Use finally to close handles from urlopen

Arguable, this is not a problem in the code as the failure case
invokes sys.exit.  However, this is more future proof as the sys.exit
may be replaced (or we may later catch another exception that is
"recoverable").

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/policies/autopkgtest.py

@@ -516,6 +516,7 @@ class AutopkgtestPolicy(BasePolicy):
         # request new results from swift
         url = os.path.join(swift_url, self.swift_container)
         url += '?' + urllib.parse.urlencode(query)
+        f = None
         try:
             f = urlopen(url, timeout=30)
             if f.getcode() == 200:
@@ -528,7 +529,6 @@ class AutopkgtestPolicy(BasePolicy):
                 # our URLS, so fail hard on those
                 raise NotImplementedError('fetch_swift_results(%s): cannot handle HTTP code %i' %
                                           (url, f.getcode()))
-            f.close()
         except IOError as e:
             # 401 "Unauthorized" is swift's way of saying "container does not exist"
             if hasattr(e, 'code') and e.code == 401:
@@ -540,6 +540,9 @@ class AutopkgtestPolicy(BasePolicy):
             # fail hard on this and let the next run retry.
             self.logger.error('Failure to fetch swift results from %s: %s', url, str(e))
             sys.exit(1)
+        finally:
+            if f is not None:
+                f.close()
 
         for p in result_paths:
             self.fetch_one_result(
@@ -552,11 +555,11 @@ class AutopkgtestPolicy(BasePolicy):
 
         Remove matching pending_tests entries.
         '''
+        f = None
         try:
             f = urlopen(url, timeout=30)
             if f.getcode() == 200:
                 tar_bytes = io.BytesIO(f.read())
-                f.close()
             else:
                 raise NotImplementedError('fetch_one_result(%s): cannot handle HTTP code %i' %
                                           (url, f.getcode()))
@@ -567,7 +570,9 @@ class AutopkgtestPolicy(BasePolicy):
             if hasattr(e, 'code') and e.code == 404:
                 return
             sys.exit(1)
-
+        finally:
+            if f is not None:
+                f.close()
         try:
             with tarfile.open(None, 'r', tar_bytes) as tar:
                 exitcode = int(tar.extractfile('exitcode').read().strip())