rewrite some osf provider urls

 * File urls of the form osf.io/<guid>/ used to redirect to a download
   url if <guid> pointed to a file. They no longer do. Now if MFR sees
   a url matching that pattern, it will add a /download/ path segment
   before the guid that *will* redirect.

Author: felliott

mfr/providers/osf/provider.py

@@ -172,11 +172,21 @@ async def _fetch_download_url(self):
                 self.download_url = self.url
                 self.metrics.add('download_url.orig_type', 'wb_v1')
             else:
-                self.metrics.add('download_url.orig_type', 'osf')
+                yum_url = None
+                import re
+                if re.compile(r'^/[a-z0-9]{5}/[^/]*', re.IGNORECASE).match(path):
+                    self.metrics.add('download_url.orig_type', 'osf-preangular')
+                    berch_url = furl.furl(self.url)
+                    berch_url.path.segments = ['download'] + berch_url.path.segments
+                    yum_url = berch_url.url
+                else:
+                    self.metrics.add('download_url.orig_type', 'osf')
+                    yum_url = self.url
+
                 # make request to osf, don't follow, store waterbutler download url
                 request = await self._make_request(
                     'GET',
-                    self.url,
+                    yum_url,
                     allow_redirects=False,
                     headers={
                         'Content-Type': 'application/json'
@@ -188,7 +198,7 @@ async def _fetch_download_url(self):
                 if request.status != 302:
                     raise exceptions.MetadataError(
                         request.reason,
-                        metadata_url=self.url,
+                        metadata_url=yum_url,
                         provider=self.NAME,
                         code=request.status,
                     )