[PATCH 06/22] block: Add bdrv_recurse_can_replace()

From: Max Reitz <mreitz@redhat.com>
To: qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Alberto Garcia <berto@igalia.com>,
	qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>
Subject: [PATCH 06/22] block: Add bdrv_recurse_can_replace()
Date: Fri, 20 Sep 2019 17:27:48 +0200	[thread overview]
Message-ID: <20190920152804.12875-7-mreitz@redhat.com> (raw)
In-Reply-To: <20190920152804.12875-1-mreitz@redhat.com>

After a couple of follow-up patches, this function will replace
bdrv_recurse_is_first_non_filter() in check_to_replace_node().

bdrv_recurse_is_first_non_filter() is both not sufficiently specific for
check_to_replace_node() (it allows cases that should not be allowed,
like replacing child nodes of quorum with dissenting data that have more
parents than just quorum), and it is too restrictive (it is perfectly
fine to replace filters).

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 include/block/block_int.h | 10 ++++++++++
 block.c                   | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 5fd4f17d93..0be7d12f04 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -103,6 +103,13 @@ struct BlockDriver {
      */
     bool (*bdrv_recurse_is_first_non_filter)(BlockDriverState *bs,
                                              BlockDriverState *candidate);
+    /*
+     * Return true if @to_replace can be replaced by a BDS with the
+     * same data as @bs without it affecting @bs's behavior (that is,
+     * without it being visible to @bs's parents).
+     */
+    bool (*bdrv_recurse_can_replace)(BlockDriverState *bs,
+                                     BlockDriverState *to_replace);
 
     int (*bdrv_probe)(const uint8_t *buf, int buf_size, const char *filename);
     int (*bdrv_probe_device)(const char *filename);
@@ -1254,6 +1261,9 @@ void bdrv_format_default_perms(BlockDriverState *bs, BdrvChild *c,
                                uint64_t perm, uint64_t shared,
                                uint64_t *nperm, uint64_t *nshared);
 
+bool bdrv_recurse_can_replace(BlockDriverState *bs,
+                              BlockDriverState *to_replace);
+
 /*
  * Default implementation for drivers to pass bdrv_co_block_status() to
  * their file.
diff --git a/block.c b/block.c
index 7d99ca692c..a2deca4ac9 100644
--- a/block.c
+++ b/block.c
@@ -6206,6 +6206,44 @@ bool bdrv_recurse_is_first_non_filter(BlockDriverState *bs,
     return false;
 }
 
+/*
+ * This function checks whether the given @to_replace is allowed to be
+ * replaced by a node that always shows the same data as @bs.  This is
+ * used for example to verify whether the mirror job can replace
+ * @to_replace by the target mirrored from @bs.
+ * To be replaceable, @bs and @to_replace may either be guaranteed to
+ * always show the same data (because they are only connected through
+ * filters), or some driver may allow replacing one of its children
+ * because it can guarantee that this child's data is not visible at
+ * all (for example, for dissenting quorum children that have no other
+ * parents).
+ */
+bool bdrv_recurse_can_replace(BlockDriverState *bs,
+                              BlockDriverState *to_replace)
+{
+    if (!bs || !bs->drv) {
+        return false;
+    }
+
+    if (bs == to_replace) {
+        return true;
+    }
+
+    /* For filters, we can recurse on our own */
+    if (bs->drv->is_filter) {
+        BdrvChild *child = bs->file ?: bs->backing;
+        return bdrv_recurse_can_replace(child->bs, to_replace);
+    }
+
+    /* See what the driver can do */
+    if (bs->drv->bdrv_recurse_can_replace) {
+        return bs->drv->bdrv_recurse_can_replace(bs, to_replace);
+    }
+
+    /* Safe default */
+    return false;
+}
+
 BlockDriverState *check_to_replace_node(BlockDriverState *parent_bs,
                                         const char *node_name, Error **errp)
 {
-- 
2.21.0


