qapi: add nbd-server-remove

Add command for removing an export. It is needed for cases when we
don't want to keep the export after the operation on it was completed.
The other example is a temporary node, created with blockdev-add.
If we want to delete it we should firstly remove any corresponding
NBD export.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180119135719.24745-3-vsementsov@virtuozzo.com>
[eblake: drop dead nb_clients code]
Signed-off-by: NEric Blake <eblake@redhat.com>

blockdev-nbd.c

@@ -189,6 +189,30 @@ void qmp_nbd_server_add(const char *device, bool has_name, const char *name,
     nbd_export_put(exp);
 }
 
+void qmp_nbd_server_remove(const char *name,
+                           bool has_mode, NbdServerRemoveMode mode,
+                           Error **errp)
+{
+    NBDExport *exp;
+
+    if (!nbd_server) {
+        error_setg(errp, "NBD server not running");
+        return;
+    }
+
+    exp = nbd_export_find(name);
+    if (exp == NULL) {
+        error_setg(errp, "Export '%s' is not found", name);
+        return;
+    }
+
+    if (!has_mode) {
+        mode = NBD_SERVER_REMOVE_MODE_SAFE;
+    }
+
+    nbd_export_remove(exp, mode, errp);
+}
+
 void qmp_nbd_server_stop(Error **errp)
 {
     nbd_export_close_all();

include/block/nbd.h

@@ -261,6 +261,7 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset, off_t size,
                           bool writethrough, BlockBackend *on_eject_blk,
                           Error **errp);
 void nbd_export_close(NBDExport *exp);
+void nbd_export_remove(NBDExport *exp, NbdServerRemoveMode mode, Error **errp);
 void nbd_export_get(NBDExport *exp);
 void nbd_export_put(NBDExport *exp);
 

nbd/server.c

@@ -1177,6 +1177,19 @@ void nbd_export_close(NBDExport *exp)
     nbd_export_put(exp);
 }
 
+void nbd_export_remove(NBDExport *exp, NbdServerRemoveMode mode, Error **errp)
+{
+    if (mode == NBD_SERVER_REMOVE_MODE_HARD || QTAILQ_EMPTY(&exp->clients)) {
+        nbd_export_close(exp);
+        return;
+    }
+
+    assert(mode == NBD_SERVER_REMOVE_MODE_SAFE);
+
+    error_setg(errp, "export '%s' still in use", exp->name);
+    error_append_hint(errp, "Use mode='hard' to force client disconnect\n");
+}
+
 void nbd_export_get(NBDExport *exp)
 {
     assert(exp->refcount > 0);

qapi/block.json

@@ -227,6 +227,47 @@
 { 'command': 'nbd-server-add',
   'data': {'device': 'str', '*name': 'str', '*writable': 'bool'} }
 
+##
+# @NbdServerRemoveMode:
+#
+# Mode for removing an NBD export.
+#
+# @safe: Remove export if there are no existing connections, fail otherwise.
+#
+# @hard: Drop all connections immediately and remove export.
+#
+# Potential additional modes to be added in the future:
+#
+# hide: Just hide export from new clients, leave existing connections as is.
+#       Remove export after all clients are disconnected.
+#
+# soft: Hide export from new clients, answer with ESHUTDOWN for all further
+#       requests from existing clients.
+#
+# Since: 2.12
+##
+{'enum': 'NbdServerRemoveMode', 'data': ['safe', 'hard']}
+
+##
+# @nbd-server-remove:
+#
+# Remove NBD export by name.
+#
+# @name: Export name.
+#
+# @mode: Mode of command operation. See @NbdServerRemoveMode description.
+#        Default is 'safe'.
+#
+# Returns: error if
+#            - the server is not running
+#            - export is not found
+#            - mode is 'safe' and there are existing connections
+#
+# Since: 2.12
+##
+{ 'command': 'nbd-server-remove',
+  'data': {'name': 'str', '*mode': 'NbdServerRemoveMode'} }
+
 ##
 # @nbd-server-stop:
 #