Use StringIO for pure python legacy batch inserts.

This reduces memory fragmentation when encoding large
batches of documents for insert. Testing shows improvements
in memory usage in pure python and reduced GC overhead in
Jython with no reduction in performance.

Author: behackett

pymongo/message.py

@@ -219,10 +219,10 @@ def _insert_message(insert_message, send_safe):
 
     send_safe = safe or not continue_on_error
     last_error = None
-    begin = struct.pack("<i", int(continue_on_error))
-    begin += bson._make_c_string(collection_name)
-    message_length = len(begin)
-    data = [begin]
+    data = StringIO()
+    data.write(struct.pack("<i", int(continue_on_error)))
+    data.write(bson._make_c_string(collection_name))
+    message_length = begin_loc = data.tell()
     has_docs = False
     for doc in docs:
         encoded = bson.BSON.encode(doc, check_keys, uuid_subtype)
@@ -231,14 +231,14 @@ def _insert_message(insert_message, send_safe):
 
         message_length += encoded_length
         if message_length < client.max_message_size and not too_large:
-            data.append(encoded)
+            data.write(encoded)
             has_docs = True
             continue
 
         if has_docs:
             # We have enough data, send this message.
             try:
-                client._send_message(_insert_message(_EMPTY.join(data),
+                client._send_message(_insert_message(data.getvalue(),
                                                      send_safe), send_safe)
             # Exception type could be OperationFailure or a subtype
             # (e.g. DuplicateKeyError)
@@ -261,13 +261,15 @@ def _insert_message(insert_message, send_safe):
                                    " bytes." %
                                    (encoded_length, client.max_bson_size))
 
-        message_length = len(begin) + encoded_length
-        data = [begin, encoded]
+        message_length = begin_loc + encoded_length
+        data.seek(begin_loc)
+        data.truncate()
+        data.write(encoded)
 
     if not has_docs:
         raise InvalidOperation("cannot do an empty bulk insert")
 
-    client._send_message(_insert_message(_EMPTY.join(data), safe), safe)
+    client._send_message(_insert_message(data.getvalue(), safe), safe)
 
     # Re-raise any exception stored due to continue_on_error
     if last_error is not None: