diff --git a/src/twisted/trial/_dist/stream.py b/src/twisted/trial/_dist/stream.py
new file mode 100644
index 0000000000..3a87c2ee1d
--- /dev/null
+++ b/src/twisted/trial/_dist/stream.py
@@ -0,0 +1,99 @@
+"""
+Buffer string streams
+"""
+
+from itertools import count
+from typing import Dict, Iterator, List, TypeVar
+
+from attrs import Factory, define
+
+from twisted.protocols.amp import AMP, Command, Integer, Unicode
+
+T = TypeVar("T")
+
+
+class StreamOpen(Command):
+    """
+    Open a new stream.
+    """
+
+    response = [(b"streamId", Integer())]
+
+
+class StreamWrite(Command):
+    """
+    Write a chunk of data to a stream.
+    """
+
+    arguments = [
+        (b"streamId", Integer()),
+        (b"data", Unicode()),
+    ]
+
+
+@define
+class StreamReceiver:
+    """
+    Buffering de-multiplexing string stream receiver.
+    """
+
+    _counter: Iterator[int] = count()
+    _streams: Dict[int, List[str]] = Factory(dict)
+
+    def open(self) -> int:
+        """
+        Open a new stream and return its unique identifier.
+        """
+        newId = next(self._counter)
+        self._streams[newId] = []
+        return newId
+
+    def write(self, streamId: int, chunk: str) -> None:
+        """
+        Write to an open stream using its unique identifier.
+
+        :raise KeyError: If there is no such open stream.
+        """
+        self._streams[streamId].append(chunk)
+
+    def finish(self, streamId: int) -> List[str]:
+        """
+        Indicate an open stream may receive no further data and return all of
+        its current contents.
+
+        :raise KeyError: If there is no such open stream.
+        """
+        return self._streams.pop(streamId)
+
+
+def chunk(data: str, chunkSize: int) -> Iterator[str]:
+    """
+    Break a string into pieces of no more than ``chunkSize`` length.
+
+    :param data: The string.
+
+    :param chunkSize: The maximum length of the resulting pieces.  All pieces
+        except possibly the last will be this length.
+
+    :return: The pieces.
+    """
+    pos = 0
+    while pos < len(data):
+        yield data[pos : pos + chunkSize]
+        pos += chunkSize
+
+
+async def stream(amp: AMP, chunks: Iterator[str]) -> int:
+    """
+    Send the given stream chunks, one by one, over the given connection.
+
+    The chunks are sent using L{StreamWrite} over a stream opened using
+    L{StreamOpen}.
+
+    :return: The identifier of the stream over which the chunks were sent.
+    """
+    streamId = (await amp.callRemote(StreamOpen))["streamId"]
+
+    for oneChunk in chunks:
+        await amp.callRemote(StreamWrite, streamId=streamId, data=oneChunk)
+    return streamId  # type: ignore[no-any-return]

diff --git a/src/twisted/trial/_dist/test/test_stream.py b/src/twisted/trial/_dist/test/test_stream.py
new file mode 100644
index 0000000000..2c506e08a0
--- /dev/null
+++ b/src/twisted/trial/_dist/test/test_stream.py
@@ -0,0 +1,206 @@
+"""
+Tests for L{twisted.trial._dist.stream}.
+"""
+
+from random import Random
+from typing import Awaitable, Dict, List, TypeVar, Union
+
+from hamcrest import (
+    all_of,
+    assert_that,
+    calling,
+    equal_to,
+    has_length,
+    is_,
+    less_than_or_equal_to,
+    raises,
+)
+from hypothesis import given
+from hypothesis.strategies import integers, just, lists, randoms, text
+
+from twisted.internet.defer import Deferred, fail
+from twisted.internet.interfaces import IProtocol
+from twisted.internet.protocol import Protocol
+from twisted.protocols.amp import AMP
+from twisted.python.failure import Failure
+from twisted.test.iosim import FakeTransport, connect
+from twisted.trial.unittest import SynchronousTestCase
+from ..stream import StreamOpen, StreamReceiver, StreamWrite, chunk, stream
+from .matchers import HasSum, IsSequenceOf
+
+T = TypeVar("T")
+
+
+class StreamReceiverTests(SynchronousTestCase):
+    """
+    Tests for L{StreamReceiver}
+    """
+
+    @given(lists(lists(text())), randoms())
+    def test_streamReceived(self, streams: List[str], random: Random) -> None:
+        """
+        All data passed to L{StreamReceiver.write} is returned by a call to
+        L{StreamReceiver.finish} with a matching C{streamId} .
+        """
+        receiver = StreamReceiver()
+        streamIds = [receiver.open() for _ in streams]
+
+        # uncorrelate the results with open() order
+        random.shuffle(streamIds)
+
+        expectedData = dict(zip(streamIds, streams))
+        for streamId, strings in expectedData.items():
+            for s in strings:
+                receiver.write(streamId, s)
+
+        # uncorrelate the results with write() order
+        random.shuffle(streamIds)
+
+        actualData = {streamId: receiver.finish(streamId) for streamId in streamIds}
+
+        assert_that(actualData, is_(equal_to(expectedData)))
+
+    @given(integers(), just("data"))
+    def test_writeBadStreamId(self, streamId: int, data: str) -> None:
+        """
+        L{StreamReceiver.write} raises L{KeyError} if called with a
+        streamId not associated with an open stream.
+        """
+        receiver = StreamReceiver()
+        assert_that(calling(receiver.write).with_args(streamId, data), raises(KeyError))
+
+    @given(integers())
+    def test_badFinishStreamId(self, streamId: int) -> None:
+        """
+        L{StreamReceiver.finish} raises L{KeyError} if called with a
+        streamId not associated with an open stream.
+        """
+        receiver = StreamReceiver()
+        assert_that(calling(receiver.finish).with_args(streamId), raises(KeyError))
+
+    def test_finishRemovesStream(self) -> None:
+        """
+        L{StreamReceiver.finish} removes the identified stream.
+        """
+        receiver = StreamReceiver()
+        streamId = receiver.open()
+        receiver.finish(streamId)
+        assert_that(calling(receiver.finish).with_args(streamId), raises(KeyError))
+
+
+class ChunkTests(SynchronousTestCase):
+    """
+    Tests for ``chunk``.
+    """
+
+    @given(data=text(), chunkSize=integers(min_value=1))
+    def test_chunk(self, data, chunkSize):
+        """
+        L{chunk} returns an iterable of L{str} where each element is no
+        longer than the given limit.  The concatenation of the strings is also
+        equal to the original input string.
+        """
+        chunks = list(chunk(data, chunkSize))
+        assert_that(
+            chunks,
+            all_of(
+                IsSequenceOf(
+                    has_length(less_than_or_equal_to(chunkSize)),
+                ),
+                HasSum(equal_to(data), data[:0]),
+            ),
+        )
+
+
+class AMPStreamReceiver(AMP):
+    """
+    A simple AMP interface to L{StreamReceiver}.
+    """
+
+    def __init__(self, streams: StreamReceiver) -> None:
+        self.streams = streams
+
+    @StreamOpen.responder
+    def streamOpen(self) -> Dict[str, object]:
+        return {"streamId": self.streams.open()}
+
+    @StreamWrite.responder
+    def streamWrite(self, streamId: int, data: str) -> Dict[str, object]:
+        self.streams.write(streamId, data)
+        return {}
+
+
+def interact(server: IProtocol, client: IProtocol, interaction: Awaitable[T]) -> T:
+    """
+    Let C{server} and C{client} exchange bytes while C{interaction} runs.
+    """
+    finished = False
+    result: Union[Failure, T]
+
+    async def to_coroutine() -> T:
+        return await interaction
+
+    def collect_result(r: Union[Failure, T]) -> None:
+        nonlocal result, finished
+        finished = True
+        result = r
+
+    pump = connect(
+        server,
+        FakeTransport(server, isServer=True),
+        client,
+        FakeTransport(client, isServer=False),
+    )
+    interacting = Deferred.fromCoroutine(to_coroutine())
+    interacting.addBoth(collect_result)
+
+    pump.flush()
+
+    if finished:
+        if isinstance(result, Failure):
+            result.raiseException()
+        return result
+    raise Exception("Interaction failed to produce a result.")
+
+
+class InteractTests(SynchronousTestCase):
+    """
+    Tests for the test helper L{interact}.
+    """
+
+    def test_failure(self):
+        """
+        If the interaction results in a failure then L{interact} raises an
+        exception.
+        """
+
+        class ArbitraryException(Exception):
+            pass
+
+        with self.assertRaises(ArbitraryException):
+            interact(Protocol(), Protocol(), fail(ArbitraryException()))
+
+    def test_incomplete(self):
+        """
+        If the interaction fails to produce a result then L{interact} raises
+        an exception.
+        """
+        with self.assertRaises(Exception):
+            interact(Protocol(), Protocol(), Deferred())
+
+
+class StreamTests(SynchronousTestCase):
+    """
+    Tests for L{stream}.
+    """
+
+    @given(lists(text()))
+    def test_stream(self, chunks):
+        """
+        All of the chunks passed to L{stream} are sent in order over a
+        stream using the given AMP connection.
+        """
+        sender = AMP()
+        streams = StreamReceiver()
+        streamId = interact(AMPStreamReceiver(streams), sender, stream(sender, chunks))
+        assert_that(streams.finish(streamId), is_(equal_to(chunks)))

disttrial needs some streaming helpers
The fix for #10314 will involve streaming arbitrarily long strings over an AMP connection.  There is no functionality in Twisted yet to support this.  Add some for trial to use.

src/twisted/trial/_dist/test/test_stream.py::StreamReceiverTests::test_badFinishStreamId src/twisted/trial/_dist/test/test_stream.py::StreamReceiverTests::test_finishRemovesStream src/twisted/trial/_dist/test/test_stream.py::StreamReceiverTests::test_streamReceived src/twisted/trial/_dist/test/test_stream.py::StreamReceiverTests::test_writeBadStreamId src/twisted/trial/_dist/test/test_stream.py::ChunkTests::test_chunk src/twisted/trial/_dist/test/test_stream.py::InteractTests::test_failure src/twisted/trial/_dist/test/test_stream.py::InteractTests::test_incomplete src/twisted/trial/_dist/test/test_stream.py::StreamTests::test_stream