fix: async_run to allow nested event loops. (#1170)

* Fix async_run to allow nested event loops.

Fixes: #1105

* Use standard asyncio.run in outer loop for tests

snakemake/common/__init__.py

@@ -37,7 +37,23 @@ def async_run(coroutine):
 
 
 else:
-    async_run = asyncio.run
+
+    def async_run(coroutine):
+        """Attaches to running event loop or creates a new one to execute a
+        coroutine.
+
+        .. seealso::
+
+             https://github.com/snakemake/snakemake/issues/1105
+             https://stackoverflow.com/a/65696398
+
+        """
+        try:
+            _ = asyncio.get_running_loop()
+        except RuntimeError:
+            asyncio.run(coroutine)
+        else:
+            asyncio.create_task(coroutine)
 
 
 # A string that prints as TBD

tests/testapi.py

@@ -2,6 +2,8 @@
 Tests for Snakemake’s API
 """
 from snakemake import snakemake
+import asyncio
+import sys
 import tempfile
 import os.path
 from textwrap import dedent
@@ -32,3 +34,26 @@ def test_run_script_directive():
                 file=f,
             )
         snakemake(path, workdir=tmpdir)
+
+
+def test_run_script_directive_async():
+    """Tests :func`snakemake.common.async_run`. The test ensure the ability to
+    execute Snakemake API even if an asyncio event loop is already running.
+
+    """
+    import tracemalloc
+    from snakemake.common import async_run
+
+    tracemalloc.start()
+
+    async def dummy_task():
+        await asyncio.sleep(0.00001)
+
+    async def main():
+        async_run(dummy_task())
+        test_run_script_directive()
+
+    if sys.version_info < (3, 7):
+        async_run(main())
+    else:
+        asyncio.run(main())