Revert "vhost-user: Introduce nested event loop in vhost_user_read()"

This reverts commit a7f523c.

The nested event loop is broken by design. It's only user was removed.
Drop the code as well so that nobody ever tries to use it again.

I had to fix a couple of trivial conflicts around return values because
of 025faa8 ("vhost-user: stick to -errno error return convention").

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20230119172424.478268-3-groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
(cherry picked from commit 4382138)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/virtio/vhost-user.c

@@ -305,37 +305,24 @@ static int vhost_user_read_header(struct vhost_dev *dev, VhostUserMsg *msg)
     return 0;
 }
 
-struct vhost_user_read_cb_data {
-    struct vhost_dev *dev;
-    VhostUserMsg *msg;
-    GMainLoop *loop;
-    int ret;
-};
-
-static gboolean vhost_user_read_cb(void *do_not_use, GIOCondition condition,
-                                   gpointer opaque)
+static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
 {
-    struct vhost_user_read_cb_data *data = opaque;
-    struct vhost_dev *dev = data->dev;
-    VhostUserMsg *msg = data->msg;
     struct vhost_user *u = dev->opaque;
     CharBackend *chr = u->user->chr;
     uint8_t *p = (uint8_t *) msg;
     int r, size;
 
     r = vhost_user_read_header(dev, msg);
     if (r < 0) {
-        data->ret = r;
-        goto end;
+        return r;
     }
 
     /* validate message size is sane */
     if (msg->hdr.size > VHOST_USER_PAYLOAD_SIZE) {
         error_report("Failed to read msg header."
                 " Size %d exceeds the maximum %zu.", msg->hdr.size,
                 VHOST_USER_PAYLOAD_SIZE);
-        data->ret = -EPROTO;
-        goto end;
+        return -EPROTO;
     }
 
     if (msg->hdr.size) {
@@ -346,53 +333,11 @@ static gboolean vhost_user_read_cb(void *do_not_use, GIOCondition condition,
             int saved_errno = errno;
             error_report("Failed to read msg payload."
                          " Read %d instead of %d.", r, msg->hdr.size);
-            data->ret = r < 0 ? -saved_errno : -EIO;
-            goto end;
+            return r < 0 ? -saved_errno : -EIO;
         }
     }
 
-end:
-    g_main_loop_quit(data->loop);
-    return G_SOURCE_REMOVE;
-}
-
-static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
-{
-    struct vhost_user *u = dev->opaque;
-    CharBackend *chr = u->user->chr;
-    GMainContext *prev_ctxt = chr->chr->gcontext;
-    GMainContext *ctxt = g_main_context_new();
-    GMainLoop *loop = g_main_loop_new(ctxt, FALSE);
-    struct vhost_user_read_cb_data data = {
-        .dev = dev,
-        .loop = loop,
-        .msg = msg,
-        .ret = 0
-    };
-
-    /*
-     * We want to be able to monitor the slave channel fd while waiting
-     * for chr I/O. This requires an event loop, but we can't nest the
-     * one to which chr is currently attached : its fd handlers might not
-     * be prepared for re-entrancy. So we create a new one and switch chr
-     * to use it.
-     */
-    qemu_chr_be_update_read_handlers(chr->chr, ctxt);
-    qemu_chr_fe_add_watch(chr, G_IO_IN | G_IO_HUP, vhost_user_read_cb, &data);
-
-    g_main_loop_run(loop);
-
-    /*
-     * Restore the previous event loop context. This also destroys/recreates
-     * event sources : this guarantees that all pending events in the original
-     * context that have been processed by the nested loop are purged.
-     */
-    qemu_chr_be_update_read_handlers(chr->chr, prev_ctxt);
-
-    g_main_loop_unref(loop);
-    g_main_context_unref(ctxt);
-
-    return data.ret;
+    return 0;
 }
 
 static int process_message_reply(struct vhost_dev *dev,