Port from cline/cline#10343: periodic gateway memory logging (#27102)

Emit a grep-friendly '[MEMORY] rss=...MB ...' line in agent.log /
gateway.log every N minutes (default 5) so slow leaks in the long-lived
gateway process show up as a time series. Based on
https://github.com/cline/cline/pull/10343
(src/standalone/memory-monitor.ts).

- gateway/memory_monitor.py: new module. Daemon thread, baseline on
  start, final snapshot on stop. Uses resource.getrusage() (stdlib)
  first, falls back to psutil, disables itself with one WARNING if
  neither is available.
- gateway/run.py: start monitor right after setup_logging() in
  start_gateway(); stop it in the shutdown block next to MCP teardown.
- hermes_cli/config.py: logging.memory_monitor { enabled, interval_seconds }
  defaults under the existing logging section.
- tests/gateway/test_memory_monitor.py: 10 unit tests covering format,
  baseline/shutdown snapshots, double-start noop, periodic timer,
  daemon thread invariant, and unavailable-RSS warn-and-skip path.

Adapted from TypeScript/Node to Python (threading.Event-based daemon
thread instead of setInterval/unref), added Python-specific gc + thread
counts to the log line (handier than ext/arrayBuffers for diagnosing
Python gateway leaks), and gated behind a config.yaml toggle so users
can silence the periodic line if they want.

No heap-snapshot-on-OOM equivalent — CPython doesn't have V8's
--heapsnapshot-near-heap-limit; tracemalloc would be the Python
equivalent but adds non-trivial overhead, so leaving that out.

gateway/memory_monitor.py

@@ -0,0 +1,230 @@
+"""Periodic process memory usage logging for the gateway.
+
+Ported from cline/cline#10343 (src/standalone/memory-monitor.ts).
+
+The gateway is a long-lived process that accumulates memory as it caches
+agent instances, session transcripts, tool schemas, memory providers, MCP
+connections, etc.  A slow leak in any of those subsystems is invisible
+in a single log line — you only see it by watching RSS climb over hours.
+
+This module emits a single structured ``[MEMORY] ...`` line every N
+minutes (default 5) so maintainers investigating a suspected leak can
+grep ``agent.log`` / ``gateway.log`` for a time series of RSS + Python
+GC stats.  The timer runs in a background thread and shuts down cleanly
+with the gateway.
+
+Design notes (parity with the Cline port):
+  * Grep-friendly single-line format beginning ``[MEMORY]``.
+  * Final snapshot logged on shutdown so "last RSS before exit" is
+    always in the log.
+  * Baseline snapshot logged immediately on start.
+  * Daemon thread — never blocks process exit.
+  * Uses ``resource`` (stdlib, Linux/macOS) first and falls back to
+    ``psutil`` when ``resource`` isn't available (Windows).  Both are
+    optional; when neither works we emit a single WARNING and disable
+    the monitor rather than crashing the gateway.
+
+Config: ``logging.memory_monitor`` in ``config.yaml`` — see
+``hermes_cli/config.py`` for the defaults block.
+"""
+
+from __future__ import annotations
+
+import gc
+import logging
+import os
+import sys
+import threading
+import time
+from typing import Optional
+
+logger = logging.getLogger(__name__)
+
+_BYTES_TO_MB = 1024 * 1024
+
+_monitor_thread: Optional[threading.Thread] = None
+_stop_event: Optional[threading.Event] = None
+_start_time: Optional[float] = None
+_interval_seconds: float = 300.0  # 5 minutes
+_lock = threading.Lock()
+
+
+def _get_rss_mb() -> Optional[int]:
+    """Return current process resident set size in MB, or None if unavailable.
+
+    Tries ``resource.getrusage`` first (Linux/macOS, no extra deps), then
+    falls back to ``psutil`` which is an optional hermes-agent dep.
+    """
+    # Linux / macOS — resource is stdlib.  On Linux ru_maxrss is in KB,
+    # on macOS it is in bytes (yes, really).  We use it as a cheap
+    # "current" RSS — ru_maxrss reports the high-water mark for the
+    # process, which is what you actually want for leak detection.
+    try:
+        import resource
+
+        maxrss = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss
+        if sys.platform == "darwin":
+            return int(maxrss / _BYTES_TO_MB)
+        # Linux / other unices: KB
+        return int(maxrss / 1024)
+    except Exception:
+        pass
+
+    # Fallback: psutil (Windows, or unusual unix without resource).
+    try:
+        import psutil  # type: ignore
+
+        rss = psutil.Process(os.getpid()).memory_info().rss
+        return int(rss / _BYTES_TO_MB)
+    except Exception:
+        return None
+
+
+def log_memory_usage(prefix: str = "") -> None:
+    """Log current memory usage in a grep-friendly ``[MEMORY] ...`` line.
+
+    Safe to call on-demand from any thread at important lifecycle
+    moments (after shutdown, after context compression, etc.).
+
+    Parameters
+    ----------
+    prefix
+        Optional extra tag inserted after ``[MEMORY]`` — e.g.
+        ``"baseline"``, ``"shutdown"``.
+    """
+    rss = _get_rss_mb()
+    uptime = int(time.monotonic() - _start_time) if _start_time else 0
+    # gc.get_stats() returns per-generation collection counts; the sum
+    # is a cheap proxy for "how much garbage have we created".
+    try:
+        gc_counts = gc.get_count()  # (gen0, gen1, gen2)
+    except Exception:
+        gc_counts = (0, 0, 0)
+    # Thread count is a handy correlate when diagnosing thread leaks.
+    try:
+        thread_count = threading.active_count()
+    except Exception:
+        thread_count = 0
+
+    tag = f"{prefix} " if prefix else ""
+    if rss is None:
+        logger.info(
+            "[MEMORY] %srss=unavailable gc=%s threads=%d uptime=%ds",
+            tag,
+            gc_counts,
+            thread_count,
+            uptime,
+        )
+    else:
+        logger.info(
+            "[MEMORY] %srss=%dMB gc=%s threads=%d uptime=%ds",
+            tag,
+            rss,
+            gc_counts,
+            thread_count,
+            uptime,
+        )
+
+
+def _monitor_loop(stop_event: threading.Event, interval: float) -> None:
+    """Background thread body — log every ``interval`` seconds until stopped."""
+    while not stop_event.wait(interval):
+        try:
+            log_memory_usage()
+        except Exception as e:
+            # Never let the monitor crash the gateway; just log and carry on.
+            logger.debug("Memory monitor iteration failed: %s", e)
+
+
+def start_memory_monitoring(interval_seconds: float = 300.0) -> bool:
+    """Start periodic memory usage logging in a daemon thread.
+
+    Logs immediately to capture a baseline, then every ``interval_seconds``.
+    Safe to call multiple times — subsequent calls are no-ops while the
+    first monitor is still running.
+
+    Parameters
+    ----------
+    interval_seconds
+        How often to log.  Default 300s (5 minutes), matching the
+        upstream cline/cline implementation.
+
+    Returns
+    -------
+    bool
+        True if a fresh monitor thread was started, False if one was
+        already running or if memory introspection isn't available.
+    """
+    global _monitor_thread, _stop_event, _start_time, _interval_seconds
+
+    with _lock:
+        if _monitor_thread is not None and _monitor_thread.is_alive():
+            return False
+
+        # Sanity-check that we can read RSS at all.  If neither resource
+        # nor psutil works, no point spinning a thread that can only log
+        # "rss=unavailable" forever — warn once and bail.
+        if _get_rss_mb() is None:
+            logger.warning(
+                "[MEMORY] Memory monitoring unavailable: neither resource.getrusage "
+                "nor psutil could read process RSS — skipping periodic logging.",
+            )
+            return False
+
+        _start_time = time.monotonic()
+        _interval_seconds = float(interval_seconds)
+        _stop_event = threading.Event()
+
+        # Baseline snapshot before the loop starts.
+        log_memory_usage(prefix="baseline")
+
+        _monitor_thread = threading.Thread(
+            target=_monitor_loop,
+            args=(_stop_event, _interval_seconds),
+            name="gateway-memory-monitor",
+            daemon=True,
+        )
+        _monitor_thread.start()
+
+        logger.info(
+            "[MEMORY] Periodic memory monitoring started (interval: %ds)",
+            int(_interval_seconds),
+        )
+        return True
+
+
+def stop_memory_monitoring(timeout: float = 2.0) -> None:
+    """Stop the monitor thread and log a final snapshot.
+
+    Safe to call even if ``start_memory_monitoring()`` was never called.
+    """
+    global _monitor_thread, _stop_event
+
+    with _lock:
+        if _stop_event is None or _monitor_thread is None:
+            return
+
+        # Final snapshot before teardown so "last RSS" is always in the log.
+        try:
+            log_memory_usage(prefix="shutdown")
+        except Exception:
+            pass
+
+        _stop_event.set()
+        thread = _monitor_thread
+        _monitor_thread = None
+        _stop_event = None
+
+    # Join outside the lock so a stuck log call can't deadlock shutdown.
+    try:
+        thread.join(timeout=timeout)
+    except Exception:
+        pass
+
+    logger.info("[MEMORY] Periodic memory monitoring stopped")
+
+
+def is_running() -> bool:
+    """True if the background monitor thread is alive."""
+    with _lock:
+        return _monitor_thread is not None and _monitor_thread.is_alive()