libxl: fix device counting race in libxl__devices_destroy

Don't have a fixed number of devices in the aodevs array, and instead
size it depending on the devices present in xenstore.  Somewhat
formalise the multiple device addition/removal machinery to make this
clearer and easier to do.

As a side-effect we fix a few "lost thread of control" bug which would
occur if there were no devices of a particular kind.  (Various if
statements which checked for there being no devices have become
redundant, but are retained to avoid making the patch bigger.)

Specifically:

 * Users of libxl__ao_devices are no longer expected to know in
   advance how many device operations they are going to do.  Instead
   they can initiate them one at a time, between bracketing calls to
   "begin" and "prepared".

 * The array of aodevs used for this is dynamically sized; to support
   this it's an array of pointers rather than of structs.

 * Users of libxl__ao_devices are presented with a more opaque interface.
   They are are no longer expected to, themselves,
      - look into the array of aodevs (this is now private)
      - know that the individual addition/removal completions are
        handled by libxl__ao_devices_callback (this callback function
        is now a private function for the multidev machinery)
      - ever deal with populating the contents of an aodevs

 * The doc comments relating to some of the members of
   libxl__ao_device are clarified.  (And the member `aodevs' is moved
   to put it with the other members with the same status.)

 * The multidev machinery allocates an aodev to represent the
   operation of preparing all of the other operations.  See
   the comment in libxl__multidev_begin.

A wrinkle is that the functions are called "multidev" but the structs
are called "libxl__ao_devices" and "aodevs".  I have given these
functions this name to distinguish them from "libxl__ao_device" and
"aodev" and so forth by more than just the use of the plural "s"
suffix.

In the next patch we will rename the structs.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

1 files changed, 46 insertions, 31 deletions

diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 2d6c71a815..07e92fbc35 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -1816,20 +1816,6 @@ typedef void libxl__device_callback(libxl__egc*, libxl__ao_device*);
  */
 _hidden void libxl__prepare_ao_device(libxl__ao *ao, libxl__ao_device *aodev);
 
-/* Prepare a bunch of devices for addition/removal. Every ao_device in
- * ao_devices is set to 'active', and the ao_device 'base' field is set to
- * the one pointed by aodevs.
- */
-_hidden void libxl__prepare_ao_devices(libxl__ao *ao,
-                                       libxl__ao_devices *aodevs);
-
-/* Generic callback to use when adding/removing several devices, this will
- * check if the given aodev is the last one, and call the callback in the
- * parent libxl__ao_devices struct, passing the appropriate error if found.
- */
-_hidden void libxl__ao_devices_callback(libxl__egc *egc,
-                                        libxl__ao_device *aodev);
-
 struct libxl__ao_device {
     /* filled in by user */
     libxl__ao *ao;
@@ -1837,32 +1823,60 @@ struct libxl__ao_device {
     libxl__device *dev;
     int force;
     libxl__device_callback *callback;
-    /* private for implementation */
-    int active;
+    /* return value, zeroed by user on entry, is valid on callback */
     int rc;
+    /* private for multidev */
+    int active;
+    libxl__ao_devices *aodevs; /* reference to the containing multidev */
+    /* private for add/remove implementation */
     libxl__ev_devstate backend_ds;
     /* Bodge for Qemu devices, also used for timeout of hotplug execution */
     libxl__ev_time timeout;
-    /* Used internally to have a reference to the upper libxl__ao_devices
-     * struct when present */
-    libxl__ao_devices *aodevs;
     /* device hotplug execution */
     const char *what;
     int num_exec;
     libxl__ev_child child;
 };
 
-/* Helper struct to simply the plug/unplug of multiple devices at the same
- * time.
- *
- * This structure holds several devices, and the callback is only called
- * when all the devices inside of the array have finished.
- */
+/*
+ * Multiple devices "multidev" handling.
+ *
+ * Firstly, you should
+ *    libxl__multidev_begin
+ *    multidev->callback = ...
+ * Then zero or more times
+ *    libxl__multidev_prepare
+ *    libal__initiate_device_{remove/addition}.
+ * Finally, once
+ *    libxl__multidev_prepared
+ * which will result (perhaps reentrantly) in one call to callback().
+ */
+
+/* Starts preparing to add/remove a bunch of devices. */
+_hidden void libxl__multidev_begin(libxl__ao *ao, libxl__ao_devices*);
+
+/* Prepares to add/remove one of many devices.  Returns a libxl__ao_device
+ * which has had libxl__prepare_ao_device called, and which has also
+ * had ->callback set.  The user should not mess with aodev->callback. */
+_hidden libxl__ao_device *libxl__multidev_prepare(libxl__ao_devices*);
+
+/* Notifies the multidev machinery that we have now finished preparing
+ * and initiating devices.  multidev->callback may then be called as
+ * soon as there are no prepared but not completed operations
+ * outstanding, perhaps reentrantly.  If rc!=0 (error should have been
+ * logged) multidev->callback will get a non-zero rc.
+ * callback may be set by the user at any point before prepared. */
+_hidden void libxl__multidev_prepared(libxl__egc*, libxl__ao_devices*, int rc);
+
 typedef void libxl__devices_callback(libxl__egc*, libxl__ao_devices*, int rc);
 struct libxl__ao_devices {
-    libxl__ao_device *array;
-    int size;
+    /* set by user: */
     libxl__devices_callback *callback;
+    /* for private use by libxl__...ao_devices... machinery: */
+    libxl__ao *ao;
+    libxl__ao_device **array;
+    int used, allocd;
+    libxl__ao_device *preparation;
 };
 
 /*
@@ -2372,10 +2386,11 @@ _hidden void libxl__devices_destroy(libxl__egc *egc,
                                     libxl__devices_remove_state *drs);
 
 /* Helper function to add a bunch of disks. This should be used when
- * the caller is inside an async op. "devices" will NOT be prepared by this
- * function, so the caller must make sure to call _prepare before calling this
- * function. The start parameter contains the position inside the aodevs array
- * that should be used to store the state of this devices.
+ * the caller is inside an async op. "devices" will NOT be prepared by
+ * this function, so the caller must make sure to call
+ * libxl__multidev_begin before calling this function. The start
+ * parameter contains the position inside the aodevs array that should
+ * be used to store the state of this devices.
  *
  * The "callback" will be called for each device, and the user is responsible
  * for calling libxl__ao_device_check_last on the callback.