nodedev: udev: Hook up virFileWaitForAccess to work around uevent race

If we find ourselves in the situation that the 'add' uevent has been
fired earlier than the sysfs tree for a device was created, we should
use the best-effort approach and give kernel some predetermined amount
of time, thus waiting for the attributes to be ready rather than
discarding the device from our device list forever. If those don't appear
in the given time frame, we need to move on, since libvirt can't wait
indefinitely.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1463285Signed-off-by: NErik Skultety <eskultet@redhat.com>

src/node_device/node_device_udev.c

@@ -1188,9 +1188,23 @@ udevProcessMediatedDevice(struct udev_device *dev,
     char *canonicalpath = NULL;
     virNodeDevCapMdevPtr data = &def->caps->data.mdev;
 
-    if (virAsprintf(&linkpath, "%s/mdev_type", udev_device_get_syspath(dev)) < 0)
+    /* Because of a kernel uevent race, we might get the 'add' event prior to
+     * the sysfs tree being ready, so any attempt to access any sysfs attribute
+     * would result in ENOENT and us dropping the device, so let's work around
+     * it by waiting for the attributes to become available.
+     */
+
+    if (virAsprintf(&linkpath, "%s/mdev_type",
+                    udev_device_get_syspath(dev)) < 0)
         goto cleanup;
 
+    if (virFileWaitForExists(linkpath, 1, 100) < 0) {
+        virReportSystemError(errno,
+                             _("failed to wait for file '%s' to appear"),
+                             linkpath);
+        goto cleanup;
+    }
+
     if (virFileResolveLink(linkpath, &canonicalpath) < 0) {
         virReportSystemError(errno, _("failed to resolve '%s'"), linkpath);
         goto cleanup;