Workaround python2.7 find_module limitation / explicitly close file (#20782)

Summary:
fix #20781 #20757
hmm I don't know an easy way to add a test to make sure it runs against a package installed as .egg. But i tested it locally with torchvision.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20782

Differential Revision: D15443600

Pulled By: ailzhang

fbshipit-source-id: 285eb0d9a44d6edb8e93618fa293f4feb431d2ae

torch/hub.py

@@ -159,12 +159,12 @@ def _get_cache_or_reload(github, force_reload):
         url = _git_archive_link(repo_owner, repo_name, branch)
         _download_archive_zip(url, cached_file)
 
-        cached_zipfile = zipfile.ZipFile(cached_file)
-        extraced_repo_name = cached_zipfile.infolist()[0].filename
-        extracted_repo = os.path.join(hub_dir, extraced_repo_name)
-        _remove_if_exists(extracted_repo)
-        # Unzip the code and rename the base folder
-        cached_zipfile.extractall(hub_dir)
+        with zipfile.ZipFile(cached_file) as cached_zipfile:
+            extraced_repo_name = cached_zipfile.infolist()[0].filename
+            extracted_repo = os.path.join(hub_dir, extraced_repo_name)
+            _remove_if_exists(extracted_repo)
+            # Unzip the code and rename the base folder
+            cached_zipfile.extractall(hub_dir)
 
         _remove_if_exists(cached_file)
         _remove_if_exists(repo_dir)
@@ -182,14 +182,35 @@ def _check_module_exists(name):
         import importlib.find_loader
         return importlib.find_loader(name) is not None
     else:
-        # NB: imp doesn't handle hierarchical module names (names contains dots).
+        # NB: Python2.7 imp.find_module() doesn't respect PEP 302,
+        #     it cannot find a package installed as .egg(zip) file.
+        #     Here we use workaround from:
+        #     https://stackoverflow.com/questions/28962344/imp-find-module-which-supports-zipped-eggs?lq=1
+        #     Also imp doesn't handle hierarchical module names (names contains dots).
         try:
+            # 1. Try imp.find_module(), which searches sys.path, but does
+            # not respect PEP 302 import hooks.
             import imp
-            imp.find_module(name)
-        except Exception:
-            return False
-        return True
-
+            result = imp.find_module(name)
+            if result:
+                return True
+        except ImportError:
+            pass
+        path = sys.path
+        for item in path:
+            # 2. Scan path for import hooks. sys.path_importer_cache maps
+            # path items to optional "importer" objects, that implement
+            # find_module() etc.  Note that path must be a subset of
+            # sys.path for this to work.
+            importer = sys.path_importer_cache.get(item)
+            if importer:
+                try:
+                    result = importer.find_module(name, [item])
+                    if result:
+                        return True
+                except ImportError:
+                    pass
+        return False
 
 def _check_dependencies(m):
     dependencies = _load_attr_from_module(m, VAR_DEPENDENCY)