Implement a failing test for InternCache churn on map keys

src/test/java/com/fasterxml/jackson/failing/MapDeserializationInternStringsTest.java

@@ -0,0 +1,33 @@
+package com.fasterxml.jackson.failing;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.core.util.InternCache;
+import com.fasterxml.jackson.databind.BaseMapTest;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+// for [core#946]
+public class MapDeserializationInternStringsTest extends BaseMapTest
+{
+    public void testArbitraryKeysDontChurnCache() throws IOException {
+        ObjectMapper mapper = new ObjectMapper();
+        String json = a2q("{'1st':'onedata','2nd':'twodata'}");
+        Map<String, String> expected = new HashMap<>();
+        expected.put("1st", "onedata");
+        expected.put("2nd", "twodata");
+        // Clear the InternCache before deserialization to avoid impact from other tests. Note that the
+        // cache is static state, and this test will not be meaningful in an environment where tests are
+        // executed in parallel.
+        InternCache.instance.clear();
+        Map<String, String> deserialized = mapper.readValue(json, new TypeReference<Map<String, String>>() {});
+        // Verify our test has done what we expect before asserting state on the InternCache.
+        assertEquals(expected, deserialized);
+        assertEquals("Arbitrary map key values should not be added to the InternCache because " +
+                "map keys may have greater cardinality than the InternCache expects",
+                Collections.emptyMap(), InternCache.instance);
+    }
+}