When we fixed bulk response heartbeating in https://review.openstack.org/#/c/510715/,
code review raised the issue of moving the xml header down to after the
early-exit clauses. At the time, it didn't seem to break anything, so
it was left in place. However, that insight was correct.
The purpose of the earlier patch was to force eventlet to use chunked
transfer encoding on the response in order to prevent eventlet from
buffering the whole response, thus defeating the purpose of the
heartbeat responses.
Moving the first line of the body lower (ie after the early exit
checks), allows other headers in a chunked transfer encoding response
to be appropriately processed before sending the headers. Sending the
xml declaration early causes it to get intermingled in the 100-continue
protocol, thus breaking the chunked transfer encoding semantics.
Closes-Bug: #1819252
Change-Id: I072f4dab21cd7cdb81b9e41072eb504131411dc8
adc568c97f