monitor: fdset: Match against O_DIRECT

We're about to enable the use of O_DIRECT in the migration code and
due to the alignment restrictions imposed by filesystems we need to
make sure the flag is only used when doing aligned IO.

The migration will do parallel IO to different regions of a file, so
we need to use more than one file descriptor. Those cannot be obtained
by duplicating (dup()) since duplicated file descriptors share the
file status flags, including O_DIRECT. If one migration channel does
unaligned IO while another sets O_DIRECT to do aligned IO, the
filesystem would fail the unaligned operation.

The add-fd QMP command along with the fdset code are specifically
designed to allow the user to pass a set of file descriptors with
different access flags into QEMU to be later fetched by code that
needs to alternate between those flags when doing IO.

Extend the fdset matching to behave the same with the O_DIRECT flag.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

monitor/fds.c

@@ -424,6 +424,11 @@ int monitor_fdset_dup_fd_add(int64_t fdset_id, int flags, Error **errp)
         int fd = -1;
         int dup_fd;
         int mon_fd_flags;
+        int mask = O_ACCMODE;
+
+#ifdef O_DIRECT
+        mask |= O_DIRECT;
+#endif
 
         if (mon_fdset->id != fdset_id) {
             continue;
@@ -437,7 +442,7 @@ int monitor_fdset_dup_fd_add(int64_t fdset_id, int flags, Error **errp)
                 return -1;
             }
 
-            if ((flags & O_ACCMODE) == (mon_fd_flags & O_ACCMODE)) {
+            if ((flags & mask) == (mon_fd_flags & mask)) {
                 fd = mon_fdset_fd->fd;
                 break;
             }