aio-posix: simplify FDMonOps->update() prototype

The AioHandler *node, bool is_new arguments are more complicated to
think about than simply being given AioHandler *old_node, AioHandler
*new_node.

Furthermore, the new Linux io_uring file descriptor monitoring mechanism
added by the new patch requires access to both the old and the new
nodes.  Make this change now in preparation.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-5-stefanha@redhat.com
Message-Id: <20200305170806.1313245-5-stefanha@redhat.com>

include/block/aio.h

@@ -57,17 +57,16 @@ typedef struct {
     /*
      * update:
      * @ctx: the AioContext
-     * @node: the handler
-     * @is_new: is the file descriptor already being monitored?
+     * @old_node: the existing handler or NULL if this file descriptor is being
+     *            monitored for the first time
+     * @new_node: the new handler or NULL if this file descriptor is being
+     *            removed
      *
-     * Add/remove/modify a monitored file descriptor.  There are three cases:
-     * 1. node->pfd.events == 0 means remove the file descriptor.
-     * 2. !is_new means modify an already monitored file descriptor.
-     * 3. is_new means add a new file descriptor.
+     * Add/remove/modify a monitored file descriptor.
      *
      * Called with ctx->list_lock acquired.
      */
-    void (*update)(AioContext *ctx, AioHandler *node, bool is_new);
+    void (*update)(AioContext *ctx, AioHandler *old_node, AioHandler *new_node);
 
     /*
      * wait:

util/aio-posix.c

@@ -139,12 +139,7 @@ void aio_set_fd_handler(AioContext *ctx,
     atomic_set(&ctx->poll_disable_cnt,
                atomic_read(&ctx->poll_disable_cnt) + poll_disable_change);
 
-    if (new_node) {
-        ctx->fdmon_ops->update(ctx, new_node, is_new);
-    } else if (node) {
-        /* Unregister deleted fd_handler */
-        ctx->fdmon_ops->update(ctx, node, false);
-    }
+    ctx->fdmon_ops->update(ctx, node, new_node);
     qemu_lockcnt_unlock(&ctx->list_lock);
     aio_notify(ctx);
 

util/fdmon-epoll.c

@@ -30,21 +30,24 @@ static inline int epoll_events_from_pfd(int pfd_events)
            (pfd_events & G_IO_ERR ? EPOLLERR : 0);
 }
 
-static void fdmon_epoll_update(AioContext *ctx, AioHandler *node, bool is_new)
+static void fdmon_epoll_update(AioContext *ctx,
+                               AioHandler *old_node,
+                               AioHandler *new_node)
 {
-    struct epoll_event event;
+    struct epoll_event event = {
+        .data.ptr = new_node,
+        .events = new_node ? epoll_events_from_pfd(new_node->pfd.events) : 0,
+    };
     int r;
-    int ctl;
 
-    if (!node->pfd.events) {
-        ctl = EPOLL_CTL_DEL;
+    if (!new_node) {
+        r = epoll_ctl(ctx->epollfd, EPOLL_CTL_DEL, old_node->pfd.fd, &event);
+    } else if (!old_node) {
+        r = epoll_ctl(ctx->epollfd, EPOLL_CTL_ADD, new_node->pfd.fd, &event);
     } else {
-        event.data.ptr = node;
-        event.events = epoll_events_from_pfd(node->pfd.events);
-        ctl = is_new ? EPOLL_CTL_ADD : EPOLL_CTL_MOD;
+        r = epoll_ctl(ctx->epollfd, EPOLL_CTL_MOD, new_node->pfd.fd, &event);
     }
 
-    r = epoll_ctl(ctx->epollfd, ctl, node->pfd.fd, &event);
     if (r) {
         fdmon_epoll_disable(ctx);
     }

util/fdmon-poll.c

@@ -93,7 +93,9 @@ static int fdmon_poll_wait(AioContext *ctx, AioHandlerList *ready_list,
     return ret;
 }
 
-static void fdmon_poll_update(AioContext *ctx, AioHandler *node, bool is_new)
+static void fdmon_poll_update(AioContext *ctx,
+                              AioHandler *old_node,
+                              AioHandler *new_node)
 {
     /* Do nothing, AioHandler already contains the state we'll need */
 }