Fixup deadlock when hammering futures of a certain type

typescript/src/async-rust-call.ts

@@ -33,6 +33,12 @@ type PollFunc = (
   handle: UniffiHandle,
 ) => void;
 
+// Calls setTimeout and then resolves the promise.
+// This may be used as a simple yield.
+export async function delayPromise(delayMs: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, delayMs));
+}
+
 /**
  * This method calls an asynchronous method on the Rust side.
  *
@@ -65,13 +71,30 @@ export async function uniffiRustCallAsync<F, T>(
   // The poll, complete and free methods are specialized by the FFIType of the return value.
   try {
     let pollResult: number | undefined;
-    do {
+    while (true) {
       // Calling pollFunc with a callback that resolves the promise that pollRust
       // returns: pollRust makes the promise, uniffiFutureContinuationCallback resolves it.
       pollResult = await pollRust((handle) => {
         pollFunc(rustFuture, uniffiFutureContinuationCallback, handle);
       });
-    } while (pollResult != UNIFFI_RUST_FUTURE_POLL_READY);
+      // From https://github.com/mozilla/uniffi-rs/pull/1837/files#diff-8a28c9cf1245b4f714d406ea4044d68e1000099928eaca1afb504ccbc008fe9fR35-R37
+      //
+      // > WARNING: the call to [rust_future_poll] must be scheduled to happen soon after the callback is
+      // > called, but not inside the callback itself.  If [rust_future_poll] is called inside the
+      // > callback, some futures will deadlock and our scheduler code might as well.
+      //
+      // This delay is to ensure that `uniffiFutureContinuationCallback` returns before the next poll, i.e.
+      // so that the next poll is outside of this callback.
+      //
+      // The length of the delay seems to be significant (at least in tests which hammer a network).
+      // I would like to understand this more: I am still seeing deadlocks when this drops below its current
+      // delay, but these maybe related to a different issue, as alluded to in
+      // https://github.com/mozilla/uniffi-rs/pull/1901
+      if (pollResult == UNIFFI_RUST_FUTURE_POLL_READY) {
+        break;
+      }
+      await delayPromise(100);
+    }
 
     // Now it's ready, all we need to do is pick up the result (and error).
     return liftFunc(