[watchdog] The Eye That Never Sleeps — Nexus Health Monitor (#794)

Co-authored-by: Google AI Agent <gemini@hermes.local>
Co-committed-by: Google AI Agent <gemini@hermes.local>

nexus/heartbeat.py

@@ -0,0 +1,79 @@
+"""
+Heartbeat writer for the Nexus consciousness loop.
+
+Call write_heartbeat() at the end of each think cycle to let the
+watchdog know the mind is alive. The file is written atomically
+(write-to-temp + rename) to prevent the watchdog from reading a
+half-written file.
+
+Usage in nexus_think.py:
+    from nexus.heartbeat import write_heartbeat
+
+    class NexusMind:
+        def think_once(self):
+            # ... do the thinking ...
+            write_heartbeat(
+                cycle=self.cycle_count,
+                model=self.model,
+                status="thinking",
+            )
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import tempfile
+import time
+from pathlib import Path
+
+
+DEFAULT_HEARTBEAT_PATH = Path.home() / ".nexus" / "heartbeat.json"
+
+
+def write_heartbeat(
+    cycle: int = 0,
+    model: str = "unknown",
+    status: str = "thinking",
+    path: Path = DEFAULT_HEARTBEAT_PATH,
+) -> None:
+    """Write a heartbeat file atomically.
+
+    The watchdog monitors this file to detect stale minds — processes
+    that are technically running but have stopped thinking (e.g., hung
+    on a blocking call, deadlocked, or crashed inside a catch-all
+    exception handler).
+
+    Args:
+        cycle: Current think cycle number
+        model: Model identifier
+        status: Current state ("thinking", "perceiving", "acting", "idle")
+        path: Where to write the heartbeat file
+    """
+    path.parent.mkdir(parents=True, exist_ok=True)
+
+    data = {
+        "pid": os.getpid(),
+        "timestamp": time.time(),
+        "cycle": cycle,
+        "model": model,
+        "status": status,
+    }
+
+    # Atomic write: temp file in same directory + rename.
+    # This guarantees the watchdog never reads a partial file.
+    fd, tmp_path = tempfile.mkstemp(
+        dir=str(path.parent),
+        prefix=".heartbeat-",
+        suffix=".tmp",
+    )
+    try:
+        with os.fdopen(fd, "w") as f:
+            json.dump(data, f)
+        os.replace(tmp_path, str(path))
+    except Exception:
+        # Best effort — never crash the mind over a heartbeat failure
+        try:
+            os.unlink(tmp_path)
+        except OSError:
+            pass