Don't force PYTHON_GIL=0, instead fail if anything re-enables

.ci/install.sh

@@ -39,8 +39,8 @@ python3 -m pip install --only-binary=:all: pyarrow || true
 # PyQt6 doesn't support PyPy3
 if [[ $GHA_PYTHON_VERSION == 3.* ]]; then
     sudo apt-get -qq install libegl1 libxcb-cursor0 libxcb-icccm4 libxcb-image0 libxcb-keysyms1 libxcb-randr0 libxcb-render-util0 libxcb-shape0 libxkbcommon-x11-0
-    # TODO Update condition when pyqt6 supports free-threading
-    if ! [[ "$PYTHON_GIL" == "0" ]]; then python3 -m pip install pyqt6 ; fi
+    # pyqt6 doesn't yet support free-threading; only install if a wheel is available
+    python3 -m pip install --only-binary=:all: pyqt6 || true
 fi
 
 # webp

.github/workflows/test.yml

@@ -73,11 +73,6 @@ jobs:
           ".ci/*.sh"
           "pyproject.toml"
 
-    - name: Set PYTHON_GIL
-      if: endsWith(matrix.python-version, 't')
-      run: |
-        echo "PYTHON_GIL=0" >> $GITHUB_ENV
-
     - name: Build system information
       run: python3 .github/workflows/system-info.py
 

Tests/conftest.py

@@ -1,9 +1,17 @@
 from __future__ import annotations
 
 import io
+import sys
+import sysconfig
 
 import pytest
 
+FREE_THREADED_BUILD = bool(sysconfig.get_config_var("Py_GIL_DISABLED"))
+
+gil_enabled_at_start = True
+if FREE_THREADED_BUILD:
+    gil_enabled_at_start = sys._is_gil_enabled()  # type: ignore[attr-defined]
+
 
 def pytest_report_header(config: pytest.Config) -> str:
     try:
@@ -16,6 +24,27 @@ def pytest_report_header(config: pytest.Config) -> str:
         return f"pytest_report_header failed: {e}"
 
 
+def pytest_terminal_summary(
+    terminalreporter: pytest.TerminalReporter, exitstatus: int, config: pytest.Config
+) -> None:
+    if (
+        FREE_THREADED_BUILD
+        and not gil_enabled_at_start
+        and sys._is_gil_enabled()  # type: ignore[attr-defined]
+    ):
+        tr = terminalreporter
+        tr.ensure_newline()
+        tr.section("GIL re-enabled", sep="=", red=True, bold=True)
+        tr.line("The GIL was re-enabled at runtime during the tests.")
+        tr.line("This can happen with no test failures if the RuntimeWarning")
+        tr.line("raised by Python when this happens is filtered by a test.")
+        tr.line("")
+        tr.line("Please ensure all new C modules declare support for running")
+        tr.line("without the GIL. Any new tests that intentionally imports")
+        tr.line("code that re-enables the GIL should do so in a subprocess.")
+        pytest.exit("GIL re-enabled during tests", returncode=1)
+
+
 def pytest_configure(config: pytest.Config) -> None:
     config.addinivalue_line(
         "markers",