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, 6 insertions, 4 deletions

diff --git a/cli/dts/lib.deno.unstable.d.ts b/cli/dts/lib.deno.unstable.d.ts
index a25c1011e..6eb7153ba 100644
--- a/cli/dts/lib.deno.unstable.d.ts
+++ b/cli/dts/lib.deno.unstable.d.ts
@@ -831,20 +831,22 @@ declare namespace Deno {
     >;
 
     /**
-     * Adds one to this callback's reference counting.
+     * Adds one to this callback's reference counting and returns the
+     * new reference count.
      *
      * If the callback's reference count becomes non-zero, it will keep
      * Deno's process from exiting.
      */
-    ref(): void;
+    ref(): number;
 
     /**
-     * Removes one from this callback's reference counting.
+     * Removes one from this callback's reference counting and returns
+     * the new reference count.
      *
      * If the callback's reference counter becomes zero, it will no longer
      * keep Deno's process from exiting.
      */
-    unref(): void;
+    unref(): number;
 
     /**
      * Removes the C function pointer associated with the UnsafeCallback.