http: Stop changing umask.

Instead, use a thread-safe way. Current code using it is not threaded, but
future code will be.

caucase/http.py

@@ -44,6 +44,18 @@ _cryptography_backend = default_backend()
 
 BACKUP_SUFFIX = '.sql.caucased'
 
+def _createKey(path):
+  """
+  Open a key file, setting it to minimum permission if it gets created.
+  Does not change umask, to be thread-safe.
+
+  Returns a python file object, opened for write-only.
+  """
+  return os.fdopen(
+    os.open(path, os.O_WRONLY | os.O_CREAT, 0o600),
+    'w',
+  )
+
 class ThreadingWSGIServer(ThreadingMixIn, WSGIServer):
   """
   Threading WSGI server
@@ -179,7 +191,7 @@ def updateSSLContext(
           ),
         ),
       )
-      with open(server_key_path, 'w') as crt_file:
+      with _createKey(server_key_path) as crt_file:
         crt_file.write(new_key_pem)
         crt_file.write(new_crt_pem)
   else:
@@ -225,13 +237,9 @@ def updateSSLContext(
     cas.createCertificate(csr_id)
     new_crt_pem = cas.getCertificate(csr_id)
     new_key_pem = utils.dump_privatekey(new_key)
-    old_mask = os.umask(077)
-    try:
-      with open(server_key_path, 'w') as crt_file:
+    with _createKey(server_key_path) as crt_file:
       crt_file.write(new_key_pem)
       crt_file.write(new_crt_pem)
-    finally:
-      os.umask(old_mask)
   ssl_context.load_cert_chain(server_key_path)
   if wrap:
     https.socket = ssl_context.wrap_socket(