block/mirror: Use source as a BdrvChild

With this, the mirror_top_bs is no longer just a technically required
node in the BDS graph but actually represents the block job operation.

Also, drop MirrorBlockJob.source, as we can reach it through
mirror_top_bs->backing.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20180613181823.13618-6-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/mirror.c

@@ -41,7 +41,6 @@ typedef struct MirrorBlockJob {
     BlockJob common;
     BlockBackend *target;
     BlockDriverState *mirror_top_bs;
-    BlockDriverState *source;
     BlockDriverState *base;
 
     /* The name of the graph node to replace */
@@ -299,7 +298,6 @@ static void coroutine_fn mirror_co_read(void *opaque)
 {
     MirrorOp *op = opaque;
     MirrorBlockJob *s = op->s;
-    BlockBackend *source = s->common.blk;
     int nb_chunks;
     uint64_t ret;
     uint64_t max_bytes;
@@ -349,7 +347,8 @@ static void coroutine_fn mirror_co_read(void *opaque)
     s->bytes_in_flight += op->bytes;
     trace_mirror_one_iteration(s, op->offset, op->bytes);
 
-    ret = blk_co_preadv(source, op->offset, op->bytes, &op->qiov, 0);
+    ret = bdrv_co_preadv(s->mirror_top_bs->backing, op->offset, op->bytes,
+                         &op->qiov, 0);
     mirror_read_complete(op, ret);
 }
 
@@ -427,7 +426,7 @@ static unsigned mirror_perform(MirrorBlockJob *s, int64_t offset,
 
 static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
 {
-    BlockDriverState *source = s->source;
+    BlockDriverState *source = s->mirror_top_bs->backing->bs;
     MirrorOp *pseudo_op;
     int64_t offset;
     uint64_t delay_ns = 0, ret = 0;
@@ -601,7 +600,7 @@ static void mirror_exit(Job *job, void *opaque)
     BlockJob *bjob = &s->common;
     MirrorExitData *data = opaque;
     AioContext *replace_aio_context = NULL;
-    BlockDriverState *src = s->source;
+    BlockDriverState *src = s->mirror_top_bs->backing->bs;
     BlockDriverState *target_bs = blk_bs(s->target);
     BlockDriverState *mirror_top_bs = s->mirror_top_bs;
     Error *local_err = NULL;
@@ -716,7 +715,7 @@ static int coroutine_fn mirror_dirty_init(MirrorBlockJob *s)
 {
     int64_t offset;
     BlockDriverState *base = s->base;
-    BlockDriverState *bs = s->source;
+    BlockDriverState *bs = s->mirror_top_bs->backing->bs;
     BlockDriverState *target_bs = blk_bs(s->target);
     int ret;
     int64_t count;
@@ -798,7 +797,7 @@ static void coroutine_fn mirror_run(void *opaque)
 {
     MirrorBlockJob *s = opaque;
     MirrorExitData *data;
-    BlockDriverState *bs = s->source;
+    BlockDriverState *bs = s->mirror_top_bs->backing->bs;
     BlockDriverState *target_bs = blk_bs(s->target);
     bool need_drain = true;
     int64_t length;
@@ -1293,7 +1292,6 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
     /* The block job now has a reference to this node */
     bdrv_unref(mirror_top_bs);
 
-    s->source = bs;
     s->mirror_top_bs = mirror_top_bs;
 
     /* No resize for the target either; while the mirror is still running, a