json: Actually pop states

The JSON encoder only looked at the top state, but did not
pop it, so if we nested objects, we got stuck in whatever
the last state we pushed aside was, so in our example, we
wrongly get a comma inserted _after_ key "b":

{"a":[{}],
 "b":,[{}]
}

2 files changed, 17 insertions, 0 deletions

diff --git a/apt-private/private-json-hooks.cc b/apt-private/private-json-hooks.cc
index b61829cbf..ce1665b18 100644
--- a/apt-private/private-json-hooks.cc
+++ b/apt-private/private-json-hooks.cc
@@ -79,6 +79,7 @@ class APT_HIDDEN JsonWriter
    void popState()
    {
       this->state = old_states.top();
+      old_states.pop();
    }
 
    public:
diff --git a/test/libapt/json_test.cc b/test/libapt/json_test.cc
index e3317afc4..9c29936f6 100644
--- a/test/libapt/json_test.cc
+++ b/test/libapt/json_test.cc
@@ -43,3 +43,19 @@ TEST(JsonTest, JsonArrayAndValues)
 
    EXPECT_EQ("[0,\"value\",1,true]", os.str());
 }
+TEST(JsonTest, JsonStackRegression)
+{
+   std::ostringstream os;
+
+   JsonWriter w(os);
+
+   // Nest those things deeply such that we transition states:
+   //    object -> array -> object; -> array -> object
+   // Older versions never popped back and got stuck on array state.
+   w.beginObject();
+   w.name("a").beginArray().beginObject().endObject().endArray();
+   w.name("b").beginArray().beginObject().endObject().endArray();
+   w.endObject();
+
+   EXPECT_EQ("{\"a\":[{}],\"b\":[{}]}", os.str());
+}