fix(logging): close spdlog file handles before drop

backtrader/utils/log_message.py

@@ -797,6 +797,26 @@ def close(self):
         super().close()
 
 
+def _release_spdlog_logger(spdlog_mod, logger, name):
+    """Flush, close and unregister one spdlog file logger.
+
+    ``spdlog.drop`` unregisters the logger but does not release its file
+    handle on the Windows binding.  Closing first keeps temporary probes,
+    rollover and retention cleanup portable across supported platforms.
+    """
+    try:
+        flush = getattr(logger, "flush", None)
+        if callable(flush):
+            flush()
+    finally:
+        try:
+            close = getattr(logger, "close", None)
+            if callable(close):
+                close()
+        finally:
+            spdlog_mod.drop(name)
+
+
 def _detect_spdlog():
     """Return the ``spdlog`` module if usable, else ``None``.
 
@@ -812,6 +832,7 @@ def _detect_spdlog():
 
         with tempfile.TemporaryDirectory() as tmp:
             probe_name = f"bt_backend_probe.{os.getpid()}.{time.monotonic_ns()}"
+            probe = None
             try:
                 probe = spdlog.FileLogger(probe_name, os.path.join(tmp, "probe.log"), truncate=True)
                 probe.set_pattern("%v")
@@ -823,7 +844,10 @@ def _detect_spdlog():
                     if "probe" not in stream.read():
                         return None
             finally:
-                spdlog.drop(probe_name)
+                if probe is None:
+                    spdlog.drop(probe_name)
+                else:
+                    _release_spdlog_logger(spdlog, probe, probe_name)
         return spdlog
     except Exception:
         return None
@@ -913,8 +937,7 @@ def emit(self, record):
     def _rotate(self, today):
         path = self._path_for(today)
         if self._owner_pid == os.getpid():
-            self._logger.flush()
-            self._mod.drop(self._unique)
+            _release_spdlog_logger(self._mod, self._logger, self._unique)
         self._unique = f"bt.{self._level_name}.{os.getpid()}.{id(self):x}"
         self._logger = self._open_logger(path)
         self._path = path
@@ -942,8 +965,7 @@ def close(self):
         try:
             if getattr(self, "_owner_pid", None) == os.getpid() and not self._bt_closed:
                 try:
-                    self._logger.flush()
-                    self._mod.drop(self._unique)
+                    _release_spdlog_logger(self._mod, self._logger, self._unique)
                 except Exception:  # nosec B110
                     # Best-effort cleanup only.
                     pass

tests/unit/utils/test_logging_split_files.py

@@ -26,12 +26,10 @@
 import backtrader as bt
 from backtrader.utils import log_message
 
-try:
-    import spdlog  # noqa: F401
-
-    HAS_SPDLOG = True
-except ImportError:
-    HAS_SPDLOG = False
+# An importable package can still lack a usable native extension or fail its
+# file-write smoke probe.  Keep the optional test matrix aligned with the same
+# availability contract used by ``backend=\"auto\"`` at runtime.
+HAS_SPDLOG = log_message._detect_spdlog() is not None
 
 BACKENDS = [
     "stdlib",
@@ -79,6 +77,39 @@ def _managed_handlers(logger):
     ]
 
 
+def _symlink_or_skip(link, target):
+    """Create a directory symlink or skip when Windows policy forbids it."""
+
+    try:
+        link.symlink_to(target, target_is_directory=True)
+    except OSError as exc:
+        if os.name == "nt" and getattr(exc, "winerror", None) == 1314:
+            pytest.skip(
+                "Windows symlink creation requires Developer Mode or SeCreateSymbolicLinkPrivilege"
+            )
+        raise
+
+
+def test_release_spdlog_logger_closes_file_before_unregistering():
+    """Windows file handles must be closed before ``spdlog.drop`` unregisters them."""
+    events = []
+
+    class FakeLogger:
+        def flush(self):
+            events.append("flush")
+
+        def close(self):
+            events.append("close")
+
+    class FakeSpdlog:
+        @staticmethod
+        def drop(name):
+            events.append(("drop", name))
+
+    log_message._release_spdlog_logger(FakeSpdlog(), FakeLogger(), "probe")
+    assert events == ["flush", "close", ("drop", "probe")]
+
+
 # ---------------------------------------------------------------------------
 # layout + routing (parametrized over available backends)
 # ---------------------------------------------------------------------------
@@ -503,7 +534,7 @@ def test_retention_does_not_follow_symlink_or_delete_date_file(tmp_path):
     outside = tmp_path / "outside"
     outside.mkdir()
     (outside / "keep.txt").write_text("keep")
-    (script / "2000_01_01").symlink_to(outside, target_is_directory=True)
+    _symlink_or_skip(script / "2000_01_01", outside)
     (script / "2000_01_02").write_text("date-named file")
     _configure(tmp_path)
     assert (script / "2000_01_01").is_symlink()
@@ -516,7 +547,7 @@ def test_script_symlink_is_rejected_without_cleanup(tmp_path):
     script.parent.mkdir()
     outside = tmp_path / "outside"
     outside.mkdir()
-    script.symlink_to(outside, target_is_directory=True)
+    _symlink_or_skip(script, outside)
     with pytest.raises(ValueError, match="symbolic"):
         _configure(tmp_path)