[Qemu-devel] [PATCH V4 6/7] block: Create authorizations mechanism for external snapshots.

["Benoît Canet" <benoit@irqsave.net>]

Signed-off-by: Benoit Canet <benoit@irqsave.net>
---
 block.c                   | 68 +++++++++++++++++++++++++++++++++++++++++------
 block/blkverify.c         |  2 +-
 include/block/block.h     | 18 ++++++++++---
 include/block/block_int.h | 12 ++++++---
 4 files changed, 84 insertions(+), 16 deletions(-)

diff --git a/block.c b/block.c
index e53f24f..cb7ac88 100644
--- a/block.c
+++ b/block.c
@@ -4962,21 +4962,73 @@ int bdrv_amend_options(BlockDriverState *bs, QEMUOptionParameter *options)
     return bs->drv->bdrv_amend_options(bs, options);
 }
 
-ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs)
+/* will be used to recurse on single child block filter until first format
+ * (single child block filter will store their child in bs->file)
+ */
+ExtSnapshotPerm bdrv_generic_check_ext_snapshot(BlockDriverState *bs,
+                                                BlockDriverState *candidate)
 {
-    if (bs->drv->bdrv_check_ext_snapshot) {
-        return bs->drv->bdrv_check_ext_snapshot(bs);
+    if (!bs->drv) {
+        return EXT_SNAPSHOT_DO_NOT_CARE;
+    }
+
+    /* This double negation allow to implement the behavior without modifying
+     * all the BlockDrivers.
+     */
+    if (!bs->drv->authorizations[BS_CANT_SNAPSHOT]) {
+        if (bs == candidate) {
+            return EXT_SNAPSHOT_ALLOWED;
+        } else {
+            return EXT_SNAPSHOT_FORBIDDEN;
+        }
     }
 
-    if (bs->file && bs->file->drv && bs->file->drv->bdrv_check_ext_snapshot) {
-        return bs->file->drv->bdrv_check_ext_snapshot(bs);
+    if (!bs->drv->authorizations[BS_FILTER_PASS_DOWN]) {
+        return EXT_SNAPSHOT_DO_NOT_CARE;
+    }
+
+    if (!bs->file) {
+        return EXT_SNAPSHOT_DO_NOT_CARE;
+    }
+
+    return bdrv_recurse_check_ext_snapshot(bs->file, candidate);
+}
+
+ExtSnapshotPerm bdrv_recurse_check_ext_snapshot(BlockDriverState *bs,
+                                                BlockDriverState *candidate)
+{
+    if (bs->drv && bs->drv->bdrv_recurse_check_ext_snapshot) {
+        return bs->drv->bdrv_recurse_check_ext_snapshot(bs, candidate);
     }
 
-    /* external snapshots are allowed by default */
-    return EXT_SNAPSHOT_ALLOWED;
+    return bdrv_generic_check_ext_snapshot(bs, candidate);
 }
 
-ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs)
+/* This function check if the candidate bs has snapshots authorized by going
+ * down the forest of bs, skipping filters and stopping on the the first bses
+ * authorizing snapshots. If one of these bses is the the candidate and has
+ * !authorizations[BS_CANT_SNAPSHOT] the snapshot is authorized.
+ */
+ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *candidate)
 {
+    BlockDriverState *bs;
+
+    /* walk down the bs forest recursively */
+    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
+        ExtSnapshotPerm perm;
+
+        if (!bs->file) {
+            continue;
+        }
+
+        perm = bdrv_recurse_check_ext_snapshot(bs->file, candidate);
+
+        /* allowed in the right subtree -> stop here */
+        if (perm == EXT_SNAPSHOT_ALLOWED) {
+            return EXT_SNAPSHOT_ALLOWED;
+        }
+    }
+
+    /* external snapshots are forbidden by default */
     return EXT_SNAPSHOT_FORBIDDEN;
 }
diff --git a/block/blkverify.c b/block/blkverify.c
index a7c9da2..105d4ac 100644
--- a/block/blkverify.c
+++ b/block/blkverify.c
@@ -417,7 +417,7 @@ 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/include/block/block.h b/include/block/block.h
index 49dab83..f98f902 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -280,16 +280,26 @@ int bdrv_amend_options(BlockDriverState *bs_new, QEMUOptionParameter *options);
 /* external snapshots */
 
 typedef enum {
-    EXT_SNAPSHOT_ALLOWED,
     EXT_SNAPSHOT_FORBIDDEN,
+    EXT_SNAPSHOT_DO_NOT_CARE,
+    EXT_SNAPSHOT_ALLOWED,
 } ExtSnapshotPerm;
 
+typedef enum {
+    BS_CANT_SNAPSHOT,
+    BS_FILTER_PASS_DOWN,
+    BS_AUTHORIZATION_COUNT,
+} BsAuthorization;
+
 /* return EXT_SNAPSHOT_ALLOWED if external snapshot is allowed
  * return EXT_SNAPSHOT_FORBIDDEN if external snapshot is forbidden
+ * return EXT_SNAPSHOT_FORBIDDEN if the candidate bs is not in this bs tree
  */
-ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs);
-/* helper used to forbid external snapshots like in blkverify */
-ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs);
+ExtSnapshotPerm bdrv_generic_check_ext_snapshot(BlockDriverState *bs,
+                                                BlockDriverState *candidate);
+ExtSnapshotPerm bdrv_recurse_check_ext_snapshot(BlockDriverState *bs,
+                                                BlockDriverState *candidate);
+ExtSnapshotPerm bdrv_check_ext_snapshot(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 9e789d2..edd85a9 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_CANT_SNAPSHOT.
+     * It's purpose is to recurse on the filter children while calling
+     * bdrv_recurse_check_ext_snapshot on them.
+     * For a sample implentation look in the future Quorum block filter.
      */
-    ExtSnapshotPerm (*bdrv_check_ext_snapshot)(BlockDriverState *bs);
+    ExtSnapshotPerm (*bdrv_recurse_check_ext_snapshot)(BlockDriverState *bs,
+                                                   BlockDriverState *candidate);
 
     int (*bdrv_probe)(const uint8_t *buf, int buf_size, const char *filename);
     int (*bdrv_probe_device)(const char *filename);
-- 
1.8.3.2