Recognize text/javascript as valid js mimetype for caching purposes (t…

…ensorflow#5746)

Recently the test core_plugin_test.py::test_js_cache began to fail in some environments with the following error:

```
... core_plugin_test.py", line 196, in test_js_cache
self.assertEqual(
AssertionError: 
- private, max-age=86400
+ no-cache, must-revalidate
```

The root cause is that these systems have now been configured to return 'text/javascript' as the mimetype for js files, where they were previously returning 'application/javascript'. This means the 'Cache-Control' header was not set by this line:

https://cs.opensource.google/tensorflow/tensorboard/+/master:tensorboard/plugins/core/core_plugin.py;l=137;drc=c061b659f50cde9dd9da5aa906ecb682ecdc68d4

```
        expires = (
            JS_CACHE_EXPIRATION_IN_SECS
            if request.args.get("_file_hash")
            and mimetype == "application/javascript"
            else 0
        )
```

After further investigation (thanks @nfelt!) we determined that the change in mimetype is likely intentional after the recent publishing of RFC 9239 (https://www.rfc-editor.org/rfc/rfc9239), where 'text/javascript' becomes the standard mimetype for js files.

We decided, therefore, to change the Cache-Control-setting logic to also recognize 'text/javascript'.

Note, in practice, the change in mimetype had no impact on running tensorboards thanks to the logic added in tensorflow#3128, which always ensures ".js" is mapped to mimetype "application/javascript" but does it at a higher level (ie outside either core plugin or the http utils).

tensorboard/plugins/core/core_plugin.py

@@ -39,6 +39,11 @@
 # If no port is specified, try to bind to this port. See help for --port
 # for more details.
 DEFAULT_PORT = 6006
+# Valid javascript mimetypes that we have seen configured, in practice.
+# Historically (~2020-2022) we saw "application/javascript" exclusively but with
+# RFC 9239 (https://www.rfc-editor.org/rfc/rfc9239) we saw some systems quickly
+# transition to 'text/javascript'.
+JS_MIMETYPES = ["text/javascript", "application/javascript"]
 JS_CACHE_EXPIRATION_IN_SECS = 86400
 
 
@@ -133,8 +138,7 @@ def _serve_asset(self, path, gzipped_asset_bytes, request):
         # Cache JS resources while keep others do not cache.
         expires = (
             JS_CACHE_EXPIRATION_IN_SECS
-            if request.args.get("_file_hash")
-            and mimetype == "application/javascript"
+            if request.args.get("_file_hash") and mimetype in JS_MIMETYPES
             else 0
         )
 

tensorboard/plugins/core/core_plugin_test.py

@@ -19,6 +19,7 @@
 import contextlib
 import io
 import json
+import mimetypes
 import os
 from unittest import mock
 import zipfile
@@ -150,6 +151,9 @@ def setUp(self):
         app = application.TensorBoardWSGI([self.plugin])
         self.server = werkzeug_test.Client(app, wrappers.Response)
 
+    def tearDown(self):
+        mimetypes.init()
+
     def _add_run(self, run_name):
         run_path = os.path.join(self.logdir, run_name)
         with test_util.FileWriter(run_path) as writer:
@@ -191,6 +195,34 @@ def test_js_no_cache(self):
         )
 
     def test_js_cache(self):
+        """Tests cache header for js files marked for caching.
+
+        The test relies on local system's defaults for the javascript mimetype
+        which, in practice, should be one of 'text/javascript' or
+        'application/javascript'. It could be either, depending on the system.
+
+        See test_js_cache_with_text_javascript() and
+        test_js_cache_with_application_javascript() for test cases where
+        the javascript mimetype have been explicitly configured.
+        """
+        response = self.server.get("/index.js?_file_hash=meow")
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(
+            ONE_DAY_CACHE_CONTROL_VALUE, response.headers.get("Cache-Control")
+        )
+
+    def test_js_cache_with_text_javascript(self):
+        """Tests cache header when js mimetype is 'text/javascript'."""
+        mimetypes.add_type("text/javascript", ".js")
+        response = self.server.get("/index.js?_file_hash=meow")
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(
+            ONE_DAY_CACHE_CONTROL_VALUE, response.headers.get("Cache-Control")
+        )
+
+    def test_js_cache_with_application_javascript(self):
+        """Tests cache header when js mimetype is 'application/javascript'."""
+        mimetypes.add_type("application/javascript", ".js")
         response = self.server.get("/index.js?_file_hash=meow")
         self.assertEqual(200, response.status_code)
         self.assertEqual(