Client.get_dataset to always create Futures attached to itself

.github/workflows/ci-windows.yaml

@@ -23,6 +23,13 @@ jobs:
           activate-environment: dask-distributed
           auto-activate-base: false
 
+      - name: Install contextvars
+        shell: bash -l {0}
+        run: |
+          if [[ "${{ matrix.python-version }}" = "3.6" ]]; then
+            conda install -c conda-forge contextvars
+          fi
+
       - name: Install tornado
         shell: bash -l {0}
         run: |

continuous_integration/travis/install.sh

@@ -67,6 +67,10 @@ conda create -n dask-distributed -c conda-forge -c defaults \
 
 source activate dask-distributed
 
+if [[ $PYTHON == 3.6 ]]; then
+  conda install -c conda-forge -c defaults contextvars
+fi
+
 # stacktrace is not currently avaiable for Python 3.8.
 # Remove the version check block below when it is avaiable.
 if [[ $PYTHON != 3.8 ]]; then

distributed/client.py

@@ -5,6 +5,7 @@
 from concurrent.futures import ThreadPoolExecutor
 from concurrent.futures._base import DoneAndNotDoneFutures
 from contextlib import contextmanager
+from contextvars import ContextVar
 import copy
 import errno
 from functools import partial
@@ -89,6 +90,7 @@
 _global_clients = weakref.WeakValueDictionary()
 _global_client_index = [0]
 
+_current_client = ContextVar("_current_client", default=None)
 
 DEFAULT_EXTENSIONS = [PubSubClientExtension]
 
@@ -162,7 +164,7 @@ def __init__(self, key, client=None, inform=True, state=None):
         self.key = key
         self._cleared = False
         tkey = tokey(key)
-        self.client = client or _get_global_client()
+        self.client = client or Client.current()
         self.client._inc_ref(tkey)
         self._generation = self.client.generation
 
@@ -353,11 +355,14 @@ def release(self, _in_destructor=False):
                 pass  # Shutting down, add_callback may be None
 
     def __getstate__(self):
-        return (self.key, self.client.scheduler.address)
+        return self.key, self.client.scheduler.address
 
     def __setstate__(self, state):
         key, address = state
-        c = get_client(address)
+        try:
+            c = Client.current(allow_global=False)
+        except ValueError:
+            c = get_client(address)
         Future.__init__(self, key, c)
         c._send_to_scheduler(
             {
@@ -727,10 +732,42 @@ def __init__(
 
         ReplayExceptionClient(self)
 
+    @contextmanager
+    def as_current(self):
+        """Thread-local, Task-local context manager that causes the Client.current class
+        method to return self. This is used when a method of Client needs to propagate a
+        reference to self deep into the stack through generic methods that shouldn't be
+        aware of this class.
+        """
+        # Python 3.6; contextvars are thread-local but not Task-local.
+        # We can still detect a race condition.
+        if sys.version_info < (3, 7) and _current_client.get() not in (self, None):
+            raise RuntimeError(
+                "Detected race condition where get_dataset() is invoked in "
+                "parallel by multiple asynchronous clients. "
+                "Please upgrade to Python 3.7+."
+            )
+
+        tok = _current_client.set(self)
+        try:
+            yield
+        finally:
+            _current_client.reset(tok)
+
     @classmethod
-    def current(cls):
-        """ Return global client if one exists, otherwise raise ValueError """
-        return default_client()
+    def current(cls, allow_global=True):
+        """When running within the context of `as_client`, return the context-local
+        current client. Otherwise, return the latest initialised Client.
+        If no Client instances exist, raise ValueError.
+        If allow_global is set to False, raise ValueError if running outside of the
+        `as_client` context manager.
+        """
+        out = _current_client.get()
+        if out:
+            return out
+        if allow_global:
+            return default_client()
+        raise ValueError("Not running inside the `as_current` context manager")
 
     @property
     def asynchronous(self):
@@ -2178,8 +2215,7 @@ def retry(self, futures, asynchronous=None):
         """
         return self.sync(self._retry, futures, asynchronous=asynchronous)
 
-    @gen.coroutine
-    def _publish_dataset(self, *args, name=None, **kwargs):
+    async def _publish_dataset(self, *args, name=None, **kwargs):
         with log_errors():
             coroutines = []
 
@@ -2205,7 +2241,7 @@ def add_coro(name, data):
             for name, data in kwargs.items():
                 add_coro(name, data)
 
-            yield coroutines
+            await asyncio.gather(*coroutines)
 
     def publish_dataset(self, *args, **kwargs):
         """
@@ -2285,13 +2321,12 @@ def list_datasets(self, **kwargs):
         return self.sync(self.scheduler.publish_list, **kwargs)
 
     async def _get_dataset(self, name):
-        out = await self.scheduler.publish_get(name=name, client=self.id)
-        if out is None:
-            raise KeyError("Dataset '%s' not found" % name)
+        with self.as_current():
+            out = await self.scheduler.publish_get(name=name, client=self.id)
 
-        with temp_default_client(self):
-            data = out["data"]
-        return data
+        if out is None:
+            raise KeyError(f"Dataset '{name}' not found")
+        return out["data"]
 
     def get_dataset(self, name, **kwargs):
         """
@@ -4697,6 +4732,14 @@ def __exit__(self, typ, value, traceback):
 def temp_default_client(c):
     """ Set the default client for the duration of the context
 
+    .. note::
+       This function should be used exclusively for unit testing the default client
+       functionality. In all other cases, please use ``Client.as_current`` instead.
+
+    .. note::
+       Unlike ``Client.as_current``, this context manager is neither thread-local nor
+       task-local.
+
     Parameters
     ----------
     c : Client

distributed/lock.py

@@ -3,7 +3,7 @@
 import logging
 import uuid
 
-from .client import _get_global_client
+from .client import Client
 from .utils import log_errors, TimeoutError
 from .worker import get_worker
 
@@ -93,7 +93,11 @@ class Lock:
     """
 
     def __init__(self, name=None, client=None):
-        self.client = client or _get_global_client() or get_worker().client
+        try:
+            self.client = client or Client.current()
+        except ValueError:
+            # Initialise new client
+            self.client = get_worker().client
         self.name = name or "lock-" + uuid.uuid4().hex
         self.id = uuid.uuid4().hex
         self._locked = False

distributed/publish.py

@@ -6,10 +6,10 @@
 class PublishExtension:
     """ An extension for the scheduler to manage collections
 
-    *  publish-list
-    *  publish-put
-    *  publish-get
-    *  publish-delete
+    *  publish_list
+    *  publish_put
+    *  publish_get
+    *  publish_delete
     """
 
     def __init__(self, scheduler):
@@ -59,21 +59,60 @@ class Datasets(MutableMapping):
 
     """
 
+    __slots__ = ("_client",)
+
     def __init__(self, client):
-        self.__client = client
+        self._client = client
 
     def __getitem__(self, key):
-        return self.__client.get_dataset(key)
+        # When client is asynchronous, it returns a coroutine
+        return self._client.get_dataset(key)
 
     def __setitem__(self, key, value):
-        self.__client.publish_dataset(value, name=key)
+        if self._client.asynchronous:
+            # 'await obj[key] = value' is not supported by Python as of 3.8
+            raise TypeError(
+                "Can't use 'client.datasets[name] = value' when client is "
+                "asynchronous; please use 'client.publish_dataset(name=value)' instead"
+            )
+        self._client.publish_dataset(value, name=key)
 
     def __delitem__(self, key):
-        self.__client.unpublish_dataset(key)
+        if self._client.asynchronous:
+            # 'await del obj[key]' is not supported by Python as of 3.8
+            raise TypeError(
+                "Can't use 'del client.datasets[name]' when client is asynchronous; "
+                "please use 'client.unpublish_dataset(name)' instead"
+            )
+        return self._client.unpublish_dataset(key)
 
     def __iter__(self):
-        for key in self.__client.list_datasets():
+        if self._client.asynchronous:
+            raise TypeError(
+                "Can't invoke iter() or 'for' on client.datasets when client is "
+                "asynchronous; use 'async for' instead"
+            )
+        for key in self._client.list_datasets():
             yield key
 
+    def __aiter__(self):
+        if not self._client.asynchronous:
+            raise TypeError(
+                "Can't invoke 'async for' on client.datasets when client is "
+                "synchronous; use iter() or 'for' instead"
+            )
+
+        async def _():
+            for key in await self._client.list_datasets():
+                yield key
+
+        return _()
+
     def __len__(self):
-        return len(self.__client.list_datasets())
+        if self._client.asynchronous:
+            # 'await len(obj)' is not supported by Python as of 3.8
+            raise TypeError(
+                "Can't use 'len(client.datasets)' when client is asynchronous; "
+                "please use 'len(await client.list_datasets())' instead"
+            )
+        return len(self._client.list_datasets())

distributed/queues.py

@@ -3,7 +3,7 @@
 import logging
 import uuid
 
-from .client import Future, _get_global_client, Client
+from .client import Future, Client
 from .utils import tokey, sync, thread_state
 from .worker import get_client
 
@@ -148,7 +148,7 @@ class Queue:
         not given, a random name will be generated.
     client: Client (optional)
         Client used for communication with the scheduler. Defaults to the
-        value of ``_get_global_client()``.
+        value of ``Client.current()``.
     maxsize: int (optional)
         Number of items allowed in the queue. If 0 (the default), the queue
         size is unbounded.
@@ -167,7 +167,7 @@ class Queue:
     """
 
     def __init__(self, name=None, client=None, maxsize=0):
-        self.client = client or _get_global_client()
+        self.client = client or Client.current()
         self.name = name or "queue-" + uuid.uuid4().hex
         self._event_started = asyncio.Event()
         if self.client.asynchronous or getattr(