Add support for increasing the soft limit on open file handles (#4893)

Add some minimal support for increasing the soft limit
on open file handles, default to at least 4096.

This hopefully fixes https://github.com/jupyterlab/jupyterlab/issues/6727

notebook/notebookapp.py

@@ -32,8 +32,13 @@ import time
 import warnings
 import webbrowser
 
-from base64 import encodebytes
+try:
+    import resource
+except ImportError:
+    # Windows
+    resource = None
 
+from base64 import encodebytes
 
 from jinja2 import Environment, FileSystemLoader
 
@@ -794,6 +799,14 @@ class NotebookApp(JupyterApp):
         """
     )
 
+    min_open_files_limit = Integer(4096, config=True,
+        help="""
+        Gets or sets a lower bound on the open file handles process resource
+        limit. This may need to be increased if you run into an
+        OSError: [Errno 24] Too many open files.
+        This is not applicable when running on Windows.
+        """)
+
     @observe('token')
     def _token_changed(self, change):
         self._token_generated = False
@@ -1371,6 +1384,23 @@ class NotebookApp(JupyterApp):
         logger.parent = self.log
         logger.setLevel(self.log.level)
     
+    def init_resources(self):
+        """initialize system resources"""
+        if resource is None:
+            self.log.debug('Ignoring min_open_files_limit because the limit cannot be adjusted (for example, on Windows)')
+            return
+
+        old_soft, old_hard = resource.getrlimit(resource.RLIMIT_NOFILE)
+        soft = self.min_open_files_limit
+        hard = old_hard
+        if old_soft < soft:
+            if hard < soft:
+                hard = soft
+            self.log.debug(
+                'Raising open file limit: soft {}->{}; hard {}->{}'.format(old_soft, soft, old_hard, hard)
+            )
+            resource.setrlimit(resource.RLIMIT_NOFILE, (soft, hard))
+
     def init_webapp(self):
         """initialize tornado webapp and httpserver"""
         self.tornado_settings['allow_origin'] = self.allow_origin
@@ -1665,6 +1695,7 @@ class NotebookApp(JupyterApp):
         self.init_logging()
         if self._dispatching:
             return
+        self.init_resources()
         self.init_configurables()
         self.init_server_extension_config()
         self.init_components()