nbd: Simplify qemu bitmap context name

Each dirty bitmap already knows its name; by reducing the scope of the
places where we construct "qemu:dirty-bitmap:NAME" strings, tracking
the name is more localized, and there are fewer per-export fields to
worry about.  This in turn will make it easier for an upcoming patch
to export more than one bitmap at once.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201027050556.269064-6-eblake@redhat.com>

nbd/server.c

@@ -95,7 +95,6 @@ struct NBDExport {
     Notifier eject_notifier;
 
     BdrvDirtyBitmap *export_bitmap;
-    char *export_bitmap_context;
 };
 
 static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
@@ -871,14 +870,15 @@ static bool nbd_meta_qemu_query(NBDClient *client, NBDExportMetaContexts *meta,
     }
 
     if (nbd_strshift(&query, "dirty-bitmap:")) {
+        const char *bm_name;
+
         trace_nbd_negotiate_meta_query_parse("dirty-bitmap:");
         if (!meta->exp->export_bitmap) {
             trace_nbd_negotiate_meta_query_skip("no dirty-bitmap exported");
             return true;
         }
-        if (nbd_meta_empty_or_pattern(client,
-                                      meta->exp->export_bitmap_context +
-                                      strlen("qemu:dirty-bitmap:"), query)) {
+        bm_name = bdrv_dirty_bitmap_name(meta->exp->export_bitmap);
+        if (nbd_meta_empty_or_pattern(client, bm_name, query)) {
             meta->bitmap = true;
         }
         return true;
@@ -1004,8 +1004,11 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
     }
 
     if (meta->bitmap) {
-        ret = nbd_negotiate_send_meta_context(client,
-                                              meta->exp->export_bitmap_context,
+        const char *bm_name = bdrv_dirty_bitmap_name(meta->exp->export_bitmap);
+        g_autofree char *context = g_strdup_printf("qemu:dirty-bitmap:%s",
+                                                   bm_name);
+
+        ret = nbd_negotiate_send_meta_context(client, context,
                                               NBD_META_ID_DIRTY_BITMAP,
                                               errp);
         if (ret < 0) {
@@ -1576,9 +1579,6 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
         bdrv_dirty_bitmap_set_busy(bm, true);
         exp->export_bitmap = bm;
         assert(strlen(bitmap) <= BDRV_BITMAP_MAX_NAME_SIZE);
-        exp->export_bitmap_context = g_strdup_printf("qemu:dirty-bitmap:%s",
-                                                     bitmap);
-        assert(strlen(exp->export_bitmap_context) < NBD_MAX_STRING_SIZE);
     }
 
     blk_add_aio_context_notifier(blk, blk_aio_attached, blk_aio_detach, exp);
@@ -1656,7 +1656,6 @@ static void nbd_export_delete(BlockExport *blk_exp)
 
     if (exp->export_bitmap) {
         bdrv_dirty_bitmap_set_busy(exp->export_bitmap, false);
-        g_free(exp->export_bitmap_context);
     }
 }