[RFC PATCH 4/8] blockjob: implement .change_aio_ctx in child_job

[Emanuele Giuseppe Esposito]

child_job_change_aio_ctx() is very similar to
child_job_can_set_aio_ctx(), but it implements a new transaction
so that if all check pass, the new transaction's .commit()
will take care of changin the BlockJob AioContext.
child_job_set_aio_ctx_commit() is similar to child_job_set_aio_ctx(),
but it doesn't need to invoke the recursion, as this is already
taken care by child_job_change_aio_ctx().

Note: bdrv_child_try_change_aio_context() is not called by
anyone at this point.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 blockjob.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/blockjob.c b/blockjob.c
index bb82e8d04c..9e26b06ed4 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -124,6 +124,50 @@ static void child_job_drained_end(BdrvChild *c, int 
*drained_end_counter)
     }
 }
 
+typedef struct BdrvStateChildJobContext {
+    AioContext *new_ctx;
+    BlockJob *job;
+} BdrvStateChildJobContext;
+
+static void child_job_set_aio_ctx_commit(void *opaque)
+{
+    BdrvStateChildJobContext *s = opaque;
+    BlockJob *job = s->job;
+
+    job_set_aio_context(&job->job, s->new_ctx);
+}
+
+static TransactionActionDrv change_child_job_context = {
+    .commit = child_job_set_aio_ctx_commit,
+    .clean = g_free,
+};
+
+static bool child_job_change_aio_ctx(BdrvChild *c, AioContext *ctx,
+                                     GSList **visited, Transaction *tran,
+                                     Error **errp)
+{
+    BlockJob *job = c->opaque;
+    BdrvStateChildJobContext *s;
+    GSList *l;
+
+    for (l = job->nodes; l; l = l->next) {
+        BdrvChild *sibling = l->data;
+        if (!bdrv_child_change_aio_context(sibling, ctx, visited,
+                                           tran, errp)) {
+            return false;
+        }
+    }
+
+    s = g_new(BdrvStateChildJobContext, 1);
+    *s = (BdrvStateChildJobContext) {
+        .new_ctx = ctx,
+        .job = job,
+    };
+
+    tran_add_tail(tran, &change_child_job_context, s);
+    return true;
+}
+
 static bool child_job_can_set_aio_ctx(BdrvChild *c, AioContext *ctx,
                                       GSList **ignore, Error **errp)
 {
@@ -172,6 +216,7 @@ static const BdrvChildClass child_job = {
     .drained_end        = child_job_drained_end,
     .can_set_aio_ctx    = child_job_can_set_aio_ctx,
     .set_aio_ctx        = child_job_set_aio_ctx,
+    .change_aio_ctx     = child_job_change_aio_ctx,
     .stay_at_node       = true,
     .get_parent_aio_context = child_job_get_parent_aio_context,
 };
-- 
2.31.1