stackpilot/backend/services/backup_service.py

"""Stack backup & restore — compose files, bind-mount data and named volumes.

A backup is a single ``.tar.gz`` with this layout::

    manifest.json            metadata + full inventory of what was captured
    compose/...              the stack directory as StackPilot can see it
    binds/<n>.tar            contents of each captured bind-mount source
    volumes/<full>.tar       contents of each captured named volume

Bind sources and volumes are read/written through a throwaway helper container
(see :mod:`services.stack_assets_service`) so that host paths this container
cannot see are still captured — without that, a stack whose data directories
live outside StackPilot's own mount would back up as "just the compose file".

What makes a backup and its restore trustworthy, and what this module keeps to:

* **Consistent.** Whenever data is captured — including data folders that ride
  along inside the stack directory — the stack's running containers are
  stopped first and exactly those are started again afterwards. A stack that
  cannot be stopped is not backed up half-written. The stack is locked for the
  duration, so a deploy, update or second backup cannot interleave.
* **Nothing is lost that the backup does not contain.** A restore replaces what
  the backup holds and leaves alone what it does not: folders left out (too
  large, deselected, remote storage) and anything mounted into the stack
  directory from another filesystem survive an overwrite.
* **Verified before destructive.** The whole archive is read once before a
  restore touches anything, and the stack directory is unpacked next to the
  old one first.
* **A restore under a new name never writes into the original's data**:
  folders outside the stack directory and volumes not named after the stack
  are skipped rather than overwritten.
* **Remote storage** (NFS/CIFS/...) is neither archived nor wiped.
"""
from __future__ import annotations

import asyncio
import io
import json
import logging
import os
import re
import shutil
import stat
import tarfile
import tempfile
import zlib
from contextlib import contextmanager
from datetime import timedelta
from typing import Optional

from sqlmodel import Session

from database import engine
from docker_client import DockerError, get_long_client, safe_call
from services import (
    compose_service,
    stack_job_service,
    stack_lock_service,
    stack_assets_service as assets,
)

logger = logging.getLogger("stackpilot.backup")

COMPOSE_PROJECT_LABEL = assets.COMPOSE_PROJECT_LABEL
COMPOSE_VOLUME_LABEL = assets.COMPOSE_VOLUME_LABEL
MANIFEST_NAME = "manifest.json"
#: 3 added compose_binds/excluded (what the compose/ tree does and does not
#: carry) and each volume's driver/options/external. Older ones restore too.
BACKUP_FORMAT_VERSION = 3

#: How long a backup or restore may hold the stack lock before another
#: operation may take it over. Generous: a large volume can take hours.
LOCK_TTL = timedelta(hours=12)


class BackupError(Exception):
    pass


class StackBusyError(BackupError):
    """The stack is locked by another operation (deploy, update, backup)."""


class BackupCancelled(BackupError):
    """Somebody stopped this backup from the outside.

    Not a failure: the archive is thrown away, the stack is started again and
    the lock goes, exactly as after an error — but nothing is reported as
    broken and no "backup failed" notification goes out.
    """


# --------------------------------------------------------------------------- #
# Backup filename convention
# --------------------------------------------------------------------------- #


def backup_basename(stack_id: str, prefix: Optional[str] = None) -> str:
    """Filename stem used to group a stack's backups (and match for retention).

    ``prefix`` (e.g. a remote host slug) keeps backups of same-named stacks on
    different hosts from colliding / pruning each other on a shared destination.
    """
    return f"backup-{prefix}-{stack_id}" if prefix else f"backup-{stack_id}"


def backup_filename(
    stack_id: str,
    include_volumes: bool,
    prefix: Optional[str] = None,
    tz: Optional[str] = None,
) -> str:
    """``backup-<stack>-<full|config>-YYYYmmdd-HHMMSS.tar.gz``, the time on the
    wall clock of ``tz`` — the schedule's zone, or the browser's for a backup
    made by hand. It used to be UTC, so a backup taken at 12:21 in Berlin was
    called …-102154 and seemed to be from two hours earlier. The format is
    unchanged, so retention still matches older files (it orders by mtime)."""
    from services.schedule_service import zone

    date = compose_service.now().astimezone(zone(tz)).strftime("%Y%m%d-%H%M%S")
    suffix = "full" if include_volumes else "config"
    return f"{backup_basename(stack_id, prefix)}-{suffix}-{date}.tar.gz"


def backup_name_pattern(stack_id: str, prefix: Optional[str] = None) -> re.Pattern:
    """Matches exactly this stack's backup files.

    A plain prefix match on ``backup-app-`` also takes ``backup-app-db-…``, so
    retention for stack "app" deleted the backups of stack "app-db".
    """
    base = re.escape(backup_basename(stack_id, prefix))
    return re.compile(rf"^{base}-(?:full|config)-\d{{8}}-\d{{6}}\.tar\.gz$")


# --------------------------------------------------------------------------- #
# Locking and stopping
# --------------------------------------------------------------------------- #


@contextmanager
def _locked(stack_id: str, action: str, owner: str = ""):
    """Hold the stack lock, and register the job so it can be stopped.

    The lock alone made a stuck backup fatal: nothing could touch the stack
    until its twelve-hour TTL ran out. The registration is what gives the
    unlock endpoint something to cancel.
    """
    with Session(engine) as session:
        try:
            stack_lock_service.acquire(session, stack_id, action, owner or action, ttl=LOCK_TTL)
        except stack_lock_service.StackBusy as exc:
            raise StackBusyError(str(exc)) from exc
        try:
            with stack_job_service.track(stack_id, action, owner or action):
                yield
        except stack_job_service.JobCancelled as exc:
            raise BackupCancelled(str(exc)) from exc
        finally:
            try:
                stack_lock_service.release(session, stack_id)
            except Exception:  # noqa: BLE001 - never mask the original error
                logger.exception("Failed to release the lock on '%s'", stack_id)


async def _running(stack_id: str) -> list:
    try:
        containers = await asyncio.to_thread(compose_service.containers_for_stack, stack_id)
    except DockerError as exc:
        raise BackupError(f"Docker is not reachable: {exc.detail or exc.error}") from exc
    return [c for c in containers if c.state == "running"]


def _tail(result: dict) -> str:
    text = (result.get("stderr") or result.get("stdout") or "").strip()
    return text.splitlines()[-1] if text else f"exit {result.get('returncode')}"


def _docker_each(ids: list[str], action: str) -> None:
    client = get_long_client()
    for container_id in ids:
        container = safe_call(client.containers.get, container_id)
        safe_call(getattr(container, action))


async def _compose(stack_id: str, args: list[str]) -> Optional[str]:
    """Run a compose command; None on success, else what went wrong."""
    try:
        result = await compose_service.run_compose(stack_id, args)
    except Exception as exc:  # noqa: BLE001 - timeouts, missing binary
        return str(exc)
    return None if result["returncode"] == 0 else _tail(result)


async def _stop(stack_id: str, running: list) -> None:
    """Stop exactly these containers, or raise and leave them running."""
    services = sorted({c.service for c in running})
    error = await _compose(stack_id, ["stop", *services])
    if error is None:
        return
    # Compose fails on a compose file that was edited (or broken) since the
    # deploy; the containers themselves can still be stopped directly.
    logger.warning("compose stop failed for %s (%s); stopping containers directly", stack_id, error)
    try:
        await asyncio.to_thread(_docker_each, [c.id for c in running], "stop")
    except Exception as exc:  # noqa: BLE001
        await _start(stack_id, running)  # put back whatever did stop
        raise BackupError(
            f"Could not stop '{stack_id}' for a consistent snapshot: {error}"
        ) from exc


async def _start(stack_id: str, running: list) -> Optional[str]:
    """Start exactly these containers again; None on success."""
    if not running:
        return None
    services = sorted({c.service for c in running})
    error = await _compose(stack_id, ["start", *services])
    if error is None:
        return None
    try:
        await asyncio.to_thread(_docker_each, [c.id for c in running], "start")
        return None
    except Exception as exc:  # noqa: BLE001
        logger.warning("Could not restart %s: %s / %s", stack_id, error, exc)
        return f"{error}; {exc}"


# --------------------------------------------------------------------------- #
# Selection
# --------------------------------------------------------------------------- #


def _decide(
    items: list[dict], key: str, chosen: Optional[list[str]], enabled: bool
) -> list[dict]:
    """Mark each inventory item ``selected`` (with a reason when it is not).

    ``chosen`` is an explicit list from the caller; without one the inventory's
    own defaults apply (everything except system paths, oversized directories
    and remote-backed storage).
    """
    for item in items:
        if not enabled:
            item["selected"], item["reason"] = False, "not requested"
        elif chosen is not None:
            selected = item[key] in chosen
            item["selected"] = selected
            item["reason"] = None if selected else "not selected"
        else:
            item["selected"] = bool(item.get("include_default"))
            item["reason"] = None if item["selected"] else (item.get("reason") or "not selected")
        # Never archive plumbing, whatever the caller asked for.
        if item["selected"] and (item.get("system") or item.get("kind") in ("special", "unknown")):
            item["selected"] = False
            item["reason"] = item.get("reason") or "not a regular file or directory"
    return items


def plan(
    stack_id: str,
    include_volumes: bool = True,
    include_binds: bool = True,
    binds: Optional[list[str]] = None,
    volumes: Optional[list[str]] = None,
) -> dict:
    """Decide what a backup captures. Returned as-is by the inventory endpoint."""
    inv = assets.inventory(stack_id)
    _decide(inv["binds"], "source", binds, include_binds)
    _decide(inv["volumes"], "name", volumes, include_volumes)
    return inv


# --------------------------------------------------------------------------- #
# Writing the archive
# --------------------------------------------------------------------------- #


class _ExactReader:
    """Hands tarfile exactly ``size`` bytes of a file that may change while it
    is read. A file that shrinks (a log rotated mid-backup) is padded with
    zeros instead of aborting inside the member, which would leave the whole
    archive unreadable; ``short`` records that it happened."""

    def __init__(self, fh, size: int):
        self._fh = fh
        self._remaining = size
        self.short = False

    def read(self, n: int = -1) -> bytes:
        if n < 0 or n > self._remaining:
            n = self._remaining
        data = self._fh.read(n)
        if len(data) < n:
            self.short = True
            data += b"\0" * (n - len(data))
        self._remaining -= n
        return data


def _add_tree(
    tar: tarfile.TarFile, root: str, arcroot: str, skip: set[str], warnings: list[str]
) -> None:
    """Add the stack directory, tolerating a directory that is in use.

    ``skip`` holds paths (relative to ``root``) whose *contents* stay out: the
    directory entry itself is kept so the mount point exists after a restore.
    Files that vanish mid-walk are left out and unreadable ones reported,
    rather than one of them failing the whole backup. A remote filesystem
    mounted somewhere inside is not descended into.
    """

    def add(path: str, rel: str, parent_dev: int) -> None:
        # A stack directory with a data folder in it can take as long as a
        # volume, so a cancellation lands here too — between two files, with
        # the half-written archive thrown away by the caller.
        stack_job_service.check()
        arc = f"{arcroot}/{rel}" if rel else arcroot
        label = rel or "."
        try:
            st = os.lstat(path)
            if stat.S_ISSOCK(st.st_mode):
                return  # a runtime artefact, meaningless in an archive
            if stat.S_ISREG(st.st_mode):
                if rel in skip:
                    return
                with open(path, "rb") as fh:
                    info = tar.gettarinfo(arcname=arc, fileobj=fh)
                    if info.isreg():
                        reader = _ExactReader(fh, info.size)
                        tar.addfile(info, reader)
                        if reader.short:
                            warnings.append(f"{label}: changed while it was read")
                    else:  # a hard link to a file already in the archive
                        tar.addfile(info)
                return
            info = tar.gettarinfo(path, arcname=arc)
        except FileNotFoundError:
            return
        except OSError as exc:
            warnings.append(f"{label}: {exc.strerror or exc}")
            return
        if info is None:
            return
        tar.addfile(info)
        if not stat.S_ISDIR(st.st_mode) or rel in skip:
            return
        if rel and st.st_dev != parent_dev:
            remote = assets.remote_mount_type(path)
            if remote:
                warnings.append(f"{label}: remote storage ({remote}) mounted here, not included")
                return
        try:
            names = sorted(os.listdir(path))
        except OSError as exc:
            warnings.append(f"{label}: {exc.strerror or exc}")
            return
        for name in names:
            add(os.path.join(path, name), f"{rel}/{name}" if rel else name, st.st_dev)

    add(root, "", os.lstat(root).st_dev)


def _write_archive(
    dest: str,
    manifest: dict,
    directory: str,
    skip: set[str],
    parts: list[tuple[str, str]],
    warnings: list[str],
) -> None:
    with tarfile.open(dest, "w:gz") as tar:
        data = json.dumps(manifest, indent=2).encode("utf-8")
        info = tarfile.TarInfo(MANIFEST_NAME)
        info.size = len(data)
        info.mtime = int(compose_service.now().timestamp())
        tar.addfile(info, io.BytesIO(data))
        _add_tree(tar, directory, "compose", skip, warnings)
        for part, arcname in parts:
            tar.add(part, arcname=arcname)
            os.unlink(part)  # keep the peak disk use to one copy of the data


# --------------------------------------------------------------------------- #
# Backup
# --------------------------------------------------------------------------- #


def _rel_inside(path: str, directory: str) -> Optional[str]:
    rel = os.path.relpath(path, directory)
    if rel == "." or rel.startswith(".." + os.sep) or rel == "..":
        return None
    return rel


async def create_backup_ex(
    stack_id: str,
    name: str,
    include_volumes: bool = True,
    stop_first: bool = True,
    include_binds: bool = True,
    binds: Optional[list[str]] = None,
    volumes: Optional[list[str]] = None,
) -> tuple[str, dict]:
    """Create a backup tar.gz. Returns (path, report).

    The report's ``errors`` lists data that should have been captured and was
    not, and ``restart_error`` a stack that could not be started again — both
    mean the backup must not be reported as a plain success.
    """
    directory = compose_service.stack_dir(stack_id)
    if not os.path.isdir(directory):
        raise BackupError("Stack directory missing")
    with _locked(stack_id, "backup"):
        return await _create(
            stack_id, name, directory, include_volumes, stop_first, include_binds, binds, volumes
        )


async def _create(
    stack_id: str,
    name: str,
    directory: str,
    include_volumes: bool,
    stop_first: bool,
    include_binds: bool,
    binds: Optional[list[str]],
    volumes: Optional[list[str]],
) -> tuple[str, dict]:
    stack_job_service.phase("looking at what to capture")
    selection = await asyncio.to_thread(
        plan, stack_id, include_volumes, include_binds, binds, volumes
    )
    selected_binds = [b for b in selection["binds"] if b["selected"]]
    # Selected bind sources this process cannot reach through the filesystem
    # need their own archive; the rest already ride along in compose/.
    cap_binds = [b for b in selected_binds if b["via"] == "archive"]
    tree_binds = [b for b in selected_binds if b["via"] != "archive"]
    cap_volumes = [v for v in selection["volumes"] if v["selected"]]
    skipped_binds = [b for b in selection["binds"] if not b["selected"]]
    skipped_volumes = [v for v in selection["volumes"] if not v["selected"]]

    # Deselected data that sits inside the stack folder must not sneak into
    # the archive through compose/ (that is how a 200 GB downloads folder
    # ends up in a "config only" backup). A restore leaves these alone.
    excluded = sorted(
        {
            rel
            for b in skipped_binds
            if b.get("inside_stack_dir") and (rel := _rel_inside(b["source"], directory))
        }
    )
    compose_binds = sorted(
        {rel for b in tree_binds if (rel := _rel_inside(b["source"], directory))}
    )

    # Consistent snapshot: stop what runs — data folders inside the stack
    # directory count as data too — and start exactly that again afterwards.
    stopped: list = []
    if stop_first and (cap_volumes or selected_binds):
        running = await _running(stack_id)
        if running:
            await _stop(stack_id, running)
            stopped = running

    workdir = tempfile.mkdtemp(prefix="sp-backup-")
    archive_path: Optional[str] = None
    warnings: list[str] = []
    restart_error: Optional[str] = None
    try:
        parts: list[tuple[str, str]] = []
        for index, bind in enumerate(cap_binds):
            # Between two items is where a cancelled backup gives up: never in
            # the middle of one, and never after the stack has been stopped
            # without the finally below starting it again.
            stack_job_service.check()
            stack_job_service.phase(f"bind {bind['source']}")
            part = os.path.join(workdir, f"bind-{index:03d}.tar")
            try:
                bind["bytes"] = await asyncio.to_thread(
                    assets.export_path, bind["source"], bind["kind"], part
                )
                bind["archive"] = f"binds/{index:03d}.tar"
                parts.append((part, bind["archive"]))
            except Exception as exc:  # noqa: BLE001
                # Cancelling pulls the helper container out from under the
                # read, so the failure here *is* the cancellation.
                stack_job_service.check()
                logger.warning("Could not archive bind %s: %s", bind["source"], exc)
                bind["archive"], bind["error"] = None, str(exc)

        for index, vol in enumerate(cap_volumes):
            stack_job_service.check()
            stack_job_service.phase(f"volume {vol['name']}")
            part = os.path.join(workdir, f"vol-{index:03d}.tar")
            try:
                vol["bytes"] = await asyncio.to_thread(assets.export_volume, vol["name"], part)
                vol["archive"] = f"volumes/{vol['name']}.tar"
                parts.append((part, vol["archive"]))
            except Exception as exc:  # noqa: BLE001
                stack_job_service.check()
                logger.warning("Could not archive volume %s: %s", vol["name"], exc)
                vol["archive"], vol["error"] = None, str(exc)

        failed = [b for b in cap_binds if b.get("error")] + [
            v for v in cap_volumes if v.get("error")
        ]
        manifest = {
            "format_version": BACKUP_FORMAT_VERSION,
            "stack_id": stack_id,
            "name": name,
            "created_at": compose_service.now().isoformat(),
            "include_volumes": include_volumes,
            "include_binds": include_binds,
            "stack_dir": directory,
            "path_mismatch": selection.get("path_mismatch"),
            "stopped": sorted({c.service for c in stopped}),
            # Data folders carried inside compose/, and folders whose contents
            # were left out of it (kept as they are by a restore).
            "compose_binds": compose_binds,
            "excluded": excluded,
            # Volumes keep the v1 shape (full/short/labels) so older StackPilots
            # can still read the manifest they care about.
            "volumes": [
                {
                    "full": v["name"],
                    "short": v["short"],
                    "labels": v["labels"],
                    "driver": v.get("driver"),
                    "options": v.get("options") or {},
                    "external": v.get("external", False),
                    "archive": v["archive"],
                    "remote": v.get("remote", False),
                    "bytes": v.get("bytes"),
                }
                for v in cap_volumes
                if v.get("archive")
            ],
            "binds": [
                {
                    "source": b["source"],
                    "kind": b["kind"],
                    "mounts": b["mounts"],
                    "inside_stack_dir": b["inside_stack_dir"],
                    "archive": b["archive"],
                    "bytes": b.get("bytes"),
                }
                for b in cap_binds
                if b.get("archive")
            ],
            "skipped": [
                {"kind": "bind", "source": b["source"], "reason": b.get("reason")}
                for b in skipped_binds
            ]
            + [
                {"kind": "volume", "name": v["name"], "reason": v.get("reason")}
                for v in skipped_volumes
            ]
            + [
                {
                    "kind": "volume" if "name" in item else "bind",
                    **({"name": item["name"]} if "name" in item else {"source": item["source"]}),
                    "reason": f"backup failed: {item['error']}",
                }
                for item in failed
            ],
        }

        stack_job_service.check()
        stack_job_service.phase("writing the archive")
        fd, archive_path = tempfile.mkstemp(prefix=f"sp-{stack_id}-", suffix=".tar.gz")
        os.close(fd)
        await asyncio.to_thread(
            _write_archive, archive_path, manifest, directory, set(excluded), parts, warnings
        )
    except BaseException:
        if archive_path and os.path.exists(archive_path):
            os.unlink(archive_path)
        raise
    finally:
        shutil.rmtree(workdir, ignore_errors=True)
        if stopped:
            restart_error = await _start(stack_id, stopped)

    errors = [f"{item.get('source') or item.get('name')}: {item['error']}" for item in failed]
    report = {
        "file": archive_path,
        "binds": [
            {"source": b["source"], "bytes": b.get("bytes") or b.get("size"), "error": b.get("error")}
            for b in tree_binds + cap_binds
        ],
        "volumes": [
            {"name": v["name"], "bytes": v.get("bytes"), "error": v.get("error")}
            for v in cap_volumes
        ],
        "skipped": manifest["skipped"],
        "path_mismatch": selection.get("path_mismatch"),
        "stopped": manifest["stopped"],
        "errors": errors,
        "warnings": warnings,
        "restart_error": restart_error,
        "size": os.path.getsize(archive_path),
    }
    return archive_path, report


async def create_backup(
    stack_id: str,
    name: str,
    include_volumes: bool = True,
    stop_first: bool = True,
    include_binds: bool = True,
    binds: Optional[list[str]] = None,
    volumes: Optional[list[str]] = None,
) -> str:
    path, _report = await create_backup_ex(
        stack_id, name, include_volumes, stop_first, include_binds, binds, volumes
    )
    return path


# --------------------------------------------------------------------------- #
# Reading and verifying
# --------------------------------------------------------------------------- #


def read_manifest(tar_path: str) -> dict:
    """The manifest, read without decompressing the rest (it comes first)."""
    try:
        with tarfile.open(tar_path, "r:gz") as tar:
            for member in tar:
                if member.name == MANIFEST_NAME:
                    fh = tar.extractfile(member)
                    if fh is None:
                        break
                    return json.loads(fh.read().decode("utf-8"))
    except (tarfile.TarError, OSError, EOFError, zlib.error, ValueError) as exc:
        raise BackupError(f"Backup archive is damaged or incomplete: {exc}") from exc
    raise BackupError("Backup is missing its manifest")


def verify_archive(tar_path: str) -> None:
    """Read the whole archive once, so a truncated or corrupt file is refused
    before a restore deletes anything. Draining the gzip stream to its end is
    what checks its CRC."""
    try:
        with tarfile.open(tar_path, "r:gz") as tar:
            for member in tar:
                if member.isreg():
                    fh = tar.extractfile(member)
                    while fh is not None and fh.read(1024 * 1024):
                        pass
            while tar.fileobj.read(1024 * 1024):
                pass
    except (tarfile.TarError, OSError, EOFError, zlib.error) as exc:
        raise BackupError(f"Backup archive is damaged or incomplete: {exc}") from exc


# --------------------------------------------------------------------------- #
# Restore — stack directory
# --------------------------------------------------------------------------- #


def _safe_target(dest_dir: str, rel: str) -> str:
    """Resolve a member path inside dest_dir, refusing traversal *and* writes
    through a symlink planted earlier in the same archive."""
    root = os.path.abspath(dest_dir)
    target = os.path.normpath(os.path.join(root, rel))
    if target != root and not target.startswith(root + os.sep):
        raise BackupError(f"Refusing unsafe path in backup: {rel}")
    parent = os.path.dirname(target)
    if os.path.exists(parent):
        real_parent = os.path.realpath(parent)
        if real_parent != root and not real_parent.startswith(root + os.sep):
            raise BackupError(f"Refusing unsafe path in backup: {rel}")
    return target


def _apply_meta(path: str, member: tarfile.TarInfo) -> None:
    """Restore mode/ownership/mtime — *arr-style images run as PUID/PGID and
    break when their config comes back root-owned with default permissions."""
    try:
        os.chown(path, member.uid, member.gid)
    except (OSError, AttributeError):
        pass
    try:
        # After chown, which clears setuid/setgid bits.
        os.chmod(path, member.mode)
    except OSError:
        pass
    try:
        os.utime(path, (member.mtime, member.mtime))
    except OSError:
        pass


def _under(rel: str, prefixes: set[str]) -> bool:
    return any(rel.startswith(p + "/") for p in prefixes)


def _extract_tree(
    tar: tarfile.TarFile, prefix: str, dest_dir: str, skip: frozenset | set = frozenset()
) -> None:
    """Extract one subtree of the archive, preserving metadata and symlinks.

    Contents under a ``skip`` path (relative) are not extracted; the path's own
    directory entry still is.
    """
    os.makedirs(dest_dir, exist_ok=True)
    root_name = prefix.rstrip("/")
    dirs: list[tuple[str, tarfile.TarInfo]] = []
    for member in tar.getmembers():
        if member.name == root_name:
            if member.isdir():
                dirs.append((dest_dir, member))
            continue
        if not member.name.startswith(prefix):
            continue
        rel = member.name[len(prefix) :].lstrip("/")
        if not rel or _under(rel, skip):
            continue
        target = _safe_target(dest_dir, rel)
        existing_dir = os.path.isdir(target) and not os.path.islink(target)
        if member.isdir():
            if os.path.lexists(target) and not existing_dir:
                os.unlink(target)
            os.makedirs(target, exist_ok=True)
            dirs.append((target, member))
            continue
        os.makedirs(os.path.dirname(target), exist_ok=True)
        # Never write through (or over) something an earlier member put here:
        # open() would follow a symlink to wherever it points.
        if existing_dir:
            shutil.rmtree(target)
        elif os.path.lexists(target):
            os.unlink(target)
        if member.issym():
            os.symlink(member.linkname, target)
            continue  # chmod/utime would follow the link
        if member.islnk():
            source = _safe_target(dest_dir, member.linkname[len(prefix) :].lstrip("/"))
            if os.path.exists(source):
                os.link(source, target)
            continue
        if not member.isreg():
            continue  # devices/fifos/sockets are runtime artefacts
        src = tar.extractfile(member)
        if src is None:
            continue
        with open(target, "wb") as out:
            shutil.copyfileobj(src, out)
        _apply_meta(target, member)
    # Directory metadata last: a 0500 directory would block writing its files.
    for target, member in sorted(dirs, key=lambda d: len(d[0]), reverse=True):
        _apply_meta(target, member)


def _remove_tree(path: str, dev: int, rel: str, warnings: list[str]) -> None:
    """rmtree that never crosses into another filesystem (a mounted disk or
    share inside the stack folder is left as it is)."""
    with os.scandir(path) as it:
        entries = list(it)
    for entry in entries:
        entry_rel = f"{rel}/{entry.name}"
        if entry.is_dir(follow_symlinks=False):
            if entry.stat(follow_symlinks=False).st_dev != dev:
                warnings.append(f"{entry_rel}: another filesystem is mounted here, left untouched")
                continue
            _remove_tree(entry.path, dev, entry_rel, warnings)
        else:
            os.unlink(entry.path)
    try:
        os.rmdir(path)
    except OSError:
        pass  # still holds a mount point


def _clear(directory: str, preserve: set[str], warnings: list[str]) -> None:
    """Empty the stack directory for the restored tree, except what the backup
    does not contain: ``preserve`` paths and other filesystems mounted inside."""
    root_dev = os.lstat(directory).st_dev

    def walk(path: str, rel: str) -> None:
        with os.scandir(path) as it:
            entries = list(it)
        for entry in entries:
            entry_rel = f"{rel}/{entry.name}" if rel else entry.name
            if entry_rel in preserve:
                continue
            if entry.is_dir(follow_symlinks=False):
                if entry.stat(follow_symlinks=False).st_dev != root_dev:
                    warnings.append(
                        f"{entry_rel}: another filesystem is mounted here, left untouched"
                    )
                    continue
                if any(p.startswith(entry_rel + "/") for p in preserve):
                    walk(entry.path, entry_rel)
                else:
                    _remove_tree(entry.path, root_dev, entry_rel, warnings)
            else:
                os.unlink(entry.path)

    walk(directory, "")


def _copy_dir_meta(src: str, dst: str) -> None:
    st = os.lstat(src)
    try:
        os.chown(dst, st.st_uid, st.st_gid)
        os.chmod(dst, stat.S_IMODE(st.st_mode))
        os.utime(dst, (st.st_atime, st.st_mtime))
    except OSError:
        pass


def _merge(src: str, dst: str, rel: str, warnings: list[str]) -> None:
    """Move the staged tree into the (cleared) stack directory."""
    for name in sorted(os.listdir(src)):
        s, d = os.path.join(src, name), os.path.join(dst, name)
        entry_rel = f"{rel}/{name}" if rel else name
        s_dir = os.path.isdir(s) and not os.path.islink(s)
        if os.path.lexists(d):
            d_dir = os.path.isdir(d) and not os.path.islink(d)
            if s_dir and d_dir:
                _merge(s, d, entry_rel, warnings)
                _copy_dir_meta(s, d)
                continue
            if d_dir:
                # Only kept (preserved or mounted) directories survive _clear.
                warnings.append(f"{entry_rel}: kept the existing folder, the backup has a file here")
                continue
            os.unlink(d)
        shutil.move(s, d)


def _restore_tree(
    tar: tarfile.TarFile,
    directory: str,
    preserve: set[str],
    skip_extract: set[str],
    warnings: list[str],
) -> None:
    """Replace the stack directory with the backup's, staged next to it first
    so a failing extraction leaves the old one untouched."""
    root = os.path.dirname(directory)
    os.makedirs(root, exist_ok=True)
    staging = tempfile.mkdtemp(prefix=f".restore-{os.path.basename(directory)}-", dir=root)
    try:
        _extract_tree(tar, "compose/", staging, skip=skip_extract)
        if not os.path.isdir(directory):
            shutil.move(staging, directory)
            return
        _clear(directory, preserve, warnings)
        _merge(staging, directory, "", warnings)
        _copy_dir_meta(staging, directory)
    finally:
        shutil.rmtree(staging, ignore_errors=True)


def _tree_paths(manifest: dict) -> tuple[set[str], set[str]]:
    """(excluded, compose_binds) relative to the stack directory.

    Format 3 records them. For older backups the excluded folders are worked
    out from the skipped binds; which data folders compose/ carries is unknown.
    """
    if "excluded" in manifest:
        return set(manifest.get("excluded") or []), set(manifest.get("compose_binds") or [])
    old_dir = manifest.get("stack_dir") or ""
    excluded = set()
    for item in manifest.get("skipped", []):
        if item.get("kind") == "bind" and old_dir and item.get("source"):
            rel = _rel_inside(item["source"], old_dir)
            if rel:
                excluded.add(rel)
    return excluded, set()


# --------------------------------------------------------------------------- #
# Restore
# --------------------------------------------------------------------------- #


def _target_id(manifest: dict, target_id: Optional[str]) -> str:
    raw = target_id or manifest.get("stack_id") or ""
    if not raw.strip():
        raise BackupError("Backup manifest has no stack id")
    # Slugify whichever id we end up using — the manifest comes from an
    # uploaded file, so its stack_id must never be able to escape STACKS_DIR.
    return compose_service.slugify(raw)


def _extract_member(tar: tarfile.TarFile, name: str) -> Optional[str]:
    """Copy one archive member to a temp file; None if it is missing."""
    try:
        member = tar.getmember(name)
    except KeyError:
        return None
    fh = tar.extractfile(member)
    if fh is None:
        return None
    with tempfile.NamedTemporaryFile(delete=False, suffix=".tar") as tmp:
        shutil.copyfileobj(fh, tmp)
        return tmp.name


def restore_backup(
    tar_path: str,
    target_id: Optional[str] = None,
    overwrite: bool = False,
    restore_volumes: bool = True,
    restore_binds: bool = True,
    verified: bool = False,
) -> dict:
    """Restore a backup's files, folders and volumes. Returns a report.

    This is the part that writes; :func:`restore` wraps it with the lock and
    with stopping and starting the stack.
    """
    manifest = read_manifest(tar_path)
    stack_id = _target_id(manifest, target_id)
    old_id = compose_service.slugify(manifest.get("stack_id") or stack_id)
    renamed = stack_id != old_id
    # Volumes are named after the compose *project*, which differs from the id
    # for a folder compose had to normalise ("my.app" -> "myapp_data").
    old_project = compose_service.project_name(manifest.get("stack_id") or stack_id)
    new_project = compose_service.project_name(stack_id)
    old_dir = manifest.get("stack_dir") or ""

    directory = compose_service.stack_dir(stack_id)
    if os.path.isdir(directory) and not overwrite:
        raise BackupError(f"Stack '{stack_id}' already exists")
    if not verified:
        verify_archive(tar_path)

    excluded, compose_binds = _tree_paths(manifest)
    # Not restoring data folders means keeping the ones there are, not
    # emptying them: compose/ carries those inside the stack directory.
    held_back = compose_binds if not restore_binds else set()
    volumes_restored = 0
    binds_restored = 0
    skipped: list[dict] = []
    warnings: list[str] = []

    with tarfile.open(tar_path, "r:gz") as tar:
        _restore_tree(tar, directory, excluded | held_back, held_back, warnings)
        skipped += [{"kind": "bind", "source": rel, "reason": "not requested"} for rel in held_back]

        for bind in manifest.get("binds", []):
            archive = bind.get("archive")
            if not archive:
                continue
            source = bind["source"]
            if not restore_binds:
                skipped.append({"kind": "bind", "source": source, "reason": "not requested"})
                continue
            if bind.get("inside_stack_dir") and old_dir:
                # A renamed stack must not write into the old stack's folder.
                source = os.path.normpath(os.path.join(directory, os.path.relpath(source, old_dir)))
            elif renamed:
                skipped.append({
                    "kind": "bind",
                    "source": source,
                    "reason": f"belongs to the original stack '{old_id}'; "
                    "restore under that name to write it back",
                })
                continue
            if assets.is_system_path(source):
                skipped.append({"kind": "bind", "source": source, "reason": "system path"})
                continue
            part = _extract_member(tar, archive)
            if part is None:
                skipped.append({"kind": "bind", "source": source, "reason": "missing from archive"})
                continue
            try:
                assets.import_path(source, bind.get("kind", "dir"), part)
                binds_restored += 1
            except Exception as exc:  # noqa: BLE001
                logger.warning("Could not restore bind %s: %s", source, exc)
                skipped.append({"kind": "bind", "source": source, "reason": str(exc)})
            finally:
                os.unlink(part)

        for v in manifest.get("volumes", []):
            full = v["full"]
            if not restore_volumes:
                skipped.append({"kind": "volume", "name": full, "reason": "not requested"})
                continue
            retargeted = False
            if old_project != new_project and full.startswith(old_project + "_"):
                full = new_project + full[len(old_project) :]
                retargeted = True
            labels = dict(v.get("labels") or {})
            if not v.get("external"):
                # Re-target compose's labels to the (possibly new) project.
                labels[COMPOSE_PROJECT_LABEL] = new_project
            reason = _volume_conflict(full, new_project, old_id, renamed and not retargeted)
            if reason:
                skipped.append({"kind": "volume", "name": full, "reason": reason})
                continue
            part = _extract_member(tar, v.get("archive") or f"volumes/{v['full']}.tar")
            if part is None:
                skipped.append({"kind": "volume", "name": full, "reason": "missing from archive"})
                continue
            try:
                # Remote-backed volumes (NFS/CIFS) are never wiped: that
                # would delete the share the volume points at.
                assets.import_volume(
                    full,
                    labels,
                    part,
                    wipe=not v.get("remote", False),
                    driver=v.get("driver"),
                    driver_opts=v.get("options") or None,
                )
                volumes_restored += 1
            except Exception as exc:  # noqa: BLE001
                logger.warning("Could not restore volume %s: %s", full, exc)
                skipped.append({"kind": "volume", "name": full, "reason": str(exc)})
            finally:
                os.unlink(part)

    # What went wrong now, as opposed to what the backup itself left out
    # (listed in skipped as well, for the full picture).
    problems = [
        f"{item.get('name') or item.get('source')}: {item['reason']}"
        for item in skipped
        if item["reason"] != "not requested"
    ] + warnings
    skipped += manifest.get("skipped", [])
    return {
        "stack_id": stack_id,
        # Restored under a new id, the copy must not carry the original's
        # display name — two stacks called the same in the list is how the
        # wrong one gets stopped.
        "name": stack_id if renamed else manifest.get("name", stack_id),
        "volumes_restored": volumes_restored,
        "binds_restored": binds_restored,
        "skipped": skipped,
        "warnings": warnings,
        "problems": problems,
    }


def _volume_conflict(full: str, project: str, old_id: str, shared_with_original: bool) -> Optional[str]:
    """Why a volume must not be overwritten, or None if it may be."""
    attrs = assets.volume_attrs(full)
    if attrs is None:
        return None  # created fresh
    owner = (attrs.get("Labels") or {}).get(COMPOSE_PROJECT_LABEL)
    if owner and owner != project:
        return f"belongs to stack '{owner}'; not overwritten"
    if shared_with_original:
        return (
            f"is not named after the stack, so it is still the original "
            f"'{old_id}' stack's volume; not overwritten"
        )
    users = assets.running_users(full)
    if users:
        return f"in use by running container(s) {', '.join(users)}; not overwritten"
    return None


async def restore(
    tar_path: str,
    target_id: Optional[str] = None,
    overwrite: bool = False,
    restore_volumes: bool = True,
    restore_binds: bool = True,
) -> dict:
    """Restore with the stack locked, stopped while its data is replaced, and
    brought back up afterwards if it was running."""
    manifest = await asyncio.to_thread(read_manifest, tar_path)
    stack_id = _target_id(manifest, target_id)
    directory = compose_service.stack_dir(stack_id)
    if os.path.isdir(directory) and not overwrite:
        raise BackupError(f"Stack '{stack_id}' already exists")

    with _locked(stack_id, "restore"):
        await asyncio.to_thread(verify_archive, tar_path)
        try:
            running = await _running(stack_id)
        except BackupError:
            running = []  # no Docker: the volume part will say so
        if running:
            # Replacing a database's files under it corrupts it.
            await _stop(stack_id, running)
        try:
            result = await asyncio.to_thread(
                restore_backup, tar_path, target_id, overwrite,
                restore_volumes, restore_binds, True,
            )
        except BaseException:
            if running:
                await _start(stack_id, running)
            raise
        result["restarted"] = False
        if running:
            # up, not start: the restored compose file may differ from what ran.
            error = await _compose(stack_id, ["up", "-d", "--remove-orphans"])
            result["restarted"] = error is None
            if error:
                result["restart_error"] = error
                result["problems"].append(f"stack did not start again: {error}")
        return result