fix(render): defer glfw.init() on macOS until a GLFWContext is created

dm_control/_render/glfw_renderer.py

@@ -15,6 +15,8 @@
 
 """An OpenGL renderer backed by GLFW."""
 
+import sys
+
 from dm_control._render import base
 from dm_control._render import executor
 
@@ -24,10 +26,29 @@
   import glfw  # pylint: disable=g-import-not-at-top
 except (ImportError, IOError, OSError) as exc:
   raise ImportError from exc
-try:
-  glfw.init()
-except glfw.GLFWError as exc:
-  raise ImportError from exc
+
+
+def _init_glfw():
+  """Calls `glfw.init()`, re-raising failures as `ImportError`.
+
+  `glfwInit` is a no-op if GLFW has already been initialized, so this is safe
+  to call multiple times.
+  """
+  try:
+    glfw.init()
+  except glfw.GLFWError as exc:
+    raise ImportError from exc
+
+
+# On macOS, initializing GLFW at import time disrupts the shared NSApplication
+# state used by other GUI toolkits, most notably matplotlib's default 'MacOSX'
+# backend (https://github.com/google-deepmind/dm_control/issues/377), so we
+# defer initialization until a context is actually created. On other platforms
+# we initialize at import time so that the automatic backend selection in
+# `dm_control/_render/__init__.py` can still fall back to EGL/OSMesa when GLFW
+# cannot create a context (e.g. on headless Linux machines).
+if sys.platform != 'darwin':
+  _init_glfw()
 
 
 class GLFWContext(base.ContextBase):
@@ -37,6 +58,7 @@ def __init__(self, max_width, max_height):
     # GLFWContext always uses `PassthroughRenderExecutor` rather than offloading
     # rendering calls to a separate thread because GLFW can only be safely used
     # from the main thread.
+    _init_glfw()
     super().__init__(max_width, max_height, executor.PassthroughRenderExecutor)
 
   def _platform_init(self, max_width, max_height):

dm_control/_render/glfw_renderer_test.py

@@ -15,6 +15,11 @@
 
 """Tests for GLFWContext."""
 
+import os
+import re
+import subprocess
+import sys
+import textwrap
 import unittest
 from absl.testing import absltest
 from dm_control import _render
@@ -64,5 +69,74 @@ def test_repeatedly_create_and_destroy_context(self):
     renderer = _render.Renderer(MAX_WIDTH, MAX_HEIGHT)
     wrapper.MjrContext(wrapper.MjModel.from_xml_string('<mujoco/>'), renderer)
 
+
+_COUNT_INIT_CALLS_SCRIPT = textwrap.dedent("""
+    import sys
+    import types
+    sys.platform = {platform}
+    stub = types.ModuleType('glfw')
+    stub.GLFWError = type('GLFWError', (Exception,), {{}})
+    stub.VISIBLE = 1
+    stub.DOUBLEBUFFER = 2
+    stub._init_calls = []
+    stub.init = lambda: stub._init_calls.append(1)
+    stub.window_hint = lambda *args, **kwargs: None
+    stub.create_window = lambda **kwargs: object()
+    stub.make_context_current = lambda window: None
+    stub.get_current_context = lambda: None
+    stub.destroy_window = lambda window: None
+    sys.modules['glfw'] = stub
+    import dm_control._render.glfw_renderer as glfw_renderer
+    print('INIT_AT_IMPORT', len(stub._init_calls))
+    glfw_renderer.GLFWContext(max_width=64, max_height=64)
+    print('INIT_AFTER_CONTEXT', len(stub._init_calls))
+""")
+
+
+class GlfwInitAtImportTest(absltest.TestCase):
+  """Tests when `glfw.init()` runs relative to importing `glfw_renderer`.
+
+  Initializing GLFW at import time interferes with other GUI toolkits that
+  share the process's NSApplication on macOS, most notably matplotlib's
+  default 'MacOSX' backend (https://github.com/google-deepmind/dm_control/
+  issues/377), so on macOS the initialization must be deferred until a
+  `GLFWContext` is actually created. On other platforms it must still happen
+  at import time so that the automatic backend selection in
+  `dm_control/_render/__init__.py` can fall back to EGL/OSMesa when GLFW
+  cannot create a context (e.g. on headless Linux machines).
+  """
+
+  def _run_with_stubbed_glfw(self, forced_platform):
+    script = _COUNT_INIT_CALLS_SCRIPT.format(platform=repr(forced_platform))
+    env = dict(os.environ, MUJOCO_GL='glfw')
+    completed_process = subprocess.run(
+        args=[sys.executable, '-c', script],
+        env=env,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        # The stub module injected here only defines the attributes that
+        # `glfw_renderer` accesses at import time and at initialization time.
+        timeout=120,
+        check=True,
+        text=True)
+    init_at_import = re.search(r'^INIT_AT_IMPORT (\d+)$',
+                               completed_process.stdout, re.M)
+    init_after_context = re.search(r'^INIT_AFTER_CONTEXT (\d+)$',
+                                   completed_process.stdout, re.M)
+    assert init_at_import and init_after_context, (
+        completed_process.stdout + completed_process.stderr)
+    return int(init_at_import.group(1)), int(init_after_context.group(1))
+
+  def test_defers_initialization_on_macos(self):
+    init_at_import, init_after_context = self._run_with_stubbed_glfw('darwin')
+    self.assertEqual(init_at_import, 0)
+    self.assertEqual(init_after_context, 1)
+
+  def test_initializes_at_import_on_other_platforms(self):
+    init_at_import, init_after_context = self._run_with_stubbed_glfw('linux')
+    self.assertEqual(init_at_import, 1)
+    self.assertEqual(init_after_context, 2)
+
+
 if __name__ == '__main__':
   absltest.main()