Merge branch 'develop' of github.com:matrix-org/synapse into neilj/server_notices_on_blocking

1 files changed, 440 insertions, 0 deletions

diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
new file mode 100644
index 0000000000..9b3f2f4b96
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+++ b/synapse/util/async_helpers.py
@@ -0,0 +1,440 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2018 New Vector Ltd.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import collections
+import logging
+from contextlib import contextmanager
+
+from six.moves import range
+
+from twisted.internet import defer
+from twisted.internet.defer import CancelledError
+from twisted.python import failure
+
+from synapse.util import Clock, logcontext, unwrapFirstError
+
+from .logcontext import (
+    PreserveLoggingContext,
+    make_deferred_yieldable,
+    run_in_background,
+)
+
+logger = logging.getLogger(__name__)
+
+
+class ObservableDeferred(object):
+    """Wraps a deferred object so that we can add observer deferreds. These
+    observer deferreds do not affect the callback chain of the original
+    deferred.
+
+    If consumeErrors is true errors will be captured from the origin deferred.
+
+    Cancelling or otherwise resolving an observer will not affect the original
+    ObservableDeferred.
+
+    NB that it does not attempt to do anything with logcontexts; in general
+    you should probably make_deferred_yieldable the deferreds
+    returned by `observe`, and ensure that the original deferred runs its
+    callbacks in the sentinel logcontext.
+    """
+
+    __slots__ = ["_deferred", "_observers", "_result"]
+
+    def __init__(self, deferred, consumeErrors=False):
+        object.__setattr__(self, "_deferred", deferred)
+        object.__setattr__(self, "_result", None)
+        object.__setattr__(self, "_observers", set())
+
+        def callback(r):
+            object.__setattr__(self, "_result", (True, r))
+            while self._observers:
+                try:
+                    # TODO: Handle errors here.
+                    self._observers.pop().callback(r)
+                except Exception:
+                    pass
+            return r
+
+        def errback(f):
+            object.__setattr__(self, "_result", (False, f))
+            while self._observers:
+                try:
+                    # TODO: Handle errors here.
+                    self._observers.pop().errback(f)
+                except Exception:
+                    pass
+
+            if consumeErrors:
+                return None
+            else:
+                return f
+
+        deferred.addCallbacks(callback, errback)
+
+    def observe(self):
+        """Observe the underlying deferred.
+
+        Can return either a deferred if the underlying deferred is still pending
+        (or has failed), or the actual value. Callers may need to use maybeDeferred.
+        """
+        if not self._result:
+            d = defer.Deferred()
+
+            def remove(r):
+                self._observers.discard(d)
+                return r
+            d.addBoth(remove)
+
+            self._observers.add(d)
+            return d
+        else:
+            success, res = self._result
+            return res if success else defer.fail(res)
+
+    def observers(self):
+        return self._observers
+
+    def has_called(self):
+        return self._result is not None
+
+    def has_succeeded(self):
+        return self._result is not None and self._result[0] is True
+
+    def get_result(self):
+        return self._result[1]
+
+    def __getattr__(self, name):
+        return getattr(self._deferred, name)
+
+    def __setattr__(self, name, value):
+        setattr(self._deferred, name, value)
+
+    def __repr__(self):
+        return "<ObservableDeferred object at %s, result=%r, _deferred=%r>" % (
+            id(self), self._result, self._deferred,
+        )
+
+
+def concurrently_execute(func, args, limit):
+    """Executes the function with each argument conncurrently while limiting
+    the number of concurrent executions.
+
+    Args:
+        func (func): Function to execute, should return a deferred.
+        args (list): List of arguments to pass to func, each invocation of func
+            gets a signle argument.
+        limit (int): Maximum number of conccurent executions.
+
+    Returns:
+        deferred: Resolved when all function invocations have finished.
+    """
+    it = iter(args)
+
+    @defer.inlineCallbacks
+    def _concurrently_execute_inner():
+        try:
+            while True:
+                yield func(next(it))
+        except StopIteration:
+            pass
+
+    return logcontext.make_deferred_yieldable(defer.gatherResults([
+        run_in_background(_concurrently_execute_inner)
+        for _ in range(limit)
+    ], consumeErrors=True)).addErrback(unwrapFirstError)
+
+
+class Linearizer(object):
+    """Limits concurrent access to resources based on a key. Useful to ensure
+    only a few things happen at a time on a given resource.
+
+    Example:
+
+        with (yield limiter.queue("test_key")):
+            # do some work.
+
+    """
+    def __init__(self, name=None, max_count=1, clock=None):
+        """
+        Args:
+            max_count(int): The maximum number of concurrent accesses
+        """
+        if name is None:
+            self.name = id(self)
+        else:
+            self.name = name
+
+        if not clock:
+            from twisted.internet import reactor
+            clock = Clock(reactor)
+        self._clock = clock
+        self.max_count = max_count
+
+        # key_to_defer is a map from the key to a 2 element list where
+        # the first element is the number of things executing, and
+        # the second element is an OrderedDict, where the keys are deferreds for the
+        # things blocked from executing.
+        self.key_to_defer = {}
+
+    def queue(self, key):
+        # we avoid doing defer.inlineCallbacks here, so that cancellation works correctly.
+        # (https://twistedmatrix.com/trac/ticket/4632 meant that cancellations were not
+        # propagated inside inlineCallbacks until Twisted 18.7)
+        entry = self.key_to_defer.setdefault(key, [0, collections.OrderedDict()])
+
+        # If the number of things executing is greater than the maximum
+        # then add a deferred to the list of blocked items
+        # When one of the things currently executing finishes it will callback
+        # this item so that it can continue executing.
+        if entry[0] >= self.max_count:
+            res = self._await_lock(key)
+        else:
+            logger.info(
+                "Acquired uncontended linearizer lock %r for key %r", self.name, key,
+            )
+            entry[0] += 1
+            res = defer.succeed(None)
+
+        # once we successfully get the lock, we need to return a context manager which
+        # will release the lock.
+
+        @contextmanager
+        def _ctx_manager(_):
+            try:
+                yield
+            finally:
+                logger.info("Releasing linearizer lock %r for key %r", self.name, key)
+
+                # We've finished executing so check if there are any things
+                # blocked waiting to execute and start one of them
+                entry[0] -= 1
+
+                if entry[1]:
+                    (next_def, _) = entry[1].popitem(last=False)
+
+                    # we need to run the next thing in the sentinel context.
+                    with PreserveLoggingContext():
+                        next_def.callback(None)
+                elif entry[0] == 0:
+                    # We were the last thing for this key: remove it from the
+                    # map.
+                    del self.key_to_defer[key]
+
+        res.addCallback(_ctx_manager)
+        return res
+
+    def _await_lock(self, key):
+        """Helper for queue: adds a deferred to the queue
+
+        Assumes that we've already checked that we've reached the limit of the number
+        of lock-holders we allow. Creates a new deferred which is added to the list, and
+        adds some management around cancellations.
+
+        Returns the deferred, which will callback once we have secured the lock.
+
+        """
+        entry = self.key_to_defer[key]
+
+        logger.info(
+            "Waiting to acquire linearizer lock %r for key %r", self.name, key,
+        )
+
+        new_defer = make_deferred_yieldable(defer.Deferred())
+        entry[1][new_defer] = 1
+
+        def cb(_r):
+            logger.info("Acquired linearizer lock %r for key %r", self.name, key)
+            entry[0] += 1
+
+            # if the code holding the lock completes synchronously, then it
+            # will recursively run the next claimant on the list. That can
+            # relatively rapidly lead to stack exhaustion. This is essentially
+            # the same problem as http://twistedmatrix.com/trac/ticket/9304.
+            #
+            # In order to break the cycle, we add a cheeky sleep(0) here to
+            # ensure that we fall back to the reactor between each iteration.
+            #
+            # (This needs to happen while we hold the lock, and the context manager's exit
+            # code must be synchronous, so this is the only sensible place.)
+            return self._clock.sleep(0)
+
+        def eb(e):
+            logger.info("defer %r got err %r", new_defer, e)
+            if isinstance(e, CancelledError):
+                logger.info(
+                    "Cancelling wait for linearizer lock %r for key %r",
+                    self.name, key,
+                )
+
+            else:
+                logger.warn(
+                    "Unexpected exception waiting for linearizer lock %r for key %r",
+                    self.name, key,
+                )
+
+            # we just have to take ourselves back out of the queue.
+            del entry[1][new_defer]
+            return e
+
+        new_defer.addCallbacks(cb, eb)
+        return new_defer
+
+
+class ReadWriteLock(object):
+    """A deferred style read write lock.
+
+    Example:
+
+        with (yield read_write_lock.read("test_key")):
+            # do some work
+    """
+
+    # IMPLEMENTATION NOTES
+    #
+    # We track the most recent queued reader and writer deferreds (which get
+    # resolved when they release the lock).
+    #
+    # Read: We know its safe to acquire a read lock when the latest writer has
+    # been resolved. The new reader is appeneded to the list of latest readers.
+    #
+    # Write: We know its safe to acquire the write lock when both the latest
+    # writers and readers have been resolved. The new writer replaces the latest
+    # writer.
+
+    def __init__(self):
+        # Latest readers queued
+        self.key_to_current_readers = {}
+
+        # Latest writer queued
+        self.key_to_current_writer = {}
+
+    @defer.inlineCallbacks
+    def read(self, key):
+        new_defer = defer.Deferred()
+
+        curr_readers = self.key_to_current_readers.setdefault(key, set())
+        curr_writer = self.key_to_current_writer.get(key, None)
+
+        curr_readers.add(new_defer)
+
+        # We wait for the latest writer to finish writing. We can safely ignore
+        # any existing readers... as they're readers.
+        yield make_deferred_yieldable(curr_writer)
+
+        @contextmanager
+        def _ctx_manager():
+            try:
+                yield
+            finally:
+                new_defer.callback(None)
+                self.key_to_current_readers.get(key, set()).discard(new_defer)
+
+        defer.returnValue(_ctx_manager())
+
+    @defer.inlineCallbacks
+    def write(self, key):
+        new_defer = defer.Deferred()
+
+        curr_readers = self.key_to_current_readers.get(key, set())
+        curr_writer = self.key_to_current_writer.get(key, None)
+
+        # We wait on all latest readers and writer.
+        to_wait_on = list(curr_readers)
+        if curr_writer:
+            to_wait_on.append(curr_writer)
+
+        # We can clear the list of current readers since the new writer waits
+        # for them to finish.
+        curr_readers.clear()
+        self.key_to_current_writer[key] = new_defer
+
+        yield make_deferred_yieldable(defer.gatherResults(to_wait_on))
+
+        @contextmanager
+        def _ctx_manager():
+            try:
+                yield
+            finally:
+                new_defer.callback(None)
+                if self.key_to_current_writer[key] == new_defer:
+                    self.key_to_current_writer.pop(key)
+
+        defer.returnValue(_ctx_manager())
+
+
+class DeferredTimeoutError(Exception):
+    """
+    This error is raised by default when a L{Deferred} times out.
+    """
+
+
+def add_timeout_to_deferred(deferred, timeout, reactor, on_timeout_cancel=None):
+    """
+    Add a timeout to a deferred by scheduling it to be cancelled after
+    timeout seconds.
+
+    This is essentially a backport of deferred.addTimeout, which was introduced
+    in twisted 16.5.
+
+    If the deferred gets timed out, it errbacks with a DeferredTimeoutError,
+    unless a cancelable function was passed to its initialization or unless
+    a different on_timeout_cancel callable is provided.
+
+    Args:
+        deferred (defer.Deferred): deferred to be timed out
+        timeout (Number): seconds to time out after
+        reactor (twisted.internet.reactor): the Twisted reactor to use
+
+        on_timeout_cancel (callable): A callable which is called immediately
+            after the deferred times out, and not if this deferred is
+            otherwise cancelled before the timeout.
+
+            It takes an arbitrary value, which is the value of the deferred at
+            that exact point in time (probably a CancelledError Failure), and
+            the timeout.
+
+            The default callable (if none is provided) will translate a
+            CancelledError Failure into a DeferredTimeoutError.
+    """
+    timed_out = [False]
+
+    def time_it_out():
+        timed_out[0] = True
+        deferred.cancel()
+
+    delayed_call = reactor.callLater(timeout, time_it_out)
+
+    def convert_cancelled(value):
+        if timed_out[0]:
+            to_call = on_timeout_cancel or _cancelled_to_timed_out_error
+            return to_call(value, timeout)
+        return value
+
+    deferred.addBoth(convert_cancelled)
+
+    def cancel_timeout(result):
+        # stop the pending call to cancel the deferred if it's been fired
+        if delayed_call.active():
+            delayed_call.cancel()
+        return result
+
+    deferred.addBoth(cancel_timeout)
+
+
+def _cancelled_to_timed_out_error(value, timeout):
+    if isinstance(value, failure.Failure):
+        value.trap(CancelledError)
+        raise DeferredTimeoutError(timeout, "Deferred")
+    return value