Force MLflow URI override in xdist workers to prevent yaml race

The previous attempt set MLFLOW_TRACKING_URI per-worker in
pytest_configure, guarded by 'if MLFLOW_TRACKING_URI not in os.environ'.
That guard hid a subtle bug:

  1. The main pytest process runs pytest_configure first, sees the
     env var is unset, and exports file:///tmp/flaml_mlruns_main_PID.
  2. xdist worker subprocesses (gw0, gw1) inherit that env var from
     the main process.
  3. When each worker re-runs pytest_configure, the guard now
     evaluates to True (the var IS set), so workers skip the
     override and end up sharing the main process's tracking dir.
  4. Concurrent writes from both workers to the same
     EXP/RUN/meta.yaml produce partial files, triggering
     'yaml.parser.ParserError: while parsing a block mapping' in
     every subsequent FLAML+MLflow test on that worker.

Fix: in xdist workers, force-override the URI when the inherited
value is one we previously set (path contains 'flaml_mlruns_');
externally-set URIs (e.g. by the user) are still respected. The new
per-worker path is 'flaml_mlruns_WORKER_PID'.

Verified locally: 'pytest test/fabric/test_mlflow_coverage.py -n 2
--dist=loadfile' now creates per-worker dirs (flaml_mlruns_gw0_*
distinct from main) and all 66 tests pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: thinkall

test/conftest.py

@@ -22,22 +22,33 @@ def pytest_configure(config):
     # file. Concurrent writers can leave the file half-written, which then
     # surfaces as ``yaml.parser.ParserError: while parsing a block mapping``
     # in every subsequent test that talks to MLflow on the same worker.
-    if "MLFLOW_TRACKING_URI" not in os.environ:
+    #
+    # ``pytest_configure`` runs in the main pytest process AND in every
+    # xdist worker. The main process runs first and exports
+    # ``MLFLOW_TRACKING_URI``; workers then inherit that value. Without
+    # the override below, all workers would share the main process's
+    # tracking dir and race on writes. We therefore force-override the
+    # URI in xdist workers when the inherited value is one we previously
+    # set ourselves (path contains ``flaml_mlruns_``); a URI set
+    # externally by the user is left untouched.
+    worker_id = os.environ.get("PYTEST_XDIST_WORKER")
+    existing_uri = os.environ.get("MLFLOW_TRACKING_URI", "")
+    inherited_from_main = bool(worker_id) and "flaml_mlruns_" in existing_uri
+    if not existing_uri or inherited_from_main:
         import tempfile
+        from pathlib import Path
 
-        worker_id = os.environ.get("PYTEST_XDIST_WORKER", "main")
+        tag = worker_id or "main"
         # IMPORTANT: do NOT pre-create this directory. ``FileStore.__init__``
         # only bootstraps the default ``Experiment(id="0")`` (which several
         # tests in ``test/fabric/test_mlflow_coverage.py`` implicitly rely on
         # via ``mlflow.start_run()``) when the root directory does not yet
         # exist. Pre-creating the dir would leave the store without a
         # Default experiment, breaking those tests with
         # ``MlflowException: Could not find experiment with ID 0``.
-        tracking_dir = os.path.join(tempfile.gettempdir(), f"flaml_mlruns_{worker_id}_{os.getpid()}")
+        tracking_dir = os.path.join(tempfile.gettempdir(), f"flaml_mlruns_{tag}_{os.getpid()}")
         # ``file:`` URIs need POSIX-style forward slashes even on Windows;
         # ``Path.as_uri()`` handles the platform-specific prefix correctly.
-        from pathlib import Path
-
         os.environ["MLFLOW_TRACKING_URI"] = Path(tracking_dir).as_uri()