Lazy initialize tensorboard

Summary:
Pull Request resolved: #4852

Every time a summary writer object is created it'll end up creating a file in the passed in logdir with just metadata of 40 bytes. This will cause a lot of spam with every rank initializing the file aggressively (I've done this myself when adding support for Adhoc logging to PyPer to a lot of angry complaints).

Report at https://fb.workplace.com/groups/723537759122220/posts/745046770304652/

Sample code that accesses "_writer" directly: https://www.internalfb.com/code/fbsource/[6b39948cd8e179fcc49f08526efa5dc26fc00843]/fbcode/mobile-vision/d2go/d2go/utils/visualization.py?lines=180

Reviewed By: Reubend, crassirostris

Differential Revision: D43863071

fbshipit-source-id: 3fb749199772837697911d802d7e43d157597573

detectron2/utils/events.py

@@ -6,6 +6,7 @@
 import time
 from collections import defaultdict
 from contextlib import contextmanager
+from functools import cached_property
 from typing import Optional
 import torch
 from fvcore.common.history_buffer import HistoryBuffer
@@ -142,10 +143,14 @@ def __init__(self, log_dir: str, window_size: int = 20, **kwargs):
             kwargs: other arguments passed to `torch.utils.tensorboard.SummaryWriter(...)`
         """
         self._window_size = window_size
+        self._writer_args = {"log_dir": log_dir, **kwargs}
+        self._last_write = -1
+
+    @cached_property
+    def _writer(self):
         from torch.utils.tensorboard import SummaryWriter
 
-        self._writer = SummaryWriter(log_dir, **kwargs)
-        self._last_write = -1
+        return SummaryWriter(**self._writer_args)
 
     def write(self):
         storage = get_event_storage()
@@ -174,7 +179,7 @@ def write(self):
             storage.clear_histograms()
 
     def close(self):
-        if hasattr(self, "_writer"):  # doesn't exist when the code fails at import
+        if "_writer" in self.__dict__:
             self._writer.close()
 
 

tests/utils/test_tensorboardx.py

@@ -0,0 +1,23 @@
+import os
+import tempfile
+import unittest
+
+from detectron2.utils.events import TensorboardXWriter
+
+
+# TODO Fix up capitalization
+class TestTensorboardXWriter(unittest.TestCase):
+    def test_no_files_created(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            writer = TensorboardXWriter(tmp_dir)
+            writer.close()
+
+            self.assertFalse(os.listdir(tmp_dir))
+
+    def test_single_write(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            writer = TensorboardXWriter(tmp_dir)
+            writer._writer.add_scalar("testing", 1, 1)
+            writer.close()
+
+            self.assertTrue(os.listdir(tmp_dir))