Set JPY_NOTEBOOK_DIR env variable in the kernel

This is done to get the dir of the notebook where notebook initialising the kernel belongs.

#980

jupyter_server/services/sessions/sessionmanager.py

@@ -268,14 +268,22 @@ async def create_session(
         self._pending_sessions.remove(record)
         return result
 
+    def _session_path_to_notebook_dir(self, session_path):
+        """Remove the tail of the path because it is just a uuid string obtained when starting the kernel,
+        the head is the dir of the notebook starting the kernel.
+        """
+        notebook_dir, _ = os.path.split(session_path)
+        return notebook_dir
+
     async def start_kernel_for_session(self, session_id, path, name, type, kernel_name):
         """Start a new kernel for a given session."""
         # allow contents manager to specify kernels cwd
         kernel_path = await ensure_async(self.contents_manager.get_kernel_path(path=path))
+        notebook_path = self._session_path_to_notebook_dir(path)
         kernel_id = await self.kernel_manager.start_kernel(
             path=kernel_path,
             kernel_name=kernel_name,
-            env={**os.environ, "JPY_SESSION_NAME": path},
+            env={**os.environ, "JPY_SESSION_NAME": path, "JPY_NOTEBOOK_DIR": notebook_path},
         )
         return kernel_id