runtime: fix memory leak/growth when creating many runtimes (#3564)

tests-integration/Cargo.toml

@@ -12,9 +12,15 @@ name = "test-cat"
 name = "test-mem"
 required-features = ["rt-net"]
 
+[[bin]]
+name = "test-process-signal"
+required-features = ["rt-process-signal"]
+
 [features]
 # For mem check
 rt-net = ["tokio/rt", "tokio/rt-multi-thread", "tokio/net"]
+# For test-process-signal
+rt-process-signal = ["rt", "tokio/process", "tokio/signal"]
 
 full = [
     "macros",

tests-integration/src/bin/test-process-signal.rs

@@ -0,0 +1,11 @@
+// https://github.com/tokio-rs/tokio/issues/3550
+fn main() {
+    for _ in 0..1000 {
+        let rt = tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap();
+
+        drop(rt);
+    }
+}