fix: allow awaiting logical size for root timelines (#8604)

Currently if `GET
/v1/tenant/x/timeline/y?force-await-initial-logical-size=true` is
requested for a root timeline created within the current pageserver
session, the request handler panics hitting the debug assertion. These
timelines will always have an accurate (at initdb import) calculated
logical size. Fix is to never attempt prioritizing timeline size
calculation if we already have an exact value.

Split off from #8528.

pageserver/src/tenant/timeline.rs

@@ -4727,6 +4727,12 @@ impl Timeline {
             return;
         }
 
+        if self.current_logical_size.current_size().is_exact() {
+            // root timelines are initialized with exact count, but never start the background
+            // calculation
+            return;
+        }
+
         if let Some(await_bg_cancel) = self
             .current_logical_size
             .cancel_wait_for_background_loop_concurrency_limit_semaphore

pageserver/src/tenant/timeline/logical_size.rs

@@ -122,6 +122,10 @@ impl CurrentLogicalSize {
             Self::Exact(_) => Accuracy::Exact,
         }
     }
+
+    pub(crate) fn is_exact(&self) -> bool {
+        matches!(self, Self::Exact(_))
+    }
 }
 
 impl LogicalSize {

test_runner/regress/test_timeline_size.py

@@ -936,6 +936,9 @@ def test_timeline_logical_size_task_priority(neon_env_builder: NeonEnvBuilder):
     tenant_id = env.initial_tenant
     timeline_id = env.initial_timeline
 
+    # just make sure this doesn't hit an assertion
+    client.timeline_detail(tenant_id, timeline_id, force_await_initial_logical_size=True)
+
     # load in some data
     endpoint = env.endpoints.create_start("main", tenant_id=tenant_id)
     endpoint.safe_psql_many(