Postpone filter is an essential part of subrequest functionality. In absence
of it a subrequest response body is sent to the client out of order with
respect to the main request header and body, as well as other subrequests.
For in-memory subrequests the response is also sent to the client instead of
being stored in memory.
Currently the postpone filter is automatically enabled if one of the following
standard modules which are known to create subrequests is enabled: ssi, slice,
addition. However a third-party module that creates subrequests can still be
built without the postpone filter or be dynamically loaded in nginx built
without it.
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