Stop parsing JSON after first finished construct.

Fix https://github.com/bitcoin/bitcoin/issues/6558.  In particular, stop
parsing JSON after the first object or array is finished.  Check that no
other garbage follows, and fail the parser if it does.

src/test/univalue_tests.cpp

@@ -314,6 +314,21 @@ BOOST_AUTO_TEST_CASE(univalue_readwrite)
     BOOST_CHECK(obj["key3"].isObject());
 
     BOOST_CHECK_EQUAL(strJson1, v.write());
+
+    /* Check for (correctly reporting) a parsing error if the initial
+       JSON construct is followed by more stuff.  Note that whitespace
+       is, of course, exempt.  */
+
+    BOOST_CHECK(v.read("  {}\n  "));
+    BOOST_CHECK(v.isObject());
+    BOOST_CHECK(v.read("  []\n  "));
+    BOOST_CHECK(v.isArray());
+
+    BOOST_CHECK(!v.read("@{}"));
+    BOOST_CHECK(!v.read("{} garbage"));
+    BOOST_CHECK(!v.read("[]{}"));
+    BOOST_CHECK(!v.read("{}[]"));
+    BOOST_CHECK(!v.read("{} 42"));
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/univalue/univalue_read.cpp

@@ -244,16 +244,16 @@ bool UniValue::read(const char *raw)
     bool expectColon = false;
     vector<UniValue*> stack;
 
+    string tokenVal;
+    unsigned int consumed;
     enum jtokentype tok = JTOK_NONE;
     enum jtokentype last_tok = JTOK_NONE;
-    while (1) {
+    do {
         last_tok = tok;
 
-        string tokenVal;
-        unsigned int consumed;
         tok = getJsonToken(tokenVal, consumed, raw);
         if (tok == JTOK_NONE || tok == JTOK_ERR)
-            break;
+            return false;
         raw += consumed;
 
         switch (tok) {
@@ -377,9 +377,11 @@ bool UniValue::read(const char *raw)
         default:
             return false;
         }
-    }
+    } while (!stack.empty ());
 
-    if (stack.size() != 0)
+    /* Check that nothing follows the initial construct (parsed above).  */
+    tok = getJsonToken(tokenVal, consumed, raw);
+    if (tok != JTOK_NONE)
         return false;
 
     return true;