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Switch the JSON byte producer from a pull to a push producer #8116

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Aug 19, 2020
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Switch the JSON byte producer from a pull to a push producer #8116

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02cace6
Switch the JSON byte producer to a push producer instead of a pull pr…
clokep Aug 18, 2020
d54f1f5
Fix tests.
clokep Aug 18, 2020
2afe301
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clokep Aug 19, 2020
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1 change: 1 addition & 0 deletions changelog.d/8116.feature
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Iteratively encode JSON to avoid blocking the reactor.
73 changes: 41 additions & 32 deletions synapse/http/server.py
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Expand Up @@ -500,7 +500,7 @@ class RootOptionsRedirectResource(OptionsResource, RootRedirect):
pass


@implementer(interfaces.IPullProducer)
@implementer(interfaces.IPushProducer)
class _ByteProducer:
"""
Iteratively write bytes to the request.
Expand All @@ -515,52 +515,62 @@ def __init__(
):
self._request = request
self._iterator = iterator
self._paused = False

def start(self) -> None:
self._request.registerProducer(self, False)
# Register the producer and start producing data.
self._request.registerProducer(self, True)
self.resumeProducing()

def _send_data(self, data: List[bytes]) -> None:
"""
Send a list of strings as a response to the request.
Send a list of bytes as a chunk of a response.
"""
if not data:
return
self._request.write(b"".join(data))

def pauseProducing(self) -> None:
self._paused = True

def resumeProducing(self) -> None:
# We've stopped producing in the meantime (note that this might be
# re-entrant after calling write).
if not self._request:
return

# Get the next chunk and write it to the request.
#
# The output of the JSON encoder is coalesced until min_chunk_size is
# reached. (This is because JSON encoders produce a very small output
# per iteration.)
#
# Note that buffer stores a list of bytes (instead of appending to
# bytes) to hopefully avoid many allocations.
buffer = []
buffered_bytes = 0
while buffered_bytes < self.min_chunk_size:
try:
data = next(self._iterator)
buffer.append(data)
buffered_bytes += len(data)
except StopIteration:
# The entire JSON object has been serialized, write any
# remaining data, finalize the producer and the request, and
# clean-up any references.
self._send_data(buffer)
self._request.unregisterProducer()
self._request.finish()
self.stopProducing()
return

self._send_data(buffer)
self._paused = False

# Write until there's backpressure telling us to stop.
while not self._paused:
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@clokep clokep Aug 18, 2020 •
edited
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This definitely looks like an infinite loop, but I don't think it is (see a previous discussion about this description here). I think when we call write we give up control to the reactor which might call pauseProducing before control is returned to resumeProducing.

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When you call write() you go through the Request -> HTTPChannel -> iocpreactor.tcp.Connection (this is HTTPChannel.transport) -> iocpreactor.abstract.FileHandle, which might cause producer.pauseProducing (which goes back through the chain in the opposite direction, essentially).

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This does mean that if we write slower than the TCP buffer drains we will write everything in one go. I don't know whether we want to put some limit to the amount written in one reactor tick? Maybe something like 64KB (or even 512KB)? A reactor tick generally takes less than 1ms, so those values would mean maxing out at ~64MB/s (or 512MB/s).

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Aha, but FileHandle has a write buffer of 64KB and doesn't actually pull from the buffer until a reactor tick, so we don't need to worry about it. The flow becomes: 1) write data, 2) pause producing is called if over 64kb of data has been written, 3) we exit the loop until the FileHandle buffer has been drained.

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That was my understanding as well! 👍 This is some fun spelunking into Twisted.

# Get the next chunk and write it to the request.
#
# The output of the JSON encoder is coalesced until min_chunk_size is
# reached. (This is because JSON encoders produce a very small output
# per iteration.)
#
# Note that buffer stores a list of bytes (instead of appending to
# bytes) to hopefully avoid many allocations.
buffer = []
buffered_bytes = 0
while buffered_bytes < self.min_chunk_size:
try:
data = next(self._iterator)
buffer.append(data)
buffered_bytes += len(data)
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Looking at the code this is basically what iocpreactor.abstract.FileHandle is doing, so I think we can just skip all this and call self._request.write(data) for each chunk?

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@clokep clokep Aug 19, 2020 •
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The issue is that the http.Request calls toChunk so each time we call write it will get turned into an HTTP chunk.

The goal of this code is to avoid b"{" being emitted from the JSONProducer becoming the HTTP chunk of 1\r\n{\r\n, which is a large explosion in the streamed bytes.

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Oh gah I hate that http.Request gets in the way :/

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Can you add a comment to that effect please?

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I expanded the comment above this section in 2afe301.

except StopIteration:
# The entire JSON object has been serialized, write any
# remaining data, finalize the producer and the request, and
# clean-up any references.
self._send_data(buffer)
self._request.unregisterProducer()
self._request.finish()
self.stopProducing()
return

self._send_data(buffer)
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def stopProducing(self) -> None:
# Clear a circular reference.
self._request = None


Expand Down Expand Up @@ -620,8 +630,7 @@ def respond_with_json(
if send_cors:
set_cors_headers(request)

producer = _ByteProducer(request, encoder(json_object))
producer.start()
_ByteProducer(request, encoder(json_object))
return NOT_DONE_YET


Expand Down
16 changes: 5 additions & 11 deletions tests/rest/client/v1/test_login.py
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Expand Up @@ -62,8 +62,7 @@ def test_POST_ratelimiting_per_address(self):
"identifier": {"type": "m.id.user", "user": "kermit" + str(i)},
"password": "monkey",
}
request_data = json.dumps(params)
request, channel = self.make_request(b"POST", LOGIN_URL, request_data)
request, channel = self.make_request(b"POST", LOGIN_URL, params)
self.render(request)

if i == 5:
Expand All @@ -76,14 +75,13 @@ def test_POST_ratelimiting_per_address(self):
# than 1min.
self.assertTrue(retry_after_ms < 6000)

self.reactor.advance(retry_after_ms / 1000.0)
self.reactor.advance(retry_after_ms / 1000.0 + 1.0)

params = {
"type": "m.login.password",
"identifier": {"type": "m.id.user", "user": "kermit" + str(i)},
"password": "monkey",
}
request_data = json.dumps(params)
request, channel = self.make_request(b"POST", LOGIN_URL, params)
self.render(request)

Expand Down Expand Up @@ -111,8 +109,7 @@ def test_POST_ratelimiting_per_account(self):
"identifier": {"type": "m.id.user", "user": "kermit"},
"password": "monkey",
}
request_data = json.dumps(params)
request, channel = self.make_request(b"POST", LOGIN_URL, request_data)
request, channel = self.make_request(b"POST", LOGIN_URL, params)
self.render(request)

if i == 5:
Expand All @@ -132,7 +129,6 @@ def test_POST_ratelimiting_per_account(self):
"identifier": {"type": "m.id.user", "user": "kermit"},
"password": "monkey",
}
request_data = json.dumps(params)
request, channel = self.make_request(b"POST", LOGIN_URL, params)
self.render(request)

Expand Down Expand Up @@ -160,8 +156,7 @@ def test_POST_ratelimiting_per_account_failed_attempts(self):
"identifier": {"type": "m.id.user", "user": "kermit"},
"password": "notamonkey",
}
request_data = json.dumps(params)
request, channel = self.make_request(b"POST", LOGIN_URL, request_data)
request, channel = self.make_request(b"POST", LOGIN_URL, params)
self.render(request)

if i == 5:
Expand All @@ -174,14 +169,13 @@ def test_POST_ratelimiting_per_account_failed_attempts(self):
# than 1min.
self.assertTrue(retry_after_ms < 6000)

self.reactor.advance(retry_after_ms / 1000.0)
self.reactor.advance(retry_after_ms / 1000.0 + 1.0)

params = {
"type": "m.login.password",
"identifier": {"type": "m.id.user", "user": "kermit"},
"password": "notamonkey",
}
request_data = json.dumps(params)
request, channel = self.make_request(b"POST", LOGIN_URL, params)
self.render(request)

Expand Down
4 changes: 2 additions & 2 deletions tests/rest/client/v2_alpha/test_register.py
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Expand Up @@ -160,7 +160,7 @@ def test_POST_ratelimiting_guest(self):
else:
self.assertEquals(channel.result["code"], b"200", channel.result)

self.reactor.advance(retry_after_ms / 1000.0)
self.reactor.advance(retry_after_ms / 1000.0 + 1.0)

request, channel = self.make_request(b"POST", self.url + b"?kind=guest", b"{}")
self.render(request)
Expand All @@ -186,7 +186,7 @@ def test_POST_ratelimiting(self):
else:
self.assertEquals(channel.result["code"], b"200", channel.result)

self.reactor.advance(retry_after_ms / 1000.0)
self.reactor.advance(retry_after_ms / 1000.0 + 1.0)

request, channel = self.make_request(b"POST", self.url + b"?kind=guest", b"{}")
self.render(request)
Expand Down
3 changes: 2 additions & 1 deletion tests/storage/test_cleanup_extrems.py
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Expand Up @@ -353,6 +353,7 @@ def test_expiry_logic(self):
self.event_creator_handler._rooms_to_exclude_from_dummy_event_insertion[
"3"
] = 300000

self.event_creator_handler._expire_rooms_to_exclude_from_dummy_event_insertion()
# All entries within time frame
self.assertEqual(
Expand All @@ -362,7 +363,7 @@ def test_expiry_logic(self):
3,
)
# Oldest room to expire
self.pump(1)
self.pump(1.01)
self.event_creator_handler._expire_rooms_to_exclude_from_dummy_event_insertion()
self.assertEqual(
len(
Expand Down