Make run_as_sync safe to use if runtime is present

Summary:
The issue with the way we use block_on() is that it tries to unconditionally create new Tokio runtime and panics if there is one already (because nested runtimes are not allowed).

Here if the runtime is present, we reuse it instead of creating a new one.

Reviewed By: ndmitchell

Differential Revision: D61635145

fbshipit-source-id: caed839c5e42ddf222d6770b1d3949228beb32c5

Cargo.toml

@@ -9,6 +9,7 @@ members = [
 
 [workspace.dependencies]
 fbinit = { git = "https://github.com/facebookexperimental/rust-shed" }
+fbinit-tokio = { git = "https://github.com/facebookexperimental/rust-shed" }
 scuba = { git = "https://github.com/facebookexperimental/rust-shed" }
 
 [profile.release]

td_util/Cargo.toml

@@ -9,6 +9,8 @@ argfile = "0.1.5"
 clap = {version = "4.1.4"}
 equivalent = "1.0.0"
 fbinit = { workspace = true }
+fbinit-tokio = { workspace = true }
+futures = "0.3.30"
 lazy_static = "1.4.0"
 static_interner.version = "0.1"
 # @oss-disable: static_interner.path = "../../buck2/shed/static_interner"

td_util/src/executor.rs

@@ -11,10 +11,35 @@
 
 use std::future::Future;
 
+// Run future within a sync context
+// Uses existing Tokio runtime if available, otherwise creates a new one.
 pub fn run_as_sync<F: Future>(future: F) -> F::Output {
-    tokio::runtime::Builder::new_current_thread()
-        .enable_all()
-        .build()
-        .unwrap()
-        .block_on(future)
+    match tokio::runtime::Handle::try_current() {
+        Ok(handle) => {
+            let _guard = handle.enter();
+            futures::executor::block_on(future)
+        }
+        Err(_) => tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap()
+            .block_on(future),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::executor::run_as_sync;
+
+    #[test]
+    fn test_run_as_sync_without_runtime() {
+        // Should not panic and should return the result of the future.
+        assert_eq!(run_as_sync(async { 42 }), 42);
+    }
+
+    #[fbinit::test]
+    async fn test_run_as_sync_with_runtime() {
+        // Tokio runtime is injected by fbinit. Still, should not panic and should return the result of the future.
+        assert_eq!(run_as_sync(async { 42 }), 42);
+    }
 }