chore: import upstream snapshot with attribution

tests/unittests/workflow/test_workflow_concurrency.py

@@ -0,0 +1,132 @@
+# Copyright 2026 Google LLC
+#
+# 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 asyncio
+from typing import Any
+from typing import AsyncGenerator
+
+from google.adk.agents.context import Context
+from google.adk.apps.app import App
+from google.adk.events.event import Event
+from google.adk.workflow import BaseNode
+from google.adk.workflow import START
+from google.adk.workflow._parallel_worker import _ParallelWorker as ParallelWorker
+from google.adk.workflow._workflow import Workflow
+from pydantic import Field
+import pytest
+from typing_extensions import override
+
+from .. import testing_utils
+from .workflow_testing_utils import create_parent_invocation_context
+
+
+@pytest.mark.asyncio
+async def test_max_concurrency_limits_running_nodes(
+    request: pytest.FixtureRequest,
+):
+  """Max concurrency limits the number of parallel graph-scheduled nodes.
+
+  Setup:
+    Workflow with 4 parallel nodes and max_concurrency=2.
+  Act:
+    - Start workflow in background.
+    - Release nodes one by one.
+  Assert:
+    - Initially only 2 nodes start.
+    - Releasing one node allows another to start.
+    - All nodes eventually complete.
+  """
+
+  class ConcurrencyWorkerNode(BaseNode):
+    """A node that signals when it starts and waits for a signal to finish."""
+
+    started_event: asyncio.Event
+    finish_event: asyncio.Event
+
+    @override
+    async def _run_impl(
+        self,
+        *,
+        ctx: Context,
+        node_input: Any,
+    ) -> AsyncGenerator[Any, None]:
+      self.started_event.set()
+      await self.finish_event.wait()
+      yield f'{self.name}_done'
+
+  class TerminalNode(BaseNode):
+
+    @override
+    async def _run_impl(
+        self, ctx: Context, node_input: Any
+    ) -> AsyncGenerator[Any, None]:
+      yield 'workflow_done'
+
+  # Given a workflow with 4 parallel nodes and max_concurrency=2
+  num_nodes = 4
+  max_concurrency = 2
+  started_events = [asyncio.Event() for _ in range(num_nodes)]
+  finish_events = [asyncio.Event() for _ in range(num_nodes)]
+
+  nodes = [
+      ConcurrencyWorkerNode(
+          name=f'Node{i}',
+          started_event=started_events[i],
+          finish_event=finish_events[i],
+      )
+      for i in range(num_nodes)
+  ]
+
+  terminal_node = TerminalNode(name='Terminal')
+  edges = [(START, tuple(nodes), terminal_node)]
+
+  agent = Workflow(
+      name='concurrency_agent',
+      max_concurrency=max_concurrency,
+      edges=edges,
+  )
+
+  app = App(name=request.function.__name__, root_agent=agent)
+  runner = testing_utils.InMemoryRunner(app=app)
+
+  # When the workflow is run in background
+  async def run_agent():
+    return await runner.run_async(testing_utils.get_user_content('start'))
+
+  run_task = asyncio.create_task(run_agent())
+
+  # Then initially only max_concurrency nodes should start
+  await asyncio.sleep(0.1)
+  started_count = sum(1 for e in started_events if e.is_set())
+  assert started_count == max_concurrency
+
+  # When one node is released
+  for i in range(num_nodes):
+    if started_events[i].is_set():
+      finish_events[i].set()
+      break
+
+  # Then another node should start, bringing total to max_concurrency + 1
+  await asyncio.sleep(0.1)
+  started_count = sum(1 for e in started_events if e.is_set())
+  assert started_count == max_concurrency + 1
+
+  # When all remaining nodes are released
+  for e in finish_events:
+    e.set()
+
+  # Then all nodes should eventually complete
+  await run_task
+  started_count = sum(1 for e in started_events if e.is_set())
+  assert started_count == num_nodes