libxl: Fix handling of timeouts

xen-unstable changeset 26469 makes changes wrt modifying and deregistering
timeouts.

First, timeout modify callbacks will only be invoked with an
abs_t of {0,0}, i.e. make the timeout fire immediately.  Prior to this
commit, timeout modify callbacks were never invoked.

Second, timeout deregister hooks will no longer be called.

This patch makes changes in the libvirt libxl driver that should be
compatible before and after changeset 26469.

While at it, fix a potential overflow in the timeout register callback.

src/libxl/libxl_driver.c

@@ -186,7 +186,15 @@ libxlTimerCallback(int timer ATTRIBUTE_UNUSED, void *timer_info)
 {
     struct libxlOSEventHookTimerInfo *info = timer_info;
 
+    /*
+     * libxl expects the event to be deregistered when calling
+     * libxl_osevent_occurred_timeout, but we dont want the event info
+     * destroyed.  Disable the timeout and only remove it after returning
+     * from libxl.
+     */
+    virEventUpdateTimeout(info->id, -1);
     libxl_osevent_occurred_timeout(info->priv->ctx, info->xl_priv);
+    virEventRemoveTimeout(info->id);
 }
 
 static void
@@ -202,6 +210,8 @@ libxlTimeoutRegisterEventHook(void *priv,
                               void *xl_priv)
 {
     struct timeval now;
+    struct timeval res;
+    static struct timeval zero;
     struct libxlOSEventHookTimerInfo *timer_info;
     int timeout, timer_id;
 
@@ -211,8 +221,15 @@ libxlTimeoutRegisterEventHook(void *priv,
     }
 
     gettimeofday(&now, NULL);
-    timeout = (abs_t.tv_usec - now.tv_usec) / 1000;
-    timeout += (abs_t.tv_sec - now.tv_sec) * 1000;
+    timersub(&abs_t, &now, &res);
+    /* Ensure timeout is not overflowed */
+    if (timercmp(&res, &zero, <)) {
+        timeout = 0;
+    } else if (res.tv_sec > INT_MAX / 1000) {
+        timeout = INT_MAX;
+    } else {
+        timeout = res.tv_sec * 1000 + (res.tv_usec + 999) / 1000;
+    }
     timer_id = virEventAddTimeout(timeout, libxlTimerCallback,
                                   timer_info, libxlTimerInfoFree);
     if (timer_id < 0) {
@@ -227,19 +244,25 @@ libxlTimeoutRegisterEventHook(void *priv,
     return 0;
 }
 
+/*
+ * Note:  There are two changes wrt timeouts starting with xen-unstable
+ * changeset 26469:
+ *
+ * 1. Timeout modify callbacks will only be invoked with an abs_t of {0,0},
+ * i.e. make the timeout fire immediately.  Prior to this commit, timeout
+ * modify callbacks were never invoked.
+ *
+ * 2. Timeout deregister hooks will no longer be called.
+ */
 static int
 libxlTimeoutModifyEventHook(void *priv ATTRIBUTE_UNUSED,
                             void **hndp,
-                            struct timeval abs_t)
+                            struct timeval abs_t ATTRIBUTE_UNUSED)
 {
-    struct timeval now;
-    int timeout;
     struct libxlOSEventHookTimerInfo *timer_info = *hndp;
 
-    gettimeofday(&now, NULL);
-    timeout = (abs_t.tv_usec - now.tv_usec) / 1000;
-    timeout += (abs_t.tv_sec - now.tv_sec) * 1000;
-    virEventUpdateTimeout(timer_info->id, timeout);
+    /* Make the timeout fire */
+    virEventUpdateTimeout(timer_info->id, 0);
     return 0;
 }