stream: tweak usage of bdrv_co_is_allocated

is_allocated_base has complex semantics that are not really usable
outside streaming.  Split the check in two parts, where the allocated
state for the top bs is moved to the caller.  The resulting function
is more generally useful.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/stream.c

@@ -101,45 +101,33 @@ static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
 /*
  * Given an image chain: [BASE] -> [INTER1] -> [INTER2] -> [TOP]
  *
- * Return true if the given sector is allocated in top.
- * Return false if the given sector is allocated in intermediate images.
- * Return true otherwise.
+ * Return true if the given sector is allocated in any image between
+ * BASE and TOP (inclusive).  BASE can be NULL to check if the given
+ * sector is allocated in any image of the chain.  Return false otherwise.
  *
  * 'pnum' is set to the number of sectors (including and immediately following
  *  the specified sector) that are known to be in the same
  *  allocated/unallocated state.
  *
  */
-static int coroutine_fn is_allocated_base(BlockDriverState *top,
+static int coroutine_fn is_allocated_above(BlockDriverState *top,
                                            BlockDriverState *base,
                                            int64_t sector_num,
                                            int nb_sectors, int *pnum)
 {
     BlockDriverState *intermediate;
-    int ret, n;
-
-    ret = bdrv_co_is_allocated(top, sector_num, nb_sectors, &n);
-    if (ret) {
-        *pnum = n;
-        return ret;
-    }
-
-    /*
-     * Is the unallocated chunk [sector_num, n] also
-     * unallocated between base and top?
-     */
-    intermediate = top->backing_hd;
+    int ret, n = nb_sectors;
 
+    intermediate = top;
     while (intermediate != base) {
         int pnum_inter;
-
         ret = bdrv_co_is_allocated(intermediate, sector_num, nb_sectors,
                                    &pnum_inter);
         if (ret < 0) {
             return ret;
         } else if (ret) {
             *pnum = pnum_inter;
-            return 0;
+            return 1;
         }
 
         /*
@@ -156,7 +144,7 @@ static int coroutine_fn is_allocated_base(BlockDriverState *top,
     }
 
     *pnum = n;
-    return 1;
+    return 0;
 }
 
 static void coroutine_fn stream_run(void *opaque)
@@ -189,6 +177,7 @@ static void coroutine_fn stream_run(void *opaque)
 
     for (sector_num = 0; sector_num < end; sector_num += n) {
         uint64_t delay_ns = 0;
+        bool copy;
 
 wait:
         /* Note that even when no rate limit is applied we need to yield
@@ -199,10 +188,20 @@ wait:
             break;
         }
 
-        ret = is_allocated_base(bs, base, sector_num,
+        ret = bdrv_co_is_allocated(bs, sector_num,
                                    STREAM_BUFFER_SIZE / BDRV_SECTOR_SIZE, &n);
+        if (ret == 1) {
+            /* Allocated in the top, no need to copy.  */
+            copy = false;
+        } else {
+            /* Copy if allocated in the intermediate images.  Limit to the
+             * known-unallocated area [sector_num, sector_num+n).  */
+            ret = is_allocated_above(bs->backing_hd, base, sector_num, n, &n);
+            copy = (ret == 1);
+        }
+
         trace_stream_one_iteration(s, sector_num, n, ret);
-        if (ret == 0) {
+        if (ret >= 0 && copy) {
             if (s->common.speed) {
                 delay_ns = ratelimit_calculate_delay(&s->limit, n);
                 if (delay_ns > 0) {