aio-posix: add fdmon_ops->dispatch()

The ppoll and epoll file descriptor monitoring implementations rely on
the event loop's generic file descriptor, timer, and BH dispatch code to
invoke user callbacks.

The io_uring file descriptor monitoring implementation will need
io_uring-specific dispatch logic for CQE handlers for custom SQEs.

Introduce a new FDMonOps ->dispatch() callback that allows file
descriptor monitoring implementations to invoke user callbacks. The next
patch will use this new callback.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20251104022933.618123-13-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/aio.h

@@ -107,6 +107,25 @@ typedef struct {
      */
     bool (*need_wait)(AioContext *ctx);
 
+    /*
+     * dispatch:
+     * @ctx: the AioContext
+     *
+     * Dispatch any work that is specific to this file descriptor monitoring
+     * implementation. Usually the event loop's generic file descriptor
+     * monitoring, BH, and timer dispatching code is sufficient, but file
+     * descriptor monitoring implementations offering additional functionality
+     * may need to implement this function for custom behavior. Called at a
+     * point in the event loop when it is safe to invoke user-defined
+     * callbacks.
+     *
+     * This function is optional and may be NULL.
+     *
+     * Returns: true if progress was made (see aio_poll()'s return value),
+     * false otherwise.
+     */
+    bool (*dispatch)(AioContext *ctx);
+
     /*
      * gsource_prepare:
      * @ctx: the AioContext

util/aio-posix.c

@@ -385,10 +385,15 @@ void aio_dispatch(AioContext *ctx)
     AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
 
     qemu_lockcnt_inc(&ctx->list_lock);
+
     aio_bh_poll(ctx);
 
     ctx->fdmon_ops->gsource_dispatch(ctx, &ready_list);
 
+    if (ctx->fdmon_ops->dispatch) {
+        ctx->fdmon_ops->dispatch(ctx);
+    }
+
     /* block_ns is 0 because polling is disabled in the glib event loop */
     aio_dispatch_ready_handlers(ctx, &ready_list, 0);
 
@@ -707,6 +712,10 @@ bool aio_poll(AioContext *ctx, bool blocking)
         block_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start;
     }
 
+    if (ctx->fdmon_ops->dispatch) {
+        progress |= ctx->fdmon_ops->dispatch(ctx);
+    }
+
     progress |= aio_bh_poll(ctx);
     progress |= aio_dispatch_ready_handlers(ctx, &ready_list, block_ns);