The server log file is truncated every time the server is started when using the --logappend option.

It looks like this is caused by http://github.com/mongodb/mongo/blob/master/util/assert_util.cpp#L106 as _append has not yet been assigned to. The commit that looks to have broken this is http://github.com/mongodb/mongo/commit/0196d8c583cc82de5a8a38ef200d2049d379b03c