Preserve live gateway log during compaction

resources/scripts/napseer_mcp_server.py

@@ -574,6 +574,7 @@ GATEWAY_LISTENER_ACTIVE_LANES = set()
 GATEWAY_LISTENER_UNSTABLE_UNTIL = 0.0
 GATEWAY_LISTENER_STABILITY_LOCK = threading.RLock()
 AUTH_RENEW_LOCK = threading.Lock()
+GATEWAY_LOG_LOCK = threading.Lock()
 GATEWAY_VAULT_SETUP_THREAD_STARTED = False
 GATEWAY_PERIODIC_LOG_STATE = {}
 WS_CLOSE_DETAILS = {}
@@ -595,6 +596,7 @@ def gateway_log(event, level="info", **fields):
             payload[key] = value
         else:
             payload[key] = str(value)
+    with GATEWAY_LOG_LOCK:
         print(json.dumps(payload, separators=(",", ":"), sort_keys=True), file=sys.stderr, flush=True)
 
 
@@ -629,15 +631,17 @@ def compact_log_file(path, max_bytes):
     newline_index = chunk.find(b"\n")
     if newline_index >= 0 and newline_index + 1 < len(chunk):
         chunk = chunk[newline_index + 1 :]
-    tmp_path = file_path.with_suffix(file_path.suffix + ".tmp")
     try:
-        tmp_path.write_bytes(chunk)
-        tmp_path.replace(file_path)
+        # Keep the inode stable: the daemon's stdout/stderr already point to
+        # this file. Replacing the pathname would leave the live process
+        # writing to a deleted inode and make `nap gateway logs` stale.
+        with GATEWAY_LOG_LOCK, file_path.open("r+b") as handle:
+            handle.seek(0)
+            handle.write(chunk)
+            handle.truncate()
+            handle.flush()
     except OSError:
-        try:
-            tmp_path.unlink(missing_ok=True)
-        except OSError:
-            pass
+        return
 
 
 def start_gateway_log_compactor_thread():

tests/test_gateway_log_compaction.py

@@ -0,0 +1,27 @@
+import importlib.util
+import pathlib
+
+
+def load_module():
+    script_path = pathlib.Path(__file__).resolve().parents[1] / "resources" / "scripts" / "napseer_mcp_server.py"
+    spec = importlib.util.spec_from_file_location("napseer_gateway_log_compaction_test", script_path)
+    module = importlib.util.module_from_spec(spec)
+    assert spec and spec.loader
+    spec.loader.exec_module(module)
+    return module
+
+
+def test_log_compaction_preserves_live_writer_inode(tmp_path):
+    mod = load_module()
+    log_path = tmp_path / "gateway.log"
+    log_path.write_bytes((b"old-line\n" * 200) + b"recent-line\n")
+    inode_before = log_path.stat().st_ino
+
+    with log_path.open("ab", buffering=0) as live_writer:
+        mod.compact_log_file(log_path, 128)
+        live_writer.write(b"after-compaction\n")
+
+    assert log_path.stat().st_ino == inode_before
+    content = log_path.read_bytes()
+    assert content.endswith(b"after-compaction\n")
+    assert b"recent-line\n" in content