tracer.trace_child_tasks() for asyncio.gather tracing

Also added documentation for datasette.tracer module.

Closes #1576

tests/plugins/my_plugin.py

@@ -1,5 +1,7 @@
+import asyncio
 from datasette import hookimpl
 from datasette.facets import Facet
+from datasette import tracer
 from datasette.utils import path_with_added_args
 from datasette.utils.asgi import asgi_send_json, Response
 import base64
@@ -270,6 +272,15 @@ def register_routes():
     def asgi_scope(scope):
         return Response.json(scope, default=repr)
 
+    async def parallel_queries(datasette):
+        db = datasette.get_database()
+        with tracer.trace_child_tasks():
+            one, two = await asyncio.gather(
+                db.execute("select coalesce(sleep(0.1), 1)"),
+                db.execute("select coalesce(sleep(0.1), 2)"),
+            )
+        return Response.json({"one": one.single_value(), "two": two.single_value()})
+
     return [
         (r"/one/$", one),
         (r"/two/(?P<name>.*)$", two),
@@ -281,6 +292,7 @@ def register_routes():
         (r"/add-message/$", add_message),
         (r"/render-message/$", render_message),
         (r"/asgi-scope$", asgi_scope),
+        (r"/parallel-queries$", parallel_queries),
     ]
 
 

tests/test_tracer.py

@@ -51,3 +51,18 @@ def test_trace(trace_debug):
     execute_manys = [trace for trace in traces if trace.get("executemany")]
     assert execute_manys
     assert all(isinstance(trace["count"], int) for trace in execute_manys)
+
+
+def test_trace_parallel_queries():
+    with make_app_client(settings={"trace_debug": True}) as client:
+        response = client.get("/parallel-queries?_trace=1")
+        assert response.status == 200
+
+    data = response.json
+    assert data["one"] == 1
+    assert data["two"] == 2
+    trace_info = data["_trace"]
+    traces = [trace for trace in trace_info["traces"] if "sql" in trace]
+    one, two = traces
+    # "two" should have started before "one" ended
+    assert two["start"] < one["end"]