[PR #7283/0e7c272f backport][3.9] Add note about updating headers in …

…on_response_prepare (#7294) **This is a backport of PR #7283 as merged into master (0e7c272).** Fixes #6443. It seems like it'll be easier just to document this. Only solution that looks like it'd work is to update the Content-Length header every time the body is changed, but that could incur a performance penalty for little reason. Co-authored-by: Sam Bull <[email protected]>
aio-libs · May 17, 2023 · 3de86c1 · 3de86c1
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diff --git a/CHANGES/7283.doc b/CHANGES/7283.doc
@@ -0,0 +1 @@
+Added a note about possibly needing to update headers when using ``on_response_prepare`` -- by :user:`Dreamsorcerer`
diff --git a/docs/web_reference.rst b/docs/web_reference.rst
@@ -1427,6 +1427,13 @@ duplicated like one using :meth:`~aiohttp.web.Application.copy`.
           async def on_prepare(request, response):
               pass
 
+      .. note::
+
+         The headers are written immediately after these callbacks are run.
+         Therefore, if you modify the content of the response, you may need to
+         adjust the `Content-Length` header or similar to match. Aiohttp will
+         not make any updates to the headers from this point.
+
    .. attribute:: on_startup
 
       A :class:`~aiosignal.Signal` that is fired on application start-up.
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		Added a note about possibly needing to update headers when using ``on_response_prepare`` -- by :user:`Dreamsorcerer`