hermes-webui/api/updates.py

"""
Hermes Web UI -- Self-update checker.

Checks if the webui and hermes-agent git repos are behind their upstream
branches. Results are cached server-side (30-min TTL) so git fetch runs
at most twice per hour regardless of client count.

Skips repos that are not git checkouts (e.g. Docker baked images where
.git does not exist).
"""
import hashlib
import json
import re
import subprocess
import threading
import time
from collections import OrderedDict
from pathlib import Path
from urllib.parse import urlparse

from api.config import REPO_ROOT, STREAMS, STREAMS_LOCK

# Lazy -- may be None if agent not found
try:
    from api.config import _AGENT_DIR
except ImportError:
    _AGENT_DIR = None

_update_cache = {'webui': None, 'agent': None, 'checked_at': 0}
_SUMMARY_CACHE_MAX = 16
_summary_cache: OrderedDict = OrderedDict()
_cache_lock = threading.Lock()
_check_in_progress = False
_apply_lock = threading.Lock()   # prevents concurrent stash/pull/pop on same repo
CACHE_TTL = 1800  # 30 minutes


def _active_stream_count() -> int:
    """Return the current in-memory chat stream count.

    Self-update schedules an in-process re-exec after git pull/reset.  That is
    restart-equivalent for live streams, even when systemd does not see a unit
    restart.  Refuse update/force-update while a stream exists so a browser
    update click cannot recreate the pending-message loss class fixed in #1543.
    """
    with STREAMS_LOCK:
        return len(STREAMS)


def _restart_blocked_response(target: str, active_streams: int) -> dict:
    plural = "s" if active_streams != 1 else ""
    return {
        'ok': False,
        'message': (
            f'Cannot update {target} while {active_streams} active chat stream{plural} '
            'is running. Wait for the response to finish, then retry the update.'
        ),
        'target': target,
        'restart_blocked': True,
        'active_streams': active_streams,
    }


def _run_git(args, cwd, timeout=10):
    """Run a git command and return (useful output, ok).

    On failure, returns stderr (or stdout as fallback) so callers can
    surface actionable git error messages instead of empty strings.
    """
    try:
        r = subprocess.run(
            ['git'] + args, cwd=str(cwd), capture_output=True,
            text=True, timeout=timeout,
        )
        stdout = r.stdout.strip()
        stderr = r.stderr.strip()
        if r.returncode == 0:
            return stdout, True
        return stderr or stdout or f"git exited with status {r.returncode}", False
    except subprocess.TimeoutExpired as exc:
        detail = (getattr(exc, 'stderr', None) or getattr(exc, 'stdout', None) or '').strip()
        return detail or f"git {' '.join(args)} timed out after {timeout}s", False
    except FileNotFoundError:
        return 'git executable not found', False
    except OSError as exc:
        return f'git failed to start: {exc}', False


def _detect_webui_version() -> str:
    """Detect the running WebUI version from git or a baked-in fallback file.

    Resolution order:
      1. ``git describe --tags --always --dirty`` — works in any git checkout.
         Returns the exact tag on tagged commits (e.g. ``v0.50.124``), a
         post-tag descriptor between releases (e.g. ``v0.50.124-1-ge91325d``),
         or a bare SHA when no tags exist (shallow clones, fresh forks).
      2. ``api/_version.py`` — a fallback written by the Docker / CI release
         workflow when ``.git`` is not present in the image.  Expected to define
         ``__version__ = 'vX.Y.Z'``.
      3. ``'unknown'`` — last resort; displayed as-is in the settings badge.
    """
    # Timeout capped at 3s: git describe on a healthy local repo is <50ms;
    # a 10s stall on import (NFS-mounted .git, broken git binary) is unacceptable.
    out, ok = _run_git(['describe', '--tags', '--always', '--dirty'], REPO_ROOT, timeout=3)
    if ok and out:
        return out

    # Docker / baked-image fallback: api/_version.py written by CI at build time.
    # Parse with regex rather than exec() — the file holds exactly one assignment
    # and regex is sufficient; exec() on a build artifact is an unnecessary surface.
    version_file = REPO_ROOT / 'api' / '_version.py'
    if version_file.exists():
        try:
            import re as _re
            m = _re.search(
                r"""__version__\s*=\s*['"]([^'"]+)['"]""",
                version_file.read_text(encoding='utf-8'),
            )
            if m:
                return m.group(1)
        except Exception:
            pass

    return 'unknown'


def _detect_agent_version() -> str:
    """Detect the running Hermes Agent version for UI display."""
    if _AGENT_DIR is None:
        return 'not detected'

    version_file = Path(_AGENT_DIR) / "VERSION"
    try:
        if version_file.exists():
            text = version_file.read_text(encoding='utf-8').strip()
            if text:
                return text
    except Exception:
        pass

    # Fallback: infer from git describe when the checkout exists but no VERSION
    # file is available (common in source checkouts and developer environments).
    if not Path(_AGENT_DIR).exists():
        return 'not detected'
    # Symmetric with _detect_webui_version() above — `--dirty` flags a
    # locally-modified checkout so operators can see when their agent has
    # uncommitted changes vs a clean tag. Per Opus advisor on stage-293.
    out, ok = _run_git(['describe', '--tags', '--always', '--dirty'], _AGENT_DIR, timeout=3)
    if ok and out:
        return out

    return 'not detected'


# Resolved once at import time — tags cannot change without a process restart.
WEBUI_VERSION: str = _detect_webui_version()
AGENT_VERSION: str = _detect_agent_version()


def _normalize_remote_url(remote_url):
    """Return the browser-facing repository URL for update compare links.

    Git remotes may be HTTPS or SSH and may include a literal ``.git`` suffix.
    Strip only that literal suffix — never use ``str.rstrip('.git')`` because it
    treats the argument as a character set and can truncate ``hermes-webui`` to
    ``hermes-webu``.
    """
    if not remote_url:
        return remote_url
    remote_url = remote_url.strip()
    if remote_url.startswith('git@'):
        remote_url = remote_url.replace(':', '/', 1).replace('git@', 'https://', 1)
    remote_url = remote_url.rstrip('/')
    if remote_url.endswith('.git'):
        remote_url = remote_url[:-4]
    return remote_url.rstrip('/')


def _build_compare_url(repo_url, current_sha, latest_sha):
    """Return a safe browser compare URL, or None when any piece is missing."""
    if not (repo_url and current_sha and latest_sha):
        return None
    parsed = urlparse(repo_url)
    if parsed.scheme not in ('http', 'https') or not parsed.netloc:
        return None
    return f"{repo_url}/compare/{current_sha}...{latest_sha}"


def _split_remote_ref(ref):
    """Split 'origin/branch-name' into ('origin', 'branch-name').

    Returns (None, ref) if ref contains no slash.
    """
    if '/' not in ref:
        return None, ref
    remote, branch = ref.split('/', 1)
    return remote, branch


def _detect_default_branch(path):
    """Detect the remote default branch (master or main)."""
    out, ok = _run_git(['symbolic-ref', 'refs/remotes/origin/HEAD'], path)
    if ok and out:
        # refs/remotes/origin/master -> master
        return out.split('/')[-1]
    # Fallback: try master, then main
    for branch in ('master', 'main'):
        _, ok = _run_git(['rev-parse', '--verify', f'origin/{branch}'], path)
        if ok:
            return branch
    return 'master'


def _check_repo(path, name):
    """Check if a git repo is behind its upstream. Returns dict or None."""
    if path is None or not (path / '.git').exists():
        return None

    # Fetch latest from origin (network call, cached by TTL)
    _, fetch_ok = _run_git(['fetch', 'origin', '--quiet'], path, timeout=15)
    if not fetch_ok:
        return {'name': name, 'behind': 0, 'error': 'fetch failed'}

    # Use the current branch's upstream tracking branch, not the repo default.
    # This avoids false "N updates behind" alerts when the user is on a feature
    # branch and master/main has moved forward with unrelated commits.
    # If no upstream is set (brand-new local branch), fall back to the default branch.
    upstream, ok = _run_git(['rev-parse', '--abbrev-ref', '@{upstream}'], path)
    if ok and upstream:
        # upstream is like "origin/feat/foo" — use it directly in rev-list
        compare_ref = upstream
    else:
        branch = _detect_default_branch(path)
        compare_ref = f'origin/{branch}'

    # Count commits behind
    out, ok = _run_git(['rev-list', '--count', f'HEAD..{compare_ref}'], path)
    behind = int(out) if ok and out.isdigit() else 0

    # Get short SHAs for display.
    #
    # latest_sha = upstream tip (compare_ref). Always exists on github.com
    # because it is literally the commit `git fetch` just pulled.
    #
    # current_sha is trickier. The intuitive choice — local HEAD — breaks
    # the "What's new?" compare URL whenever HEAD is not a public commit:
    # unpushed work, dirty stage branches, forks, in-flight rebases, or
    # release-time merge commits whose SHA only lives in the maintainer's
    # checkout. We saw exactly this in #1579: a banner reporting "17 updates"
    # linked to /compare/<localHEAD>...<upstream> and 404'd because <localHEAD>
    # was never pushed to the canonical repo.
    #
    # The right base is the merge-base between HEAD and the upstream ref —
    # that's the most recent commit both sides agree on, and (because
    # `git fetch` succeeded above) it is guaranteed to be present upstream.
    # If a user is 17 commits behind with no local-only commits, merge-base
    # equals local HEAD and the URL is identical to what we shipped before;
    # if they ARE ahead with local-only commits, the URL still resolves to
    # the public history they share with upstream. If merge-base fails for
    # any reason (e.g. shallow clone where the bases diverge before the
    # cutoff), fall back to None so the JS link guard suppresses the link
    # rather than emitting a known-broken URL.
    mb_full, mb_ok = _run_git(['merge-base', 'HEAD', compare_ref], path)
    if mb_ok and mb_full:
        short, ok = _run_git(['rev-parse', '--short', mb_full], path)
        current = short if (ok and short) else None
    else:
        current = None
    latest, _ = _run_git(['rev-parse', '--short', compare_ref], path)

    # Get repo URL for "What's new?" link
    remote_url, _ = _run_git(['remote', 'get-url', 'origin'], path)
    remote_url = _normalize_remote_url(remote_url)

    return {
        'name': name,
        'behind': behind,
        'current_sha': current,
        'latest_sha': latest,
        'branch': compare_ref,
        'repo_url': remote_url,
        'compare_url': _build_compare_url(remote_url, current, latest),
    }


def check_for_updates(force=False):
    """Return cached update status for webui and agent repos."""
    global _check_in_progress
    with _cache_lock:
        if not force and time.time() - _update_cache['checked_at'] < CACHE_TTL:
            return dict(_update_cache)
        if _check_in_progress:
            return dict(_update_cache)  # another thread is already checking
        _check_in_progress = True

    try:
        # Run checks outside the lock (network I/O)
        webui_info = _check_repo(REPO_ROOT, 'webui')
        agent_info = _check_repo(_AGENT_DIR, 'agent')

        with _cache_lock:
            _update_cache['webui'] = webui_info
            _update_cache['agent'] = agent_info
            _update_cache['checked_at'] = time.time()
            return dict(_update_cache)
    finally:
        _check_in_progress = False


def _repo_path_for_update_target(target: str):
    if target == 'webui':
        return REPO_ROOT
    if target == 'agent':
        return _AGENT_DIR
    return None


def _commit_subjects_for_update(info: dict, *, limit: int = 24) -> list[str]:
    """Return commit subjects for an update range, if the local git refs exist."""
    subjects, _truncated = _commit_subjects_for_update_with_limit(info, limit=limit)
    return subjects


def _commit_subjects_for_update_with_limit(info: dict, *, limit: int = 24) -> tuple[list[str], bool]:
    """Return recent commit subjects plus whether the local list was capped."""
    if not isinstance(info, dict):
        return [], False
    target = info.get('name')
    if target not in ('webui', 'agent'):
        target = 'webui' if info.get('repo_url', '').endswith('hermes-webui') else target
    path = _repo_path_for_update_target(target)
    if path is None or not (Path(path) / '.git').exists():
        return [], False
    current = str(info.get('current_sha') or '').strip()
    latest = str(info.get('latest_sha') or '').strip()
    if not (current and latest):
        return [], False
    probe_limit = max(1, int(limit)) + 1
    out, ok = _run_git(['log', '--format=%s', f'{current}..{latest}', f'-n{probe_limit}'], path, timeout=5)
    if not ok or not out:
        return [], False
    subjects = [line.strip() for line in out.splitlines() if line.strip()]
    truncated = len(subjects) > limit
    return subjects[:limit], truncated


def _summary_cache_key(updates: dict, details: list[dict]) -> str:
    """Stable key for the exact update range being summarized."""
    payload = []
    for item in details:
        payload.append({
            'name': item.get('name'),
            'behind': item.get('behind'),
            'current_sha': item.get('current_sha'),
            'latest_sha': item.get('latest_sha'),
            'compare_url': item.get('compare_url'),
        })
    blob = json.dumps(payload, sort_keys=True, separators=(',', ':'))
    return hashlib.sha256(blob.encode('utf-8')).hexdigest()


def _clean_summary_bullet(line: str) -> str:
    line = re.sub(r'^\s*(?:[-*•]+|\d+[.)])\s*', '', str(line or '')).strip()
    line = re.sub(r'\s+', ' ', line)
    if not line:
        return ''
    if line[-1] not in '.!?':
        line += '.'
    return line[:240]


def _summary_bullets_from_text(text: str, *, fallback_items: list[str]) -> list[str]:
    raw = str(text or '').strip()
    candidates = []
    for line in raw.splitlines():
        cleaned = _clean_summary_bullet(line)
        if cleaned:
            candidates.append(cleaned)
    if len(candidates) <= 1 and raw:
        candidates = [_clean_summary_bullet(part) for part in re.split(r'(?<=[.!?])\s+', raw)]
        candidates = [item for item in candidates if item]
    if not candidates:
        candidates = [_clean_summary_bullet(item) for item in fallback_items]
    seen = set()
    bullets = []
    for item in candidates:
        key = item.lower()
        if item and key not in seen:
            bullets.append(item)
            seen.add(key)
        if len(bullets) >= 5:
            break
    return bullets or ['Updates are available.']


def _fallback_update_bullets(details: list[dict]) -> list[str]:
    bullets = []
    for item in details:
        label = item.get('label') or item.get('name') or 'Hermes'
        behind = item.get('behind') or 0
        commits = item.get('commits') or []
        if commits:
            highlights = '; '.join(commits[:3])
            qualifier = 'recent updates' if item.get('commits_truncated') else 'updates'
            bullets.append(f"{label} has {behind} update(s), including {qualifier}: {highlights}.")
        else:
            bullets.append(f"{label} has {behind} update(s) available.")
    return bullets or ['Updates are available.']


def _worth_knowing_bullets(details: list[dict]) -> list[str]:
    items = []
    truncated = [item for item in details if item.get('commits_truncated') and item.get('commits_limit')]
    for item in truncated[:2]:
        label = item.get('label') or item.get('name') or 'Hermes'
        behind = item.get('behind') or 0
        limit = item.get('commits_limit') or len(item.get('commits') or [])
        items.append(
            f"{label} has {behind} updates; this summary uses the latest {limit} commit subjects, with the full comparison still available in the diff link."
        )
    if items:
        return items
    targets = [
        f"{item.get('label') or item.get('name') or 'Hermes'} ({item.get('behind') or 0} update{'s' if (item.get('behind') or 0) != 1 else ''})"
        for item in details
        if item.get('behind')
    ]
    if len(targets) > 1:
        return ['This summary combines updates from ' + ' and '.join(targets) + '.']
    return []


def _format_update_summary_sections(summary_text: str, details: list[dict]) -> tuple[list[dict], str]:
    bullets = _summary_bullets_from_text(summary_text, fallback_items=_fallback_update_bullets(details))
    if len(bullets) > 1:
        notice_items = bullets[:3]
        worth_items = bullets[3:]
    else:
        notice_items = bullets
        worth_items = []
    notice_keys = {item.lower() for item in notice_items}
    worth_items = [item for item in worth_items if item.lower() not in notice_keys]
    if not worth_items:
        worth_items = [
            item for item in _worth_knowing_bullets(details)
            if item.lower() not in notice_keys
        ]
    worth_items = worth_items[:2]
    sections = [
        {
            'title': "What you'll notice",
            'items': notice_items,
        },
    ]
    if worth_items:
        sections.append(
            {
                'title': 'Worth knowing',
                'items': worth_items,
            }
        )
    lines = []
    for section in sections:
        lines.append(section['title'])
        lines.extend(f"- {item}" for item in section['items'])
        lines.append('')
    return sections, '\n'.join(lines).strip()


def _fallback_update_summary(updates: dict, details: list[dict]) -> str:
    _sections, summary = _format_update_summary_sections('', details)
    return summary


def _update_summary_prompt(details: list[dict]) -> tuple[str, str]:
    system = (
        "You write human-readable release summaries for Hermes users. "
        "Focus on what the user will notice in the product. Keep it simple, specific, and short. "
        "avoid technical jargon, implementation details, SHA names, branch names, and file paths unless necessary. "
        "Return only bullets. Do not include headings, markdown tables, intro paragraphs, or closing notes."
    )
    user_lines = [
        "Summarize these available updates as 3-5 concise bullets.",
        "Use everyday language and explain visible behavior changes, not code mechanics.",
        "Return only bullets; the WebUI will add the fixed section headings separately.",
        "",
    ]
    for item in details:
        user_lines.append(f"{item['label']}: {item['behind']} commit(s) behind")
        commits = item.get('commits') or []
        if commits:
            if item.get('commits_truncated'):
                user_lines.append(
                    f"- Showing latest {len(commits)} of {item['behind']} commit subjects; summarize trends, not every commit."
                )
            user_lines.extend(f"- {subject}" for subject in commits)
        else:
            user_lines.append("- No local commit subjects available; summarize only the update count.")
        user_lines.append("")
    return system, '\n'.join(user_lines)


def summarize_update_payload(updates: dict, llm_callback=None, *, target: str | None = None, use_cache: bool = True) -> dict:
    """Build a human-readable What's New summary and keep regular diff comparison links.

    ``llm_callback`` receives ``(system_prompt, user_prompt)`` and returns text.
    The caller may wire that to AIAgent; this module keeps a deterministic
    fallback so the banner remains useful when no LLM provider is configured.
    Summaries are cached per exact update range so refreshes do not generate
    slightly different wording for the same available updates.
    """
    if not isinstance(updates, dict):
        updates = {}
    requested_target = target if target in ('webui', 'agent') else None
    details = []
    for key, label in (('webui', 'WebUI'), ('agent', 'Agent')):
        if requested_target and key != requested_target:
            continue
        info = updates.get(key)
        if not isinstance(info, dict) or int(info.get('behind') or 0) <= 0:
            continue
        commit_limit = 24
        commits, commits_truncated = _commit_subjects_for_update_with_limit({'name': key, **info}, limit=commit_limit)
        behind = int(info.get('behind') or 0)
        item = {
            'name': key,
            'label': label,
            'behind': behind,
            'current_sha': info.get('current_sha'),
            'latest_sha': info.get('latest_sha'),
            'compare_url': info.get('compare_url'),
            'commits': commits,
            'commits_limit': commit_limit,
            'commits_truncated': bool(commits_truncated or (commits and behind > len(commits))),
        }
        details.append(item)
    cache_key = _summary_cache_key(updates, details)
    if use_cache:
        with _cache_lock:
            cached = _summary_cache.get(cache_key)
            if cached:
                _summary_cache.move_to_end(cache_key)
        if cached:
            result = dict(cached)
            result['cached'] = True
            return result

    generated_by = 'fallback'
    candidate = ''
    if details and callable(llm_callback):
        system, prompt = _update_summary_prompt(details)
        try:
            candidate = (llm_callback(system, prompt) or '').strip()
            if candidate:
                generated_by = 'llm'
        except Exception:
            candidate = ''
    sections, summary = _format_update_summary_sections(candidate, details)
    result = {
        'ok': True,
        'summary': summary,
        'summary_sections': sections,
        'generated_by': generated_by,
        'cached': False,
        'cache_key': cache_key,
        'target': requested_target,
        'targets': details,
    }
    if use_cache:
        with _cache_lock:
            if len(_summary_cache) >= _SUMMARY_CACHE_MAX and cache_key not in _summary_cache:
                _summary_cache.popitem(last=False)
            _summary_cache[cache_key] = dict(result)
    return result


# ── Self-update application ───────────────────────────────────────────────────


def _schedule_restart(delay: float = 2.0) -> None:
    """Re-exec this process after *delay* seconds.

    Called after a successful update so that the freshly-pulled code is
    loaded on the next request, rather than running with a mix of old and
    new Python modules in sys.modules.

    os.execv() replaces the current process image with a fresh interpreter
    running the same argv — sessions are preserved on disk, the HTTP port
    is reclaimed within the delay window, and the client's own
    ``setTimeout(() => location.reload(), 2500)`` lands after the restart.

    Coordinates with ``_apply_lock``: when the user updates both webui
    and agent, the client POSTs them sequentially.  Without coordination
    the restart timer scheduled by the first update's success would fire
    while the second update's git-pull is still running, killing it mid-
    stream and leaving the second repo in an unknown partial state.
    Blocking on ``_apply_lock`` before ``os.execv`` means a pending
    second update always completes before the restart happens.
    """
    import os
    import sys

    def _do():
        import time
        time.sleep(delay)
        # Hold _apply_lock through os.execv so no new update can start between
        # the lock-release and the process replacement.  Any in-flight update
        # finishes first (since it holds the lock), and then the process is
        # replaced while still holding the lock — meaning no new update can
        # sneak in during the brief TOCTOU window that existed with the
        # original acquire-release-execv sequence.
        # Threads die when execv replaces the process image, so the lock is
        # released atomically by the kernel.
        with _apply_lock:
            try:
                os.execv(sys.executable, [sys.executable] + sys.argv)
            except Exception:
                # Last-resort: if execv fails (e.g. frozen binary), just exit
                # so the process supervisor (start.sh / Docker) restarts us.
                os._exit(0)

    threading.Thread(target=_do, daemon=True).start()


def apply_force_update(target: str) -> dict:
    """Force-reset the target repo to the latest remote HEAD.

    Unlike apply_update() which requires a clean working tree and refuses
    merge conflicts, this discards all local modifications (checkout .) and
    resets to origin/<branch> — equivalent to what the diverged/conflict
    error messages ask the user to run manually.

    Should only be called when apply_update() has already returned a
    response with ``conflict: True`` or ``diverged: True`` and the user
    has confirmed they want to discard local changes.
    """
    active_streams = _active_stream_count()
    if active_streams:
        return _restart_blocked_response(target, active_streams)

    if not _apply_lock.acquire(blocking=False):
        return {'ok': False, 'message': 'Update already in progress'}
    try:
        if target == 'webui':
            path = REPO_ROOT
        elif target == 'agent':
            path = _AGENT_DIR
        else:
            return {'ok': False, 'message': f'Unknown target: {target}'}

        if path is None or not (path / '.git').exists():
            return {'ok': False, 'message': 'Not a git repository'}

        _, fetch_ok = _run_git(['fetch', 'origin', '--quiet'], path, timeout=15)
        if not fetch_ok:
            return {
                'ok': False,
                'message': 'Could not reach the remote repository. Check your connection.',
            }

        upstream, ok = _run_git(['rev-parse', '--abbrev-ref', '@{upstream}'], path)
        if ok and upstream:
            compare_ref = upstream
        else:
            branch = _detect_default_branch(path)
            compare_ref = f'origin/{branch}'

        # Discard local modifications then reset to remote HEAD
        _run_git(['checkout', '.'], path)
        _, ok = _run_git(['reset', '--hard', compare_ref], path)
        if not ok:
            return {'ok': False, 'message': f'Force reset to {compare_ref} failed'}

        with _cache_lock:
            _update_cache['checked_at'] = 0

        _schedule_restart()

        return {
            'ok': True,
            'message': f'{target} force-updated to {compare_ref}',
            'target': target,
            'restart_scheduled': True,
        }
    finally:
        _apply_lock.release()


def apply_update(target):
    """Stash, pull --ff-only, pop for the given target repo."""
    active_streams = _active_stream_count()
    if active_streams:
        return _restart_blocked_response(target, active_streams)

    if not _apply_lock.acquire(blocking=False):
        return {'ok': False, 'message': 'Update already in progress'}
    try:
        return _apply_update_inner(target)
    finally:
        _apply_lock.release()


def _apply_update_inner(target):
    """Inner implementation of apply_update, called under _apply_lock."""
    if target == 'webui':
        path = REPO_ROOT
    elif target == 'agent':
        path = _AGENT_DIR
    else:
        return {'ok': False, 'message': f'Unknown target: {target}'}

    if path is None or not (path / '.git').exists():
        return {'ok': False, 'message': 'Not a git repository'}

    # Use the current branch's upstream for pull, matching the behaviour
    # of _check_repo. Falls back to default branch if no upstream is set.
    upstream, ok = _run_git(['rev-parse', '--abbrev-ref', '@{upstream}'], path)
    if ok and upstream:
        compare_ref = upstream
    else:
        branch = _detect_default_branch(path)
        compare_ref = f'origin/{branch}'

    # Fetch before attempting pull, so the remote ref is current.
    _, fetch_ok = _run_git(['fetch', 'origin', '--quiet'], path, timeout=15)
    if not fetch_ok:
        return {
            'ok': False,
            'message': (
                'Could not reach the remote repository. '
                'Check your internet connection and try again.'
            ),
        }

    # Check for dirty working tree (ignore untracked files — git stash
    # doesn't include them, so stashing on '??' alone leaves nothing to pop)
    status_out, status_ok = _run_git(
        ['status', '--porcelain', '--untracked-files=no'], path
    )
    if not status_ok:
        return {'ok': False, 'message': f'Failed to inspect repo status: {status_out[:200]}'}
    # Fail early on unresolved merge conflicts
    if any(line[:2] in {'DD', 'AU', 'UD', 'UA', 'DU', 'AA', 'UU'}
           for line in status_out.splitlines()):
        return {
            'ok': False,
            'message': (
                f'The local {target} repo has unresolved merge conflicts. '
                'To reset to the latest remote version run: '
                'git -C ' + str(path) + ' checkout . && '
                'git -C ' + str(path) + ' pull --ff-only'
            ),
            'conflict': True,
        }
    stashed = False
    if status_out:
        _, ok = _run_git(['stash'], path)
        if not ok:
            return {'ok': False, 'message': 'Failed to stash local changes'}
        stashed = True

    # Pull with ff-only (no merge commits).
    # Split tracking refs like 'origin/main' into separate remote + branch
    # arguments — git treats 'origin/main' as a repository name otherwise.
    remote, branch = _split_remote_ref(compare_ref)
    pull_args = ['pull', '--ff-only']
    if remote:
        pull_args.extend([remote, branch])
    else:
        pull_args.append(compare_ref)
    pull_out, pull_ok = _run_git(pull_args, path, timeout=30)
    if not pull_ok:
        if stashed:
            _run_git(['stash', 'pop'], path)

        # Diagnose the most common failure modes and surface actionable messages.
        pull_lower = pull_out.lower()
        if 'not possible to fast-forward' in pull_lower or 'diverged' in pull_lower:
            return {
                'ok': False,
                'message': (
                    f'The local {target} repo has commits that are not on the remote '
                    'branch, so a fast-forward update is not possible. '
                    'Run: git -C ' + str(path) + ' fetch origin && '
                    'git -C ' + str(path) + ' reset --hard ' + compare_ref
                ),
                'diverged': True,
            }
        if 'does not track' in pull_lower or 'no tracking information' in pull_lower:
            return {
                'ok': False,
                'message': (
                    f'The local {target} branch has no upstream tracking branch configured. '
                    'Run: git -C ' + str(path) + ' branch --set-upstream-to=' + compare_ref
                ),
            }
        # Generic fallback — include the raw git output for debugging.
        detail = pull_out.strip()[:300] if pull_out.strip() else '(no output from git)'
        return {'ok': False, 'message': f'Pull failed: {detail}'}

    # Pop stash if we stashed
    if stashed:
        _, pop_ok = _run_git(['stash', 'pop'], path)
        if not pop_ok:
            return {
                'ok': False,
                'message': 'Updated but stash pop failed -- manual merge needed',
                'stash_conflict': True,
            }

    # Invalidate cache
    with _cache_lock:
        _update_cache['checked_at'] = 0

    # Schedule a self-restart so the updated code is loaded fresh.  A plain
    # git pull leaves stale Python modules in sys.modules — agent imports that
    # reference new symbols (functions, classes) added in the update will fail
    # on the next request with AttributeError / ImportError.  os.execv() re-
    # execs the same interpreter with the same argv, picking up the new code
    # cleanly without requiring the user to restart manually.
    #
    # The 2 s delay gives the HTTP response time to flush to the client before
    # the process replaces itself.  The client already does
    # setTimeout(() => location.reload(), 1500) on success, so the page reload
    # and the restart land at roughly the same time.
    _schedule_restart()

    return {
        'ok': True,
        'message': f'{target} updated successfully',
        'target': target,
        'restart_scheduled': True,
    }