Check if nbviewer URL would show an error

If the nbviewer widget would show a 404 or other error we don't want to
display it. We check the status before rendering the template by making
a HEAD request to nbviewer, if the status code is >=400 we don't show
the preview.

binderhub/main.py

@@ -1,6 +1,7 @@
 """
 Main handler classes for requests
 """
+from tornado.httpclient import AsyncHTTPClient
 from tornado.web import HTTPError, authenticated
 from tornado.httputil import url_concat
 from tornado.log import app_log
@@ -15,6 +16,7 @@
     "zenodo": "Zenodo"
 }
 
+
 class MainHandler(BaseHandler):
     """Main handler for requests"""
 
@@ -35,7 +37,7 @@ class ParameterizedMainHandler(BaseHandler):
     """Main handler that allows different parameter settings"""
 
     @authenticated
-    def get(self, provider_prefix, _unescaped_spec):
+    async def get(self, provider_prefix, _unescaped_spec):
         prefix = '/v2/' + provider_prefix
         spec = self.get_spec_from_request(prefix)
         spec = spec.rstrip("/")
@@ -61,14 +63,25 @@ def get(self, provider_prefix, _unescaped_spec):
             org, repo_name, ref = spec.split('/', 2)
             # NOTE: tornado unquotes query arguments too -> notebooks%2Findex.ipynb becomes notebooks/index.ipynb
             filepath = self.get_argument('filepath', '').lstrip('/')
-           
+
             # Check if we have a JupyterLab + file path, if so then use it for the filepath
             urlpath = self.get_argument('urlpath', '').lstrip('/')
             if urlpath.startswith("lab") and "/tree/" in urlpath:
                 filepath = urlpath.split('tree/', 1)[-1]
-            
+
             blob_or_tree = 'blob' if filepath else 'tree'
             nbviewer_url = f'{nbviewer_url}/{org}/{repo_name}/{blob_or_tree}/{ref}/{filepath}'
+
+        # Check if the nbviewer URL is valid and would display something
+        # useful to the reader, if not we don't show it
+        client = AsyncHTTPClient()
+        response = await client.fetch(nbviewer_url,
+                                      method="HEAD",
+                                      user_agent="BinderHub",
+                                      raise_error=False)
+        if response.code >= 400:
+            nbviewer_url = None
+
         self.render_template(
             "loading.html",
             base_url=self.settings['base_url'],