[V6,5/8] block: Create authorizations mechanism for external snapshot and resize.

diff --git a/block.c b/block.c
index e1bc732..3e0994b 100644
--- a/block.c
+++ b/block.c
@@ -5088,21 +5088,68 @@  int bdrv_amend_options(BlockDriverState *bs, QEMUOptionParameter *options)
     return bs->drv->bdrv_amend_options(bs, options);
 }
 
-ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs)
+/* Used to recurse on single child block filters.
+ * Single child block filter will store their child in bs->file.
+ */
+bool bdrv_generic_is_first_non_filter(BlockDriverState *bs,
+                                      BlockDriverState *candidate)
 {
-    if (bs->drv->bdrv_check_ext_snapshot) {
-        return bs->drv->bdrv_check_ext_snapshot(bs);
+    if (!bs->drv) {
+        return false;
+    }
+
+    if (!bs->drv->authorizations[BS_IS_A_FILTER]) {
+        if (bs == candidate) {
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    if (!bs->drv->authorizations[BS_FILTER_PASS_DOWN]) {
+        return false;
     }
 
-    if (bs->file && bs->file->drv && bs->file->drv->bdrv_check_ext_snapshot) {
-        return bs->file->drv->bdrv_check_ext_snapshot(bs);
+    if (!bs->file) {
+        return false;
+    }
+
+    return bdrv_recurse_is_first_non_filter(bs->file, candidate);
+}
+
+bool bdrv_recurse_is_first_non_filter(BlockDriverState *bs,
+                                      BlockDriverState *candidate)
+{
+    if (bs->drv && bs->drv->bdrv_recurse_is_first_non_filter) {
+        return bs->drv->bdrv_recurse_is_first_non_filter(bs, candidate);
     }
 
-    /* external snapshots are allowed by default */
-    return EXT_SNAPSHOT_ALLOWED;
+    return bdrv_generic_is_first_non_filter(bs, candidate);
 }
 
-ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs)
+/* This function checks if the candidate is the first non filter bs down it's
+ * bs chain. Since we don't have pointers to parents it explore all bs chains
+ * from the top. Some filters can choose not to pass down the recursion.
+ */
+bool bdrv_is_first_non_filter(BlockDriverState *candidate)
 {
-    return EXT_SNAPSHOT_FORBIDDEN;
+    BlockDriverState *bs;
+
+    /* walk down the bs forest recursively */
+    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
+        bool perm;
+
+        if (!bs->file) {
+            continue;
+        }
+
+        perm = bdrv_recurse_is_first_non_filter(bs->file, candidate);
+
+        /* candidate is the first non filter */
+        if (perm) {
+            return true;
+        }
+    }
+
+    return false;
 }
diff --git a/block/blkverify.c b/block/blkverify.c
index a2e8f5f..cfcbcf4 100644
--- a/block/blkverify.c
+++ b/block/blkverify.c
@@ -404,7 +404,7 @@  static BlockDriver bdrv_blkverify = {
     .bdrv_aio_writev        = blkverify_aio_writev,
     .bdrv_aio_flush         = blkverify_aio_flush,
 
-    .bdrv_check_ext_snapshot = bdrv_check_ext_snapshot_forbidden,
+    .authorizations         = { true, false },
 };
 
 static void bdrv_blkverify_init(void)
diff --git a/blockdev.c b/blockdev.c
index 3c27911..8f95c7f 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1250,7 +1250,7 @@  static void external_snapshot_prepare(BlkTransactionState *common,
         }
     }
 
-    if (bdrv_check_ext_snapshot(state->old_bs) != EXT_SNAPSHOT_ALLOWED) {
+    if (!bdrv_is_first_non_filter(state->old_bs)) {
         error_set(errp, QERR_FEATURE_DISABLED, "snapshot");
         return;
     }
diff --git a/include/block/block.h b/include/block/block.h
index b4a77e6..59d9f12 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -287,16 +287,16 @@  int bdrv_amend_options(BlockDriverState *bs_new, QEMUOptionParameter *options);
 /* external snapshots */
 
 typedef enum {
-    EXT_SNAPSHOT_ALLOWED,
-    EXT_SNAPSHOT_FORBIDDEN,
-} ExtSnapshotPerm;
-
-/* return EXT_SNAPSHOT_ALLOWED if external snapshot is allowed
- * return EXT_SNAPSHOT_FORBIDDEN if external snapshot is forbidden
- */
-ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs);
-/* helper used to forbid external snapshots like in blkverify */
-ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs);
+    BS_IS_A_FILTER,
+    BS_FILTER_PASS_DOWN,
+    BS_AUTHORIZATION_COUNT,
+} BsAuthorization;
+
+bool bdrv_generic_is_first_non_filter(BlockDriverState *bs,
+                                      BlockDriverState *candidate);
+bool bdrv_recurse_is_first_non_filter(BlockDriverState *bs,
+                                      BlockDriverState *candidate);
+bool bdrv_is_first_non_filter(BlockDriverState *candidate);
 
 /* async block I/O */
 typedef void BlockDriverDirtyHandler(BlockDriverState *bs, int64_t sector,
diff --git a/include/block/block_int.h b/include/block/block_int.h
index f3f518c..611a955 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -69,10 +69,16 @@  struct BlockDriver {
     const char *format_name;
     int instance_size;
 
-    /* if not defined external snapshots are allowed
-     * future block filters will query their children to build the response
+    /* this table of boolean contains authorizations for the block operations */
+    bool authorizations[BS_AUTHORIZATION_COUNT];
+    /* for snapshots complex block filter like Quorum can implement the
+     * following recursive callback instead of BS_IS_A_FILTER.
+     * It's purpose is to recurse on the filter children while calling
+     * bdrv_recurse_is_first_non_filter on them.
+     * For a sample implementation look in the future Quorum block filter.
      */
-    ExtSnapshotPerm (*bdrv_check_ext_snapshot)(BlockDriverState *bs);
+    bool (*bdrv_recurse_is_first_non_filter)(BlockDriverState *bs,
+                                             BlockDriverState *candidate);
 
     int (*bdrv_probe)(const uint8_t *buf, int buf_size, const char *filename);
     int (*bdrv_probe_device)(const char *filename);