[23/29] block/export: Create BlockBackend in blk_exp_add()

diff --git a/include/block/nbd.h b/include/block/nbd.h
index a4dc1f9e54..5270b7eadd 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -332,10 +332,10 @@  typedef struct NBDClient NBDClient;
 
 int nbd_export_create(BlockExport *exp, BlockExportOptions *exp_args,
                       Error **errp);
-int nbd_export_new(BlockExport *blk_exp, BlockDriverState *bs,
+int nbd_export_new(BlockExport *blk_exp,
                    const char *name, const char *desc,
                    const char *bitmap, bool readonly, bool shared,
-                   bool writethrough, Error **errp);
+                   Error **errp);
 void nbd_export_set_on_eject_blk(BlockExport *exp, BlockBackend *blk);
 
 AioContext *nbd_export_aio_context(NBDExport *exp);
diff --git a/block/export/export.c b/block/export/export.c
index 40fc9d505f..52ec00dfcf 100644
--- a/block/export/export.c
+++ b/block/export/export.c
@@ -58,7 +58,10 @@  static const BlockExportDriver *blk_exp_find_driver(BlockExportType type)
 BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
 {
     const BlockExportDriver *drv;
-    BlockExport *exp;
+    BlockExport *exp = NULL;
+    BlockDriverState *bs;
+    BlockBackend *blk;
+    AioContext *ctx;
     int ret;
 
     if (!id_wellformed(export->id)) {
@@ -76,6 +79,33 @@  BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
         return NULL;
     }
 
+    bs = bdrv_lookup_bs(NULL, export->node_name, errp);
+    if (!bs) {
+        return NULL;
+    }
+
+    ctx = bdrv_get_aio_context(bs);
+    aio_context_acquire(ctx);
+
+    /*
+     * Block exports are used for non-shared storage migration. Make sure
+     * that BDRV_O_INACTIVE is cleared and the image is ready for write
+     * access since the export could be available before migration handover.
+     * ctx was acquired in the caller.
+     */
+    bdrv_invalidate_cache(bs, NULL);
+
+    blk = blk_new(ctx, BLK_PERM_CONSISTENT_READ, BLK_PERM_ALL);
+    ret = blk_insert_bs(blk, bs, errp);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    if (!export->has_writethrough) {
+        export->writethrough = false;
+    }
+    blk_set_enable_write_cache(blk, !export->writethrough);
+
     assert(drv->instance_size >= sizeof(BlockExport));
     exp = g_malloc0(drv->instance_size);
     *exp = (BlockExport) {
@@ -83,19 +113,30 @@  BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
         .refcount   = 1,
         .user_owned = true,
         .id         = g_strdup(export->id),
+        .ctx        = ctx,
+        .blk        = blk,
     };
 
     ret = drv->create(exp, export, errp);
     if (ret < 0) {
-        g_free(exp->id);
-        g_free(exp);
-        return NULL;
+        goto fail;
     }
 
     assert(exp->blk != NULL);
 
     QLIST_INSERT_HEAD(&block_exports, exp, next);
+
+    aio_context_release(ctx);
     return exp;
+
+fail:
+    blk_unref(blk);
+    aio_context_release(ctx);
+    if (exp) {
+        g_free(exp->id);
+        g_free(exp);
+    }
+    return NULL;
 }
 
 /* Callers must hold exp->ctx lock */
diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index 4a9a1be571..cdbbcdb958 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -177,9 +177,6 @@  int nbd_export_create(BlockExport *exp, BlockExportOptions *exp_args,
                       Error **errp)
 {
     BlockExportOptionsNbd *arg = &exp_args->u.nbd;
-    BlockDriverState *bs = NULL;
-    AioContext *aio_context;
-    int ret;
 
     assert(exp_args->type == BLOCK_EXPORT_TYPE_NBD);
 
@@ -207,38 +204,16 @@  int nbd_export_create(BlockExport *exp, BlockExportOptions *exp_args,
         return -EEXIST;
     }
 
-    bs = bdrv_lookup_bs(NULL, exp_args->node_name, errp);
-    if (!bs) {
-        return -ENOENT;
-    }
-
-    aio_context = bdrv_get_aio_context(bs);
-    aio_context_acquire(aio_context);
-
     if (!arg->has_writable) {
         arg->writable = false;
     }
-    if (bdrv_is_read_only(bs) && arg->writable) {
-        ret = -EINVAL;
+    if (blk_is_read_only(exp->blk) && arg->writable) {
         error_setg(errp, "Cannot export read-only node as writable");
-        goto out;
-    }
-
-    if (!exp_args->has_writethrough) {
-        exp_args->writethrough = false;
-    }
-
-    ret = nbd_export_new(exp, bs, arg->name, arg->description, arg->bitmap,
-                         !arg->writable, !arg->writable,
-                         exp_args->writethrough, errp);
-    if (ret < 0) {
-        goto out;
+        return -EINVAL;
     }
 
-    ret = 0;
- out:
-    aio_context_release(aio_context);
-    return ret;
+    return nbd_export_new(exp, arg->name, arg->description, arg->bitmap,
+                          !arg->writable, !arg->writable, errp);
 }
 
 void qmp_nbd_server_add(NbdServerAddOptions *arg, Error **errp)
diff --git a/nbd/server.c b/nbd/server.c
index f9af45c480..6c8532de23 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1507,56 +1507,42 @@  void nbd_export_set_on_eject_blk(BlockExport *exp, BlockBackend *blk)
     blk_add_remove_bs_notifier(blk, &nbd_exp->eject_notifier);
 }
 
-int nbd_export_new(BlockExport *blk_exp, BlockDriverState *bs,
+int nbd_export_new(BlockExport *blk_exp,
                    const char *name, const char *desc,
                    const char *bitmap, bool readonly, bool shared,
-                   bool writethrough, Error **errp)
+                   Error **errp)
 {
     NBDExport *exp = container_of(blk_exp, NBDExport, common);
-    AioContext *ctx;
-    BlockBackend *blk;
+    BlockBackend *blk = blk_exp->blk;
     int64_t size;
-    uint64_t perm;
+    uint64_t perm, shared_perm;
     int ret;
 
-    size = bdrv_getlength(bs);
+    size = blk_getlength(blk);
     if (size < 0) {
         error_setg_errno(errp, -size,
                          "Failed to determine the NBD export's length");
         return size;
     }
 
-    ctx = bdrv_get_aio_context(bs);
-    blk_exp->ctx = ctx;
-
-    /*
-     * NBD exports are used for non-shared storage migration.  Make sure
-     * that BDRV_O_INACTIVE is cleared and the image is ready for write
-     * access since the export could be available before migration handover.
-     * ctx was acquired in the caller.
-     */
     assert(name && strlen(name) <= NBD_MAX_STRING_SIZE);
 
-    bdrv_invalidate_cache(bs, NULL);
-
     /* Don't allow resize while the NBD server is running, otherwise we don't
      * care what happens with the node. */
-    perm = BLK_PERM_CONSISTENT_READ;
+    blk_get_perm(blk, &perm, &shared_perm);
+
     if (!readonly) {
         perm |= BLK_PERM_WRITE;
     }
-    blk = blk_new(ctx, perm,
-                  BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
-                  BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD);
-    ret = blk_insert_bs(blk, bs, errp);
+
+    ret = blk_set_perm(blk, perm, shared_perm & ~BLK_PERM_RESIZE, errp);
     if (ret < 0) {
-        goto fail;
+        return ret;
     }
-    blk_set_enable_write_cache(blk, !writethrough);
+
     blk_set_allow_aio_context_change(blk, true);
 
     QTAILQ_INIT(&exp->clients);
-    exp->common.blk = blk;
     exp->name = g_strdup(name);
     assert(!desc || strlen(desc) <= NBD_MAX_STRING_SIZE);
     exp->description = g_strdup(desc);
@@ -1574,6 +1560,7 @@  int nbd_export_new(BlockExport *blk_exp, BlockDriverState *bs,
     exp->size = QEMU_ALIGN_DOWN(size, BDRV_SECTOR_SIZE);
 
     if (bitmap) {
+        BlockDriverState *bs = blk_bs(blk);
         BdrvDirtyBitmap *bm = NULL;
 
         while (bs) {
@@ -1620,7 +1607,6 @@  int nbd_export_new(BlockExport *blk_exp, BlockDriverState *bs,
     return 0;
 
 fail:
-    blk_unref(blk);
     g_free(exp->name);
     g_free(exp->description);
     return ret;