show: fix corrupt json output with no bookmarks

hgext/show.py

@@ -19,6 +19,7 b' from mercurial import ('
     cmdutil,
     commands,
     error,
+    formatter,
     pycompat,
     registrar,
 )
@@ -109,8 +110,11 b' def showbookmarks(ui, repo, fm):'
     """bookmarks and their associated changeset"""
     marks = repo._bookmarks
     if not len(marks):
-        # TODO json output is corrupted; consider using formatter
-        ui.write(_('(no bookmarks set)\n'))
+        # This is a bit hacky. Ideally, templates would have a way to
+        # specify an empty output, but we shouldn't corrupt JSON while
+        # waiting for this functionality.
+        if not isinstance(fm, formatter.jsonformatter):
+            ui.write(_('(no bookmarks set)\n'))
         return
 
     active = repo._activebookmark

tests/test-show.t

@@ -114,4 +114,12 b' bookmarks JSON works'
    }
   ]
 
+JSON works with no bookmarks
+
+  $ hg book -d a-longer-bookmark
+  $ hg book -d book1
+  $ hg show bookmarks -T json
+  [
+  ]
+
   $ cd ..