Merge branch 'develop' into matthew/preview_urls

3 files changed, 74 insertions, 23 deletions

diff --git a/synapse/util/async.py b/synapse/util/async.py
index cd4d90f3cf..0d6f48e2d8 100644
--- a/synapse/util/async.py
+++ b/synapse/util/async.py
@@ -16,9 +16,13 @@
 
 from twisted.internet import defer, reactor
 
-from .logcontext import PreserveLoggingContext, preserve_fn
+from .logcontext import (
+    PreserveLoggingContext, preserve_fn, preserve_context_over_deferred,
+)
 from synapse.util import unwrapFirstError
 
+from contextlib import contextmanager
+
 
 @defer.inlineCallbacks
 def sleep(seconds):
@@ -137,3 +141,47 @@ def concurrently_execute(func, args, limit):
         preserve_fn(_concurrently_execute_inner)()
         for _ in xrange(limit)
     ], consumeErrors=True).addErrback(unwrapFirstError)
+
+
+class Linearizer(object):
+    """Linearizes access to resources based on a key. Useful to ensure only one
+    thing is happening at a time on a given resource.
+
+    Example:
+
+        with (yield linearizer.queue("test_key")):
+            # do some work.
+
+    """
+    def __init__(self):
+        self.key_to_defer = {}
+
+    @defer.inlineCallbacks
+    def queue(self, key):
+        # If there is already a deferred in the queue, we pull it out so that
+        # we can wait on it later.
+        # Then we replace it with a deferred that we resolve *after* the
+        # context manager has exited.
+        # We only return the context manager after the previous deferred has
+        # resolved.
+        # This all has the net effect of creating a chain of deferreds that
+        # wait for the previous deferred before starting their work.
+        current_defer = self.key_to_defer.get(key)
+
+        new_defer = defer.Deferred()
+        self.key_to_defer[key] = new_defer
+
+        if current_defer:
+            yield preserve_context_over_deferred(current_defer)
+
+        @contextmanager
+        def _ctx_manager():
+            try:
+                yield
+            finally:
+                new_defer.callback(None)
+                current_d = self.key_to_defer.get(key)
+                if current_d is new_defer:
+                    self.key_to_defer.pop(key, None)
+
+        defer.returnValue(_ctx_manager())
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 35544b19fd..758f5982b0 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -167,7 +167,8 @@ class CacheDescriptor(object):
                 % (orig.__name__,)
             )
 
-        self.cache = Cache(
+    def __get__(self, obj, objtype=None):
+        cache = Cache(
             name=self.orig.__name__,
             max_entries=self.max_entries,
             keylen=self.num_args,
@@ -175,14 +176,12 @@ class CacheDescriptor(object):
             tree=self.tree,
         )
 
-    def __get__(self, obj, objtype=None):
-
         @functools.wraps(self.orig)
         def wrapped(*args, **kwargs):
             arg_dict = inspect.getcallargs(self.orig, obj, *args, **kwargs)
             cache_key = tuple(arg_dict[arg_nm] for arg_nm in self.arg_names)
             try:
-                cached_result_d = self.cache.get(cache_key)
+                cached_result_d = cache.get(cache_key)
 
                 observer = cached_result_d.observe()
                 if DEBUG_CACHES:
@@ -204,7 +203,7 @@ class CacheDescriptor(object):
                 # Get the sequence number of the cache before reading from the
                 # database so that we can tell if the cache is invalidated
                 # while the SELECT is executing (SYN-369)
-                sequence = self.cache.sequence
+                sequence = cache.sequence
 
                 ret = defer.maybeDeferred(
                     preserve_context_over_fn,
@@ -213,20 +212,21 @@ class CacheDescriptor(object):
                 )
 
                 def onErr(f):
-                    self.cache.invalidate(cache_key)
+                    cache.invalidate(cache_key)
                     return f
 
                 ret.addErrback(onErr)
 
                 ret = ObservableDeferred(ret, consumeErrors=True)
-                self.cache.update(sequence, cache_key, ret)
+                cache.update(sequence, cache_key, ret)
 
                 return preserve_context_over_deferred(ret.observe())
 
-        wrapped.invalidate = self.cache.invalidate
-        wrapped.invalidate_all = self.cache.invalidate_all
-        wrapped.invalidate_many = self.cache.invalidate_many
-        wrapped.prefill = self.cache.prefill
+        wrapped.invalidate = cache.invalidate
+        wrapped.invalidate_all = cache.invalidate_all
+        wrapped.invalidate_many = cache.invalidate_many
+        wrapped.prefill = cache.prefill
+        wrapped.cache = cache
 
         obj.__dict__[self.orig.__name__] = wrapped
 
@@ -240,11 +240,12 @@ class CacheListDescriptor(object):
     the list of missing keys to the wrapped fucntion.
     """
 
-    def __init__(self, orig, cache, list_name, num_args=1, inlineCallbacks=False):
+    def __init__(self, orig, cached_method_name, list_name, num_args=1,
+                 inlineCallbacks=False):
         """
         Args:
             orig (function)
-            cache (Cache)
+            method_name (str); The name of the chached method.
             list_name (str): Name of the argument which is the bulk lookup list
             num_args (int)
             inlineCallbacks (bool): Whether orig is a generator that should
@@ -263,7 +264,7 @@ class CacheListDescriptor(object):
         self.arg_names = inspect.getargspec(orig).args[1:num_args + 1]
         self.list_pos = self.arg_names.index(self.list_name)
 
-        self.cache = cache
+        self.cached_method_name = cached_method_name
 
         self.sentinel = object()
 
@@ -277,11 +278,13 @@ class CacheListDescriptor(object):
         if self.list_name not in self.arg_names:
             raise Exception(
                 "Couldn't see arguments %r for %r."
-                % (self.list_name, cache.name,)
+                % (self.list_name, cached_method_name,)
             )
 
     def __get__(self, obj, objtype=None):
 
+        cache = getattr(obj, self.cached_method_name).cache
+
         @functools.wraps(self.orig)
         def wrapped(*args, **kwargs):
             arg_dict = inspect.getcallargs(self.orig, obj, *args, **kwargs)
@@ -297,14 +300,14 @@ class CacheListDescriptor(object):
                 key[self.list_pos] = arg
 
                 try:
-                    res = self.cache.get(tuple(key)).observe()
+                    res = cache.get(tuple(key)).observe()
                     res.addCallback(lambda r, arg: (arg, r), arg)
                     cached[arg] = res
                 except KeyError:
                     missing.append(arg)
 
             if missing:
-                sequence = self.cache.sequence
+                sequence = cache.sequence
                 args_to_call = dict(arg_dict)
                 args_to_call[self.list_name] = missing
 
@@ -327,10 +330,10 @@ class CacheListDescriptor(object):
 
                     key = list(keyargs)
                     key[self.list_pos] = arg
-                    self.cache.update(sequence, tuple(key), observer)
+                    cache.update(sequence, tuple(key), observer)
 
                     def invalidate(f, key):
-                        self.cache.invalidate(key)
+                        cache.invalidate(key)
                         return f
                     observer.addErrback(invalidate, tuple(key))
 
@@ -370,7 +373,7 @@ def cachedInlineCallbacks(max_entries=1000, num_args=1, lru=False, tree=False):
     )
 
 
-def cachedList(cache, list_name, num_args=1, inlineCallbacks=False):
+def cachedList(cached_method_name, list_name, num_args=1, inlineCallbacks=False):
     """Creates a descriptor that wraps a function in a `CacheListDescriptor`.
 
     Used to do batch lookups for an already created cache. A single argument
@@ -400,7 +403,7 @@ def cachedList(cache, list_name, num_args=1, inlineCallbacks=False):
     """
     return lambda orig: CacheListDescriptor(
         orig,
-        cache=cache,
+        cached_method_name=cached_method_name,
         list_name=list_name,
         num_args=num_args,
         inlineCallbacks=inlineCallbacks,
diff --git a/synapse/util/caches/response_cache.py b/synapse/util/caches/response_cache.py
index be310ba320..36686b479e 100644
--- a/synapse/util/caches/response_cache.py
+++ b/synapse/util/caches/response_cache.py
@@ -35,7 +35,7 @@ class ResponseCache(object):
             return None
 
     def set(self, key, deferred):
-        result = ObservableDeferred(deferred)
+        result = ObservableDeferred(deferred, consumeErrors=True)
         self.pending_result_cache[key] = result
 
         def remove(r):