safekeeper: fix panic in debug_dump. (#9097)

Panic was triggered only when dump selected no timelines.

sentry report:
https://neondatabase.sentry.io/issues/5832368589/

safekeeper/src/debug_dump.rs

@@ -253,6 +253,13 @@ pub async fn build(args: Args) -> Result<Response> {
         });
     }
 
+    // Tokio forbids to drop runtime in async context, so this is a stupid way
+    // to drop it in non async context.
+    tokio::task::spawn_blocking(move || {
+        let _r = runtime;
+    })
+    .await?;
+
     Ok(Response {
         start_time,
         finish_time: Utc::now(),

test_runner/regress/test_wal_acceptor.py

@@ -894,6 +894,13 @@ def test_timeline_status(neon_env_builder: NeonEnvBuilder, auth_enabled: bool):
     assert debug_dump_0["timelines"][0]["timeline_id"] == str(timeline_id)
     assert debug_dump_0["timelines"][0]["wal_last_modified"] != ""
 
+    # debug dump non existing tenant, should return no timelines.
+    debug_dump_non_existent = wa_http_cli_debug.debug_dump(
+        {"tenant_id": "deadbeefdeadbeefdeadbeefdeadbeef"}
+    )
+    log.info(f"debug_dump_non_existend: {debug_dump_non_existent}")
+    assert len(debug_dump_non_existent["timelines"]) == 0
+
     endpoint.safe_psql("create table t(i int)")
 
     # ensure epoch goes up after reboot