[PATCH v3 09/33] block: Add generic bdrv_inherited_options()

[Max Reitz]

After the series this patch belongs to, we want to have a common
BdrvChildClass that encompasses all of child_file, child_format, and
child_backing.  Such a single class needs a single .inherit_options()
implementation, and this patch introduces it.

The next patch will show how the existing implementations can fall back
to it just by passing appropriate BdrvChildRole and parent_is_format
values.

Signed-off-by: Max Reitz <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
---
 block.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)

diff --git a/block.c b/block.c
index c33f0e9b42..9179b9b604 100644
--- a/block.c
+++ b/block.c
@@ -998,6 +998,90 @@ static void bdrv_temp_snapshot_options(int *child_flags, 
QDict *child_options,
     *child_flags &= ~BDRV_O_NATIVE_AIO;
 }
 
+/*
+ * Returns the options and flags that a generic child of a BDS should
+ * get, based on the given options and flags for the parent BDS.
+ */
+static void __attribute__((unused))
+    bdrv_inherited_options(BdrvChildRole role, bool parent_is_format,
+                           int *child_flags, QDict *child_options,
+                           int parent_flags, QDict *parent_options)
+{
+    int flags = parent_flags;
+
+    /*
+     * First, decide whether to set, clear, or leave BDRV_O_PROTOCOL.
+     * Generally, the question to answer is: Should this child be
+     * format-probed by default?
+     */
+
+    /*
+     * Pure and non-filtered data children of non-format nodes should
+     * be probed by default (even when the node itself has BDRV_O_PROTOCOL
+     * set).  This only affects a very limited set of drivers (namely
+     * quorum and blkverify when this comment was written).
+     * Force-clear BDRV_O_PROTOCOL then.
+     */
+    if (!parent_is_format &&
+        (role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
+                 BDRV_CHILD_FILTERED)) ==
+            BDRV_CHILD_DATA)
+    {
+        flags &= ~BDRV_O_PROTOCOL;
+    }
+
+    /*
+     * All children of format nodes (except for COW children) and all
+     * metadata children in general should never be format-probed.
+     * Force-set BDRV_O_PROTOCOL then.
+     */
+    if ((parent_is_format && !(role & BDRV_CHILD_COW)) ||
+        (role & BDRV_CHILD_METADATA))
+    {
+        flags |= BDRV_O_PROTOCOL;
+    }
+
+    /*
+     * If the cache mode isn't explicitly set, inherit direct and no-flush from
+     * the parent.
+     */
+    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_DIRECT);
+    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_NO_FLUSH);
+    qdict_copy_default(child_options, parent_options, BDRV_OPT_FORCE_SHARE);
+
+    if (role & BDRV_CHILD_COW) {
+        /* backing files are always opened read-only */
+        qdict_set_default_str(child_options, BDRV_OPT_READ_ONLY, "on");
+        qdict_set_default_str(child_options, BDRV_OPT_AUTO_READ_ONLY, "off");
+    } else {
+        /* Inherit the read-only option from the parent if it's not set */
+        qdict_copy_default(child_options, parent_options, BDRV_OPT_READ_ONLY);
+        qdict_copy_default(child_options, parent_options,
+                           BDRV_OPT_AUTO_READ_ONLY);
+    }
+
+    if (parent_is_format && !(role & BDRV_CHILD_COW)) {
+        /*
+         * Our format drivers take care to send flushes and respect
+         * unmap policy, so we can default to enable both on lower
+         * layers regardless of the corresponding parent options.
+         */
+        qdict_set_default_str(child_options, BDRV_OPT_DISCARD, "unmap");
+    }
+
+    /* Clear flags that only apply to the top layer */
+    flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ);
+
+    if (role & BDRV_CHILD_METADATA) {
+        flags &= ~BDRV_O_NO_IO;
+    }
+    if (role & BDRV_CHILD_COW) {
+        flags &= ~BDRV_O_TEMPORARY;
+    }
+
+    *child_flags = flags;
+}
+
 /*
  * Returns the options and flags that bs->file should get if a protocol driver
  * is expected, based on the given options and flags for the parent BDS
-- 
2.24.1