nbd: Allow bitmap export during QMP nbd-server-add

With the experimental x-nbd-server-add-bitmap command, there was
a window of time where an NBD client could see the export but not
the associated dirty bitmap, which can cause a client that planned
on using the dirty bitmap to be forced to treat the entire image
as dirty as a safety fallback.  Furthermore, if the QMP client
successfully exports a disk but then fails to add the bitmap, it
has to take on the burden of removing the export.  Since we don't
allow changing the exposed dirty bitmap (whether to a different
bitmap, or removing advertisement of the bitmap), it is nicer to
make the bitmap tied to the export at the time the export is
created, with automatic failure to export if the bitmap is not
available.

The experimental command included an optional 'bitmap-export-name'
field for remapping the name exposed over NBD to be different from
the bitmap name stored on disk.  However, my libvirt demo code
for implementing differential backups on top of persistent bitmaps
did not need to take advantage of that feature (it is instead
possible to create a new temporary bitmap with the desired name,
use block-dirty-bitmap-merge to merge one or more persistent
bitmaps into the temporary, then associate the temporary with the
NBD export, if control is needed over the exported bitmap name).
Hence, I'm not copying that part of the experiment over to the
stable addition. For more details on the libvirt demo, see
https://www.redhat.com/archives/libvir-list/2018-October/msg01254.html,
https://kvmforum2018.sched.com/event/FzuB/facilitating-incremental-backup-eric-blake-red-hat

This patch focuses on the user interface, and reduces (but does
not completely eliminate) the window where an NBD client can see
the export but not the dirty bitmap, with less work to clean up
after errors.  Later patches will add further cleanups now that
this interface is declared stable via a single QMP command,
including removing the race window.

Update test 223 to use the new interface.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190111194720.15671-6-eblake@redhat.com>

blockdev-nbd.c

@@ -140,7 +140,8 @@ void qmp_nbd_server_start(SocketAddressLegacy *addr,
 }
 
 void qmp_nbd_server_add(const char *device, bool has_name, const char *name,
-                        bool has_writable, bool writable, Error **errp)
+                        bool has_writable, bool writable,
+                        bool has_bitmap, const char *bitmap, Error **errp)
 {
     BlockDriverState *bs = NULL;
     BlockBackend *on_eject_blk;
@@ -185,6 +186,15 @@ void qmp_nbd_server_add(const char *device, bool has_name, const char *name,
      * our only way of accessing it is through nbd_export_find(), so we can drop
      * the strong reference that is @exp. */
     nbd_export_put(exp);
+
+    if (has_bitmap) {
+        Error *err = NULL;
+        nbd_export_bitmap(exp, bitmap, bitmap, &err);
+        if (err) {
+            error_propagate(errp, err);
+            nbd_export_remove(exp, NBD_SERVER_REMOVE_MODE_HARD, NULL);
+        }
+    }
 }
 
 void qmp_nbd_server_remove(const char *name,

hmp.c

@@ -2326,7 +2326,7 @@ void hmp_nbd_server_start(Monitor *mon, const QDict *qdict)
         }
 
         qmp_nbd_server_add(info->value->device, false, NULL,
-                           true, writable, &local_err);
+                           true, writable, false, NULL, &local_err);
 
         if (local_err != NULL) {
             qmp_nbd_server_stop(NULL);
@@ -2347,7 +2347,8 @@ void hmp_nbd_server_add(Monitor *mon, const QDict *qdict)
     bool writable = qdict_get_try_bool(qdict, "writable", false);
     Error *local_err = NULL;
 
-    qmp_nbd_server_add(device, !!name, name, true, writable, &local_err);
+    qmp_nbd_server_add(device, !!name, name, true, writable,
+                       false, NULL, &local_err);
     hmp_handle_error(mon, &local_err);
 }
 

qapi/block.json

@@ -246,6 +246,10 @@
 #
 # @writable: Whether clients should be able to write to the device via the
 #     NBD connection (default false).
+
+# @bitmap: Also export the dirty bitmap reachable from @device, so the
+#          NBD client can use NBD_OPT_SET_META_CONTEXT with
+#          "qemu:dirty-bitmap:NAME" to inspect the bitmap. (since 4.0)
 #
 # Returns: error if the server is not running, or export with the same name
 #          already exists.
@@ -253,7 +257,8 @@
 # Since: 1.3.0
 ##
 { 'command': 'nbd-server-add',
-  'data': {'device': 'str', '*name': 'str', '*writable': 'bool'} }
+  'data': {'device': 'str', '*name': 'str', '*writable': 'bool',
+           '*bitmap': 'str' } }
 
 ##
 # @NbdServerRemoveMode:

tests/qemu-iotests/223

@@ -126,23 +126,20 @@ _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-start",
   "arguments":{"addr":{"type":"unix",
     "data":{"path":"'"$TEST_DIR/nbd"1'"}}}}' "error" # Attempt second server
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
-  "arguments":{"device":"n"}}' "return"
+  "arguments":{"device":"n", "bitmap":"b"}}' "return"
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
   "arguments":{"device":"nosuch"}}' "error" # Attempt to export missing node
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
   "arguments":{"device":"n"}}' "error" # Attempt to export same name twice
-_send_qemu_cmd $QEMU_HANDLE '{"execute":"x-nbd-server-add-bitmap",
-  "arguments":{"name":"n", "bitmap":"b"}}' "return"
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
-  "arguments":{"device":"n", "name":"n2"}}' "return"
-_send_qemu_cmd $QEMU_HANDLE '{"execute":"x-nbd-server-add-bitmap",
-  "arguments":{"name":"n2", "bitmap":"b2"}}' "error" # Enabled vs. read-only
-_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-remove",
-  "arguments":{"name":"n2"}}' "return"
+  "arguments":{"device":"n", "name":"n2",
+  "bitmap":"b2"}}' "error" # enabled vs. read-only
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
-  "arguments":{"device":"n", "name":"n2", "writable":true}}' "return"
-_send_qemu_cmd $QEMU_HANDLE '{"execute":"x-nbd-server-add-bitmap",
-  "arguments":{"name":"n2", "bitmap":"b2"}}' "return"
+  "arguments":{"device":"n", "name":"n2",
+  "bitmap":"b3"}}' "error" # Missing bitmap
+_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
+  "arguments":{"device":"n", "name":"n2", "writable":true,
+  "bitmap":"b2"}}' "return"
 
 echo
 echo "=== Contrast normal status to large granularity dirty-bitmap ==="

tests/qemu-iotests/223.out

@@ -33,11 +33,8 @@ wrote 2097152/2097152 bytes at offset 2097152
 {"return": {}}
 {"error": {"class": "GenericError", "desc": "Cannot find device=nosuch nor node_name=nosuch"}}
 {"error": {"class": "GenericError", "desc": "NBD server already has export named 'n'"}}
-{"return": {}}
-{"return": {}}
 {"error": {"class": "GenericError", "desc": "Enabled bitmap 'b2' incompatible with readonly export"}}
-{"return": {}}
-{"return": {}}
+{"error": {"class": "GenericError", "desc": "Bitmap 'b3' is not found"}}
 {"return": {}}
 
 === Contrast normal status to large granularity dirty-bitmap ===