aio: add errp argument to aio_context_setup()

When aio_context_new() -> aio_context_setup() fails at startup it
doesn't really matter whether errors are returned to the caller or the
process terminates immediately.

However, it is not acceptable to terminate when hotplugging --object
iothread at runtime. Refactor aio_context_setup() so that errors can be
propagated. The next commit will set errp when fdmon_io_uring_setup()
fails.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20251104022933.618123-10-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/aio.h

@@ -718,10 +718,13 @@ void qemu_set_current_aio_context(AioContext *ctx);
 /**
  * aio_context_setup:
  * @ctx: the aio context
+ * @errp: error pointer
  *
  * Initialize the aio context.
+ *
+ * Returns: true on success, false otherwise
  */
-void aio_context_setup(AioContext *ctx);
+bool aio_context_setup(AioContext *ctx, Error **errp);
 
 /**
  * aio_context_destroy:

util/aio-posix.c

@@ -718,7 +718,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
     return progress;
 }
 
-void aio_context_setup(AioContext *ctx)
+bool aio_context_setup(AioContext *ctx, Error **errp)
 {
     ctx->fdmon_ops = &fdmon_poll_ops;
     ctx->epollfd = -1;
@@ -726,10 +726,11 @@ void aio_context_setup(AioContext *ctx)
 
     /* Use the fastest fd monitoring implementation if available */
     if (fdmon_io_uring_setup(ctx)) {
-        return;
+        return true;
     }
 
     fdmon_epoll_setup(ctx);
+    return true;
 }
 
 void aio_context_destroy(AioContext *ctx)

util/aio-win32.c

@@ -419,8 +419,9 @@ bool aio_poll(AioContext *ctx, bool blocking)
     return progress;
 }
 
-void aio_context_setup(AioContext *ctx)
+bool aio_context_setup(AioContext *ctx, Error **errp)
 {
+    return true;
 }
 
 void aio_context_destroy(AioContext *ctx)

util/async.c

@@ -581,6 +581,7 @@ static void co_schedule_bh_cb(void *opaque)
 
 AioContext *aio_context_new(Error **errp)
 {
+    ERRP_GUARD();
     int ret;
     AioContext *ctx;
 
@@ -611,7 +612,10 @@ AioContext *aio_context_new(Error **errp)
      * you add any new resources to AioContext, it's probably best to acquire
      * them before aio_context_setup().
      */
-    aio_context_setup(ctx);
+    if (!aio_context_setup(ctx, errp)) {
+        event_notifier_cleanup(&ctx->notifier);
+        goto fail;
+    }
 
     g_source_set_can_recurse(&ctx->source, true);
     qemu_lockcnt_init(&ctx->list_lock);