Allow dominate to work in async contexts (#187)

* Allow dominate to work in async contexts

Using ContextVars to allow dominate to work within async contexts. Added
unit tests to ensure code works as expected.

* Small Fixes

 - Added .venv and .envrc to .gitignore (I use direnv and venv to keep
   my python environments isolated - I hope this is okay!)
 - Removed print statements I left in dom_tag during debugging
 - Replaced global incrementing int with UUID for contextvar ID
   generation - this zeroes the risk of race-hazards/collisions
 - _get_thread_context now returns a tuple vs. a hash of a tuple.
   Functionally not much changes - the underlying dictionary will still
   use the same hashing function but the only difference is that _if_
   there is a collision, the dictionary will still be able to return the
   correct element

---------

Co-authored-by: tlonny <tlonny@lonnycore.lonny.zone>

Author: tlonny

.gitignore

@@ -39,3 +39,5 @@ nosetests.xml
 
 .idea
 .idea/
+.venv/
+.envrc

dominate/dom_tag.py

@@ -23,6 +23,9 @@
 from collections import defaultdict, namedtuple
 from functools import wraps
 import threading
+from asyncio import get_event_loop
+from uuid import uuid4
+from contextvars import ContextVar
 
 try:
   # Python 3
@@ -37,19 +40,44 @@
   basestring = str
   unicode = str
 
-
 try:
   import greenlet
 except ImportError:
   greenlet = None
 
+# We want dominate to work in async contexts - however, the problem is
+# when we bind a tag using "with", we set what is essentially a global variable.
+# If we are processing multiple documents at the same time, one context
+# can "overwrite" the "bound tag" of another - this can cause documents to
+# sort of bleed into one another...
+
+# The solution is to use a ContextVar - which provides async context local storage.
+# We use this to store a unique ID for each async context. We then use thie ID to
+# form the key (in _get_thread_context) that is used to index the _with_context defaultdict.
+# The presense of this key ensures that each async context has its own stack and doesn't conflict.
+async_context_id = ContextVar('async_context_id', default = None)
+
+def _get_async_context_id():
+  if async_context_id.get() is None:
+    async_context_id.set(uuid4().hex)
+  return async_context_id.get()
 
 def _get_thread_context():
   context = [threading.current_thread()]
+  # Tag extra content information with a name to make sure
+  # a greenlet.getcurrent() == 1 doesn't get confused with a
+  # a _get_thread_context() == 1.
   if greenlet:
-    context.append(greenlet.getcurrent())
-  return hash(tuple(context))
-
+    context.append(("greenlet", greenlet.getcurrent()))
+
+  try:
+    if get_event_loop().is_running():
+      # Only add this extra information if we are actually in a running event loop
+      context.append(("async", _get_async_context_id()))
+  # A runtime error is raised if there is no async loop...
+  except RuntimeError:
+    pass
+  return tuple(context)
 
 class dom_tag(object):
   is_single = False  # Tag does not require matching end tag (ex. <hr/>)

tests/test_dom_tag_async.py

@@ -0,0 +1,75 @@
+from asyncio import gather, run, Semaphore
+from dominate.dom_tag import async_context_id
+from textwrap import dedent
+
+from dominate import tags
+
+# To simulate sleep without making the tests take a hella long time to complete
+# lets use a pair of semaphores to explicitly control when our coroutines run.
+# The order of execution will be marked as comments below:
+def test_async_bleed():
+    async def tag_routine_1(sem_1, sem_2):
+        root = tags.div(id = 1) # [1]
+        with root: # [2]
+            sem_2.release() # [3]
+            await sem_1.acquire() # [4]
+            tags.div(id = 2) # [11]
+        return str(root) # [12]
+
+    async def tag_routine_2(sem_1, sem_2):
+        await sem_2.acquire() # [5]
+        root = tags.div(id = 3) # [6]
+        with root: # [7]
+            tags.div(id = 4) # [8]
+        sem_1.release() # [9]
+        return str(root) # [10]
+
+    async def merge():
+        sem_1 = Semaphore(0)
+        sem_2 = Semaphore(0)
+        return await gather(
+            tag_routine_1(sem_1, sem_2), 
+            tag_routine_2(sem_1, sem_2)
+        )
+
+    # Set this test up for failure - pre-set the context to a non-None value.
+    # As it is already set, _get_async_context_id will not set it to a new, unique value
+    # and thus we won't be able to differentiate between the two contexts. This essentially simulates
+    # the behavior before our async fix was implemented (the bleed):
+    async_context_id.set(0)
+    tag_1, tag_2 = run(merge())
+
+    # This looks wrong - but its what we would expect if we don't
+    # properly handle async...
+    assert tag_1 == dedent("""\
+        <div id="1">
+          <div id="3">
+            <div id="4"></div>
+          </div>
+          <div id="2"></div>
+        </div>
+    """).strip()
+
+    assert tag_2 == dedent("""\
+        <div id="3">
+          <div id="4"></div>
+        </div>
+    """).strip()
+
+    # Okay, now lets do it right - lets clear the context. Now when each async function
+    # calls _get_async_context_id, it will get a unique value and we can differentiate.
+    async_context_id.set(None)
+    tag_1, tag_2 = run(merge())
+
+    # Ah, much better...
+    assert tag_1 == dedent("""\
+        <div id="1">
+          <div id="2"></div>
+        </div>
+    """).strip()
+    
+    assert tag_2 == dedent("""\
+        <div id="3">
+          <div id="4"></div>
+        </div>
+    """).strip()