[v3,12/15] copy-on-read: add filter append/drop functions

diff --git a/block/copy-on-read.c b/block/copy-on-read.c
index 4030d56..31e1619 100644
--- a/block/copy-on-read.c
+++ b/block/copy-on-read.c
@@ -24,8 +24,15 @@ 
 #include "qemu/cutils.h"
 #include "block/block_int.h"
 #include "qemu/module.h"
+#include "qapi/error.h"
+#include "qapi/qmp/qdict.h"
+#include "block/copy-on-read.h"
 
 
+typedef struct BDRVStateCOR {
+    bool active;
+} BDRVStateCOR;
+
 static int cor_open(BlockDriverState *bs, QDict *options, int flags,
                     Error **errp)
 {
@@ -57,6 +64,17 @@  static void cor_child_perm(BlockDriverState *bs, BdrvChild *c,
                            uint64_t perm, uint64_t shared,
                            uint64_t *nperm, uint64_t *nshared)
 {
+    BDRVStateCOR *s = bs->opaque;
+
+    if (!s->active) {
+        /*
+         * While the filter is being removed
+         */
+        *nperm = 0;
+        *nshared = BLK_PERM_ALL;
+        return;
+    }
+
     *nperm = perm & PERM_PASSTHROUGH;
     *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
 
@@ -150,6 +168,7 @@  static int cor_change_backing_file(BlockDriverState *bs,
 
 static BlockDriver bdrv_copy_on_read = {
     .format_name                        = "copy-on-read",
+    .instance_size                      = sizeof(BDRVStateCOR),
 
     .bdrv_open                          = cor_open,
     .bdrv_child_perm                    = cor_child_perm,
@@ -178,4 +197,83 @@  static void bdrv_copy_on_read_init(void)
     bdrv_register(&bdrv_copy_on_read);
 }
 
+
+static BlockDriverState *create_filter_node(BlockDriverState *bs,
+                                            const char *filter_node_name,
+                                            Error **errp)
+{
+    QDict *opts = qdict_new();
+
+    qdict_put_str(opts, "driver", "copy-on-read");
+    qdict_put_str(opts, "file", bdrv_get_node_name(bs));
+    if (filter_node_name) {
+        qdict_put_str(opts, "node-name", filter_node_name);
+    }
+
+    return bdrv_open(NULL, NULL, opts, BDRV_O_RDWR, errp);
+}
+
+
+BlockDriverState *bdrv_cor_filter_append(BlockDriverState *bs,
+                                         const char *filter_node_name,
+                                         Error **errp)
+{
+    BlockDriverState *cor_filter_bs;
+    BDRVStateCOR *state;
+    Error *local_err = NULL;
+
+    cor_filter_bs = create_filter_node(bs, filter_node_name, errp);
+    if (cor_filter_bs == NULL) {
+        error_prepend(errp, "Could not create filter node: ");
+        return NULL;
+    }
+
+    if (!filter_node_name) {
+        cor_filter_bs->implicit = true;
+    }
+
+    bdrv_drained_begin(bs);
+    bdrv_replace_node(bs, cor_filter_bs, &local_err);
+    bdrv_drained_end(bs);
+
+    if (local_err) {
+        bdrv_unref(cor_filter_bs);
+        error_propagate(errp, local_err);
+        return NULL;
+    }
+
+    state = cor_filter_bs->opaque;
+    state->active = true;
+
+    return cor_filter_bs;
+}
+
+
+void bdrv_cor_filter_drop(BlockDriverState *cor_filter_bs)
+{
+    BdrvChild *child;
+    BlockDriverState *bs;
+    BDRVStateCOR *s = cor_filter_bs->opaque;
+
+    child = bdrv_filtered_rw_child(cor_filter_bs);
+    if (!child) {
+        return;
+    }
+    bs = child->bs;
+
+    /* Retain the BDS until we complete the graph change. */
+    bdrv_ref(bs);
+    /* Hold a guest back from writing while permissions are being reset. */
+    bdrv_drained_begin(bs);
+    /* Drop permissions before the graph change. */
+    s->active = false;
+    bdrv_child_refresh_perms(cor_filter_bs, child, &error_abort);
+    bdrv_replace_node(cor_filter_bs, bs, &error_abort);
+
+    bdrv_drained_end(bs);
+    bdrv_unref(bs);
+    bdrv_unref(cor_filter_bs);
+}
+
+
 block_init(bdrv_copy_on_read_init);