[Qemu-devel] [RFC v2 PATCH 1/6] block: add support functions for live commit, to find and delete images.

[Jeff Cody]

Add bdrv_find_child(), and bdrv_delete_intermediate().

bdrv_find_child():  given 'bs' and the active (topmost) BDS of an image chain,
                    find the image that is the immediate top of 'bs'

bdrv_delete_intermediate():
                    Given 3 BDS (active, top, base), delete images above
                    base up to and including top, and set base to be the
                    parent of top's child node.

                    E.g., this converts:

                    bottom <- base <- intermediate <- top <- active

                    to

                    bottom <- base <- active

                    where top == active is permitted, although active
                    will not be deleted.

Signed-off-by: Jeff Cody <address@hidden>
---
 block.c | 142 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 block.h |   5 ++-
 2 files changed, 146 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 9470319..11e275c 100644
--- a/block.c
+++ b/block.c
@@ -1752,6 +1752,148 @@ int bdrv_change_backing_file(BlockDriverState *bs,
     return ret;
 }
 
+/* Finds the image layer immediately to the 'top' of bs.
+ *
+ * active is the current topmost image.
+ */
+BlockDriverState *bdrv_find_child(BlockDriverState *active,
+                                  BlockDriverState *bs)
+{
+    BlockDriverState *child = NULL;
+    BlockDriverState *intermediate;
+
+    /* if the active bs layer is the same as the new top, then there
+     * is no image above the top, so it will be returned as the child
+     */
+    if (active == bs) {
+        child = active;
+    } else {
+        intermediate = active;
+        while (intermediate->backing_hd) {
+            if (intermediate->backing_hd == bs) {
+                child = intermediate;
+                break;
+            }
+            intermediate = intermediate->backing_hd;
+        }
+    }
+
+    return child;
+}
+
+typedef struct BlkIntermediateStates {
+    BlockDriverState *bs;
+    QSIMPLEQ_ENTRY(BlkIntermediateStates) entry;
+} BlkIntermediateStates;
+
+
+/* deletes images above 'base' up to and including 'top', and sets the image
+ * above 'top' to have base as its backing file.
+ *
+ * E.g., this will convert the following chain:
+ * bottom <- base <- intermediate <- top <- active
+ *
+ * to
+ *
+ * bottom <- base <- active
+ *
+ * It is allowed for bottom==base, in which case it converts:
+ *
+ * base <- intermediate <- top <- active
+ *
+ * to
+ *
+ * base <- active
+ *
+ * It is also allowed for top==active, except in that case active is not
+ * deleted:
+ *
+ * base <- intermediate <- top
+ *
+ * becomes
+ *
+ * base <- top
+ */
+int bdrv_delete_intermediate(BlockDriverState *active, BlockDriverState *top,
+                             BlockDriverState *base)
+{
+    BlockDriverState *intermediate;
+    BlockDriverState *base_bs = NULL;
+    BlockDriverState *new_top_bs = NULL;
+    BlkIntermediateStates *intermediate_state, *next;
+    int ret = -1;
+
+    QSIMPLEQ_HEAD(states_to_delete, BlkIntermediateStates) states_to_delete;
+    QSIMPLEQ_INIT(&states_to_delete);
+
+    if (!top->drv || !base->drv) {
+        goto exit;
+    }
+
+    new_top_bs = bdrv_find_child(active, top);
+
+    /* special case of new_top_bs->backing_hd already pointing to base - 
nothing
+     * to do, no intermediate images
+     */
+    if (new_top_bs->backing_hd == base) {
+        ret = 0;
+        goto exit;
+    }
+
+    if (new_top_bs == NULL) {
+        /* we could not find the image above 'top', this is an error */
+        goto exit;
+    }
+
+    /* if the active and top image passed in are the same, then we
+     * can't delete the active, so we start one below
+     */
+    intermediate = (active == top) ? active->backing_hd : top;
+
+    /* now we will go down through the list, and add each BDS we find
+     * into our deletion queue, until we hit the 'base'
+     */
+    while (intermediate) {
+        intermediate_state = g_malloc0(sizeof(BlkIntermediateStates));
+        intermediate_state->bs = intermediate;
+        QSIMPLEQ_INSERT_TAIL(&states_to_delete, intermediate_state, entry);
+
+        if (intermediate->backing_hd == base) {
+            base_bs = intermediate->backing_hd;
+            break;
+        }
+        intermediate = intermediate->backing_hd;
+    }
+    if (base_bs == NULL) {
+        /* something went wrong, we did not end at the base. safely
+         * unravel everything, and exit with error */
+        goto exit;
+    }
+
+    /* success - we can delete the intermediate states, and link top->base */
+    ret = bdrv_change_backing_file(new_top_bs, base_bs->filename,
+                                   base_bs->drv ? base_bs->drv->format_name : 
"");
+    if (ret) {
+        goto exit;
+    }
+    new_top_bs->backing_hd = base_bs;
+
+
+    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
+        /* so that bdrv_close() does not recursively close the chain */
+        intermediate_state->bs->backing_hd = NULL;
+        bdrv_delete(intermediate_state->bs);
+    }
+    ret = 0;
+
+exit:
+    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
+        g_free(intermediate_state);
+    }
+    return ret;
+}
+
+
 static int bdrv_check_byte_request(BlockDriverState *bs, int64_t offset,
                                    size_t size)
 {
diff --git a/block.h b/block.h
index db812b1..ee76869 100644
--- a/block.h
+++ b/block.h
@@ -201,7 +201,10 @@ int bdrv_commit_all(void);
 int bdrv_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt);
 void bdrv_register(BlockDriver *bdrv);
-
+int bdrv_delete_intermediate(BlockDriverState *active, BlockDriverState *top,
+                             BlockDriverState *base);
+BlockDriverState *bdrv_find_child(BlockDriverState *active,
+                                  BlockDriverState *bs);
 
 typedef struct BdrvCheckResult {
     int corruptions;
-- 
1.7.11.2