fix(ext/ffi): Fix UnsafeCallback ref'ing making Deno enter a live-loop (#16216)

Fixes #15136

Currently `UnsafeCallback` class' `ref()` and `unref()` methods rely on
the `event_loop_middleware` implementation in core. If even a single
`UnsafeCallback` is ref'ed, then the FFI event loop middleware will
always return `true` to signify that there may still be more work for
the event loop to do.

The middleware handling in core does not wait a moment to check again,
but will instead synchronously directly re-poll the event loop and
middlewares for more work. This becomes a live-loop.

This PR introduces a `Future` implementation for the `CallbackInfo`
struct that acts as the intermediary data storage between an
`UnsafeCallback` and the `libffi` C callback. Ref'ing a callback now
means calling an async op that binds to the `CallbackInfo` Future and
only resolves once the callback is unref'ed. The `libffi` C callback
will call the waker of this Future when it fires to make sure that the
main thread wakes up to receive the callback.

1 files changed, 2 insertions, 1 deletions

diff --git a/test_ffi/tests/test.js b/test_ffi/tests/test.js
index 6bf3c47f8..0af94b906 100644
--- a/test_ffi/tests/test.js
+++ b/test_ffi/tests/test.js
@@ -515,6 +515,7 @@ logCallback.unref();
 console.log("Thread safe call counter:", counter);
 returnU8Callback.ref();
 await dylib.symbols.call_fn_ptr_return_u8_thread_safe(returnU8Callback.pointer);
+// Purposefully do not unref returnU8Callback: Instead use it to test close() unrefing.
 
 // Test statics
 console.log("Static u32:", dylib.symbols.static_u32);
@@ -585,7 +586,7 @@ After: ${postStr}`,
 })();
 
 function assertIsOptimized(fn) {
-  const status = % GetOptimizationStatus(fn);
+  const status = %GetOptimizationStatus(fn);
   assert(status & (1 << 4), `expected ${fn.name} to be optimized, but wasn't`);
 }