document that headers must be set before streaming

docs/patterns/streaming.rst

@@ -8,6 +8,21 @@ roundtrip to the filesystem?
 
 The answer is by using generators and direct responses.
 
+HTTP Response Behavior
+----------------------
+
+**Headers cannot be changed after the streaming response starts.**
+
+When using streaming, it's important to be aware of the order than an HTTP
+response is sent. All headers must be sent first, then the body. More headers
+cannot be sent after the body has begun. Therefore, you must make sure all
+headers are set before starting the response, outside the generator.
+
+In particular, if the generator will access ``session``, be sure to do so in the
+view as well so that the ``Vary: cookie`` header will be set. Do not modify the
+session in the generator, as the ``Set-Cookie`` header will already be sent.
+
+
 Basic Usage
 -----------
 

docs/templating.rst

@@ -225,5 +225,11 @@ functions to make this easier to use.
         return stream_template("timeline.html")
 
 These functions automatically apply the
-:func:`~flask.stream_with_context` wrapper if a request is active, so
-that it remains available in the template.
+:func:`~flask.stream_with_context` wrapper if a request is active, so that
+:data:`.request`, :data:`.session`, and :data:`.g` remain available in the
+template.
+
+More headers cannot be sent after the body has begun. Therefore, you must
+make sure all headers are set before starting the response. In particular,
+if the template will access ``session``, be sure to do so in the view as
+well so that the ``Vary: cookie`` header will be set.

src/flask/ctx.py

@@ -153,13 +153,27 @@ F = t.TypeVar("F", bound=t.Callable[..., t.Any])
 
 
 def copy_current_request_context(f: F) -> F:
-    """A helper function that decorates a function to retain the current
-    request context.  This is useful when working with greenlets.  The moment
-    the function is decorated a copy of the request context is created and
-    then pushed when the function is called.  The current session is also
-    included in the copied request context.
+    """Decorate a function to run inside the current request context. This can
+    be used when starting a background task, otherwise it will not see the app
+    and request objects that were active in the parent.
 
-    Example::
+    .. warning::
+
+        Due to the following caveats, it is often safer (and simpler) to pass
+        the data you need when starting the task, rather than using this and
+        relying on the context objects.
+
+    In order to avoid execution switching partially though reading data, either
+    read the request body (access ``form``, ``json``, ``data``, etc) before
+    starting the task, or use a lock. This can be an issue when using threading,
+    but shouldn't be an issue when using greenlet/gevent or asyncio.
+
+    If the task will access ``session``, be sure to do so in the parent as well
+    so that the ``Vary: cookie`` header will be set. Modifying ``session`` in
+    the task should be avoided, as it may execute after the response cookie has
+    already been written.
+
+    .. code-block:: python
 
         import gevent
         from flask import copy_current_request_context

src/flask/helpers.py

@@ -68,6 +68,18 @@ def stream_with_context(
     available, even though at the point the generator runs the request context
     will typically have ended.
 
+    .. warning::
+
+        Due to the following caveat, it is often safer to pass the data you
+        need as arguments to the generator, rather than relying on the context
+        objects.
+
+    More headers cannot be sent after the body has begun. Therefore, you must
+    make sure all headers are set before starting the response. In particular,
+    if the generator will access ``session``, be sure to do so in the view as
+    well so that the ``Vary: cookie`` header will be set. Do not modify the
+    session in the generator, as the ``Set-Cookie`` header will already be sent.
+
     Use it as a decorator on a generator function:
 
     .. code-block:: python