Cannot download files when a custom ContentsManager is implemented

[TomTJarosz]

Use case:
I have a custom ContentsManager in use (extending notebook.ContentsManager as specified here https://jupyter-notebook.readthedocs.io/en/stable/extending/contents.html#writing-a-custom-contentsmanager). I would like to go download a file by clicking on 'Download' button on the Jupyter tree page.

Problem:
As a result of having a custom content manager, FilesHandler.get will be used to serve the request GET /files/<file name>?download=1 (this is the request made when clicking the 'Download' button on the Jupyter tree page). FilesHandler.get seems to be non-functional (possibly resulting from 815ed3c). Prior to this commit, FilesHandler.get returns None. Currently, calling FilesHandler.get returns a <generator object get at 0x...>. See an attached stack trace of the error below.

[E 11:16:56.049 NotebookApp] Uncaught exception GET /files/Untitled.ipynb?download=1 (::1)
    HTTPServerRequest(protocol='http', host='localhost:8888', method='GET', uri='/files/Untitled.ipynb?download=1', version='HTTP/1.1', remote_ip='::1')
    Traceback (most recent call last):
      File "/usr/local/share/c3/conda/envs/c3notebook/lib/python3.6/site-packages/tornado/web.py", line 1597, in _execute
        result = yield result
      File "/usr/local/share/c3/conda/envs/c3notebook/lib/python3.6/site-packages/tornado/gen.py", line 1133, in run
        value = future.result()
      File "/usr/local/share/c3/conda/envs/c3notebook/lib/python3.6/site-packages/tornado/gen.py", line 1221, in handle_yield
        self.future = convert_yielded(yielded)
      File "/usr/local/share/c3/conda/envs/c3notebook/lib/python3.6/functools.py", line 807, in wrapper
        return dispatch(args[0].__class__)(*args, **kw)
      File "/usr/local/share/c3/conda/envs/c3notebook/lib/python3.6/site-packages/tornado/gen.py", line 1363, in convert_yielded
        raise BadYieldError("yielded unknown object %r" % (yielded,))
    tornado.gen.BadYieldError: yielded unknown object <generator object get at 0x113b774c0>
[D 11:16:56.049 NotebookApp] Using contents: services/contents
[D 11:16:56.050 NotebookApp] Using contents: services/contents
[E 11:16:56.050 NotebookApp] {
      "Host": "localhost:8888",
      "Connection": "keep-alive",
      "Upgrade-Insecure-Requests": "1",
      "User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.100 Safari/537.36",
      "Sec-Fetch-Mode": "navigate",
      "Sec-Fetch-User": "?1",
      "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3",
      "Sec-Fetch-Site": "same-origin",
      "Referer": "http://localhost:8888/tree",
      "Accept-Encoding": "gzip, deflate, br",
      "Accept-Language": "en-US,en;q=0.9",
      "Cookie": "_ga=GA1.1.149903520.1553274092; Idea-b94da5fd=5e449cf1-7ab6-4227-8eff-7d7c383b912d; c3canonical=false; _xsrf=2|94454110|a91b2136ddf4a67eaf5f2c917b3d232a|1566318996; c3auth=3033ac4e5232e08eacb564838fb6c565d6a1d228ee10815c8991fb2f9fee17e4a623; c3tenant=c3; c3tag=c3; username-localhost-8889=\"2|1:0|10:1567629964|23:username-localhost-8889|44:OTBjNDA3MzMxMzhiNGZiZWFlOTBmOTMwZGY0NDNlNTQ=|cc51e887661513bf91ef6ed6c591193631ec002ea614716a40fd4e35e3ccc310\"; username-localhost-8890=\"2|1:0|10:1567630655|23:username-localhost-8890|44:MDczZGY2Zjc3ZDk0NGRkNDg4ZTY0MGUwNzY5MzIzZDE=|ee874cb6d41bb551cf0f4151528c4ea57c4892c47fa5e01c8e6abc90864359b8\"; username-localhost-8888=\"2|1:0|10:1567707380|23:username-localhost-8888|44:ZGY1YTdhMzU5ODUzNDU0ZjgxYzBkMDQxNDAyNWY0Mzg=|eaa58c0e4f2934ddd76f5ad65fbcec40d8fee075bde114e37f394ba23e636db7\""
    }
[E 11:16:56.050 NotebookApp] 500 GET /files/Untitled.ipynb?download=1 (::1) 2.03ms referer=http://localhost:8888/tree

Also provided is the of values of variables method, self.path_args, self.path_kwargs, and result in the scope https://github.com/tornadoweb/tornado/blob/v5.1.1/tornado/web.py#L1591 (just prior to an exception).

method = <bound method FilesHandler.get of <notebook.files.handlers.FilesHandler object at 0x113bb2940>>
self.path_args = ['Untitled.ipynb']
self.path_kwargs = {}
result = <generator object get at 0x113b774c0>

The result of calling FilesHandler.get('Untitled.ipynb') is returning an object which is causing an exception.

Request:
In general, I would like FilesHandler.get to work. Specifically, I would recommend to decorate FilesHandler.get with the @gen.coroutine decorator. When looking at other handlers (SessionRootHandler.get, ContentsHandler.get, MainKernelSpecHandler.get v6.0.0), it seems that a method will yield maybe_future(...) iff it is decorated with @gen.coroutine (https://stackoverflow.com/questions/37197514/python-tornado-async-fetching-of-urls also supports my assumption that the @gen.coroutine decorator should be used). Adding this decorator to FilesHandler.get locally resolved this issue locally.

[TomTJarosz]

@michalc if you could provide any input, it would be greatly appreciated. I anticipate that this should be an easy fix.