diet-guard/diet_guard/_sync.py

"""Cross-device log sync orchestration for diet_guard.

Pulls every other device's pushed log from GitHub-backed dumb storage
(``crdt_sync.GitHubSyncClient``), merges with the local log via
``crdt_sync``'s shared CRDT scheme (:mod:`diet_guard._sync_merge` adapts
diet_guard's entries to/from ``crdt_sync.Record``), re-signs every persisted
entry, rebuilds the food bank, and pushes this device's own merged log back
up in the new Record-based wire format.
"""

from __future__ import annotations

import json
import logging

from crdt_sync import GitHubSyncClient, Log, merge_logs

from diet_guard._constants import (
    SYNC_DEVICE_ID,
    SYNC_REPO_NAME,
    SYNC_REPO_OWNER,
    SYNC_TIMEOUT_SECONDS,
    SYNC_TOKEN_FILE,
)
from diet_guard._foodbank import rebuild_food_bank
from diet_guard._state import DayLog, read_raw_log, resign_entry, write_raw_log
from diet_guard._sync_merge import daylog_to_log, log_to_daylog, parse_remote_log

_logger = logging.getLogger(__name__)

_DEVICES_DIR = "diet-guard-sync/devices"


class SyncError(Exception):
    """Raised when a sync run cannot even start (no usable PAT)."""


def _device_log_path(device_id: str) -> str:
    """Return the repo-relative path a device's full log is pushed to."""
    return f"{_DEVICES_DIR}/{device_id}/food_log.json"


def _read_token() -> str:
    """Return the saved sync PAT, stripped of trailing whitespace.

    Raises:
        SyncError: If the token file is missing or empty -- the user has not
            completed the one-time github.com setup step yet.
    """
    if not SYNC_TOKEN_FILE.exists():
        message = (
            f"no sync token at {SYNC_TOKEN_FILE} -- create a fine-grained "
            "GitHub PAT scoped to the syncs repo's contents and "
            f"save it there (mode 600), then re-run sync"
        )
        raise SyncError(message)
    token = SYNC_TOKEN_FILE.read_text().strip()
    if not token:
        msg = f"{SYNC_TOKEN_FILE} is empty"
        raise SyncError(msg)
    return token


def _pull_remote_logs(client: GitHubSyncClient) -> list[Log]:
    """Return every other device's last-pushed log, skipping this one.

    A device whose pushed file is corrupt, truncated, or otherwise
    unparsable (new or old wire format) is logged and skipped, same as one
    that has never pushed at all -- GitHub is an external system boundary,
    and one bad device's file must not stall merging in every other
    device's.
    """
    remote_logs: list[Log] = []
    for device_id in client.list_directory(_DEVICES_DIR):
        if device_id == SYNC_DEVICE_ID:
            continue
        text = client.get_file_text(_device_log_path(device_id))
        if text is None:
            continue
        try:
            remote_logs.append(parse_remote_log(text))
        except (TypeError, KeyError, ValueError, json.JSONDecodeError):
            _logger.warning("Unparsable log pushed by device %r, skipping", device_id)
    return remote_logs


def run_sync() -> DayLog:
    """Run one full sync tick: pull, merge, re-sign, persist, push.

    Every persisted entry is re-signed regardless of origin (not just
    phone-origin ones): a signature computed on another device cannot be
    trusted as this device's shared key sees it, and an inbound entry with no
    signature at all would otherwise be silently dropped on the very next
    read by :func:`diet_guard._state.load_log`.

    Returns:
        The merged log as it now sits on disk locally, post re-sign.

    Raises:
        SyncError: If the local PAT is missing or empty.
        crdt_sync.GitHubSyncError: Propagated from the GitHub client for any
            transport failure -- the caller (CLI/timer) decides how to
            report it.
    """
    token = _read_token()
    client = GitHubSyncClient(
        SYNC_REPO_OWNER, SYNC_REPO_NAME, token, timeout_seconds=SYNC_TIMEOUT_SECONDS
    )

    merged = daylog_to_log(read_raw_log())
    for remote_log in _pull_remote_logs(client):
        merged = merge_logs(merged, remote_log)

    merged_daylog = log_to_daylog(merged)
    resigned: DayLog = {
        day: [resign_entry(entry) for entry in entries]
        for day, entries in merged_daylog.items()
    }
    write_raw_log(resigned)
    rebuild_food_bank(resigned)

    push_log = daylog_to_log(resigned)
    push_json = json.dumps(
        {record_id: record.to_dict() for record_id, record in push_log.items()},
        indent=2,
    )
    client.put_file_text(
        _device_log_path(SYNC_DEVICE_ID),
        push_json,
        message="diet_guard sync",
    )
    return resigned


def pull_shared_log() -> str | None:
    """Run a sync tick, failing closed instead of raising.

    A thin wrapper over :func:`run_sync` for callers that must never crash on a
    sync error: the gate's automatic pre-lock refresh and the lock screen's
    manual "Fetch from sync" button.  Returns ``None`` on success, or a short
    human-readable reason when the pull could not complete (no token, network
    down, a disk error writing the merged log back), so the caller keeps its
    own lock decision rather than failing open.
    """
    try:
        run_sync()
    except Exception as exc:  # pylint: disable=broad-exception-caught
        return f"sync unavailable ({exc})"
    return None