obsolete: don't use os.stat in repo.obsstore.__nonzero__ if it's static HTTP

If a repo is accessed via static HTTP, then we obviously can't use os.stat() to
just peek at the file size. Let's download the entire file to check its size.
Yes, this feels wasteful, but:

1. If we're cloning or pulling a repo from a static HTTP server, we need the
   contents of the obsstore anyway.

2. Implementing statichttpvfs.stat() that uses HEAD will result in one more
   request to a static-only HTTP server, which is already slow. Also parsing a
   response to a HEAD request to construct os.stat_result is pretty hacky.
   There's also a question of the remote server properly supporting HEAD method
   and reporting at least file size.

3. Implementing statichttpvfs.stat() that uses GET is pretty much the same
   thing as we do here, except we can't even cache the response easily, unlike
   simply accessing obsstore._data, which is @propertycache'd.

Importing statichttprepo locally to avoid circular import.

See also: 4507bc001365 and commit message of f8f2ecdde4b5.

Differential Revision: https://phab.mercurial-scm.org/D12195

mercurial/obsolete.py

@@ -575,11 +575,17 b' class obsstore(object):'
         return len(self._all)
 
     def __nonzero__(self):
+        from . import statichttprepo
+
+        if isinstance(self.repo, statichttprepo.statichttprepository):
+            # If repo is accessed via static HTTP, then we can't use os.stat()
+            # to just peek at the file size.
+            return len(self._data) > 1
         if not self._cached('_all'):
             try:
                 return self.svfs.stat(b'obsstore').st_size > 1
             except OSError as inst:
-                if inst.errno not in (errno.ENOENT, errno.EINVAL):
+                if inst.errno != errno.ENOENT:
                     raise
                 # just build an empty _all list if no obsstore exists, which
                 # avoids further stat() syscalls