[PATCH v2 04/11] rust/qemu-api: Add wrappers to run futures in QEMU

[Kevin Wolf <kwolf@redhat.com>]

This adds helper functions that allow running Rust futures to completion
using QEMU's event loops.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 include/qemu/coroutine-rust.h | 24 +++++++++++
 rust/wrapper.h                |  1 +
 util/qemu-co-rust-async.c     | 55 +++++++++++++++++++++++++
 rust/qemu-api/meson.build     |  1 +
 rust/qemu-api/src/futures.rs  | 77 +++++++++++++++++++++++++++++++++++
 rust/qemu-api/src/lib.rs      |  1 +
 util/meson.build              |  3 ++
 7 files changed, 162 insertions(+)
 create mode 100644 include/qemu/coroutine-rust.h
 create mode 100644 util/qemu-co-rust-async.c
 create mode 100644 rust/qemu-api/src/futures.rs

diff --git a/include/qemu/coroutine-rust.h b/include/qemu/coroutine-rust.h
new file mode 100644
index 0000000000..0c5cf42a6b
--- /dev/null
+++ b/include/qemu/coroutine-rust.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Helpers to run Rust futures using QEMU coroutines
+ *
+ * Copyright Red Hat
+ *
+ * Author:
+ *   Kevin Wolf <kwolf@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ */
+
+#ifndef QEMU_COROUTINE_RUST_H
+#define QEMU_COROUTINE_RUST_H
+
+typedef struct RustBoxedFuture RustBoxedFuture;
+typedef void coroutine_fn RunFuture(RustBoxedFuture *future, void *opaque);
+
+void no_coroutine_fn rust_run_future(RustBoxedFuture *future,
+                                     RunFuture *entry,
+                                     void *opaque);
+
+#endif
diff --git a/rust/wrapper.h b/rust/wrapper.h
index 303d7bba7f..3dc385e256 100644
--- a/rust/wrapper.h
+++ b/rust/wrapper.h
@@ -58,3 +58,4 @@ typedef enum memory_order {
 #include "block/block_int.h"
 #include "block/qdict.h"
 #include "qapi/qapi-visit-block-core.h"
+#include "qemu/coroutine-rust.h"
diff --git a/util/qemu-co-rust-async.c b/util/qemu-co-rust-async.c
new file mode 100644
index 0000000000..d893dfb7bd
--- /dev/null
+++ b/util/qemu-co-rust-async.c
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Helpers to run Rust futures using QEMU coroutines
+ *
+ * Copyright Red Hat
+ *
+ * Author:
+ *   Kevin Wolf <kwolf@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+
+#include "block/aio-wait.h"
+#include "qemu/coroutine.h"
+#include "qemu/coroutine-rust.h"
+#include "qemu/main-loop.h"
+
+typedef struct FutureCo {
+    RustBoxedFuture *future;
+    RunFuture *entry;
+    void *opaque;
+    bool done;
+} FutureCo;
+
+static void coroutine_fn rust_co_run_future_entry(void *opaque)
+{
+    FutureCo *data = opaque;
+
+    data->entry(data->future, data->opaque);
+    data->done = true;
+    aio_wait_kick();
+}
+
+void no_coroutine_fn rust_run_future(RustBoxedFuture *future,
+                                     RunFuture *entry,
+                                     void *opaque)
+{
+    AioContext *ctx = qemu_get_current_aio_context();
+    Coroutine *co;
+    FutureCo data = {
+        .future = future,
+        .entry = entry,
+        .opaque = opaque,
+        .done = false,
+    };
+
+    GLOBAL_STATE_CODE();
+
+    co = qemu_coroutine_create(rust_co_run_future_entry, &data);
+    aio_co_enter(ctx, co);
+    AIO_WAIT_WHILE(ctx, !data.done);
+}
diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
index e0a3052c79..44fd34e193 100644
--- a/rust/qemu-api/meson.build
+++ b/rust/qemu-api/meson.build
@@ -22,6 +22,7 @@ sources_core = [
   'src/chardev.rs',
   'src/c_str.rs',
   'src/errno.rs',
+  'src/futures.rs',
   'src/module.rs',
   'src/offset_of.rs',
   'src/prelude.rs',
diff --git a/rust/qemu-api/src/futures.rs b/rust/qemu-api/src/futures.rs
new file mode 100644
index 0000000000..cd307a1d62
--- /dev/null
+++ b/rust/qemu-api/src/futures.rs
@@ -0,0 +1,77 @@
+use crate::bindings;
+use std::ffi::c_void;
+use std::future::Future;
+use std::mem::MaybeUninit;
+use std::sync::Arc;
+use std::task::{Context, Poll, Wake, Waker};
+
+struct RunFutureWaker {
+    co: *mut bindings::Coroutine,
+}
+unsafe impl Send for RunFutureWaker {}
+unsafe impl Sync for RunFutureWaker {}
+
+impl Wake for RunFutureWaker {
+    fn wake(self: Arc<Self>) {
+        unsafe {
+            bindings::aio_co_wake(self.co);
+        }
+    }
+}
+
+/// Use QEMU's event loops to run a Rust [`Future`] to completion and return its result.
+///
+/// This function must be called in coroutine context. If the future isn't ready yet, it yields.
+pub fn qemu_co_run_future<F: Future>(future: F) -> F::Output {
+    let waker = Waker::from(Arc::new(RunFutureWaker {
+        co: unsafe { bindings::qemu_coroutine_self() },
+    }));
+    let mut cx = Context::from_waker(&waker);
+
+    let mut pinned_future = std::pin::pin!(future);
+    loop {
+        match pinned_future.as_mut().poll(&mut cx) {
+            Poll::Ready(res) => return res,
+            Poll::Pending => unsafe {
+                bindings::qemu_coroutine_yield();
+            },
+        }
+    }
+}
+
+/// Wrapper around [`qemu_co_run_future`] that can be called from C.
+///
+/// # Safety
+///
+/// `future` must be a valid pointer to an owned `F` (it will be freed in this function).  `output`
+/// must be a valid pointer representing a mutable reference to an `F::Output` where the result can
+/// be stored.
+unsafe extern "C" fn rust_co_run_future<F: Future>(
+    future: *mut bindings::RustBoxedFuture,
+    output: *mut c_void,
+) {
+    let future = unsafe { Box::from_raw(future.cast::<F>()) };
+    let output = output.cast::<F::Output>();
+    let ret = qemu_co_run_future(*future);
+    unsafe {
+        output.write(ret);
+    }
+}
+
+/// Use QEMU's event loops to run a Rust [`Future`] to completion and return its result.
+///
+/// This function must be called outside of coroutine context to avoid deadlocks. It blocks and
+/// runs a nested even loop until the future is ready and returns a result.
+pub fn qemu_run_future<F: Future>(future: F) -> F::Output {
+    let future_ptr = Box::into_raw(Box::new(future));
+    let mut output = MaybeUninit::<F::Output>::uninit();
+    unsafe {
+        bindings::rust_run_future(
+            future_ptr.cast::<bindings::RustBoxedFuture>(),
+            #[allow(clippy::as_underscore)]
+            Some(rust_co_run_future::<F> as _),
+            output.as_mut_ptr().cast::<c_void>(),
+        );
+        output.assume_init()
+    }
+}
diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
index 825443abde..84928905f1 100644
--- a/rust/qemu-api/src/lib.rs
+++ b/rust/qemu-api/src/lib.rs
@@ -20,6 +20,7 @@
 pub mod cell;
 pub mod chardev;
 pub mod errno;
+pub mod futures;
 #[cfg(feature = "system")]
 pub mod irq;
 #[cfg(feature = "system")]
diff --git a/util/meson.build b/util/meson.build
index 780b5977a8..14a2ae17fd 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -101,6 +101,9 @@ if have_block
   util_ss.add(files('qemu-coroutine-sleep.c'))
   util_ss.add(files('qemu-co-shared-resource.c'))
   util_ss.add(files('qemu-co-timeout.c'))
+  if have_rust
+    util_ss.add(files('qemu-co-rust-async.c'))
+  endif
   util_ss.add(files('readline.c'))
   util_ss.add(files('throttle.c'))
   util_ss.add(files('timed-average.c'))
-- 
2.48.1