
        return None
    try:
        return os.path.getsize(path_str) >= logical
    except OSError:
        return None


def read_header_bytes_preopen(path: Path | str, *, length: int = 100, force: bool = False) -> Optional[bytes]:
    """Read the first *length* bytes of *path* -- only when no connection is live. The ONLY
    sanctioned byte-level read of a database file, for first-open validation (real SQLite? zeroed?
    overwritten?). Check and open/read/close run together under ``_live_lock`` so a connection
    cannot be opened between deciding "nothing is live" and closing this descriptor."""
    with _live_lock:
        if not force and _live_main_key(_key(path)) is not None:
            logger.debug(
                "refusing byte-level read of %s: a live connection exists in "
                "this process and close() would cancel its POSIX locks",
                path)
            return None
        try:
            with open(path, "rb") as handle:
                return handle.read(length)
        except OSError:
            return None


@contextlib.contextmanager
def offline_file_access(path: Path | str, *, what: str = "read"):
    """Hold the connection-lifecycle lock across a raw read of a database file: checking
    :func:`has_live_connection` and *then* doing raw I/O is a check/use race (a connection opened
    in between loses its POSIX locks to the raw ``close()``). Held only for the raw I/O."""
    with _live_lock:
        main = _live_main_key(_key(path))
        if main is not None:
            subject = "it" if main == _key(path) else f"its main database {main}"
            raise LiveConnectionError(
                f"Refusing to {what} {path}: a connection to {subject} is still open "
                "in this process, and raw file access would cancel that "
                "connection's POSIX advisory locks. Close all database "
                "handles (stop the gateway/dashboard) and retry.")
        yield