Allow metadata to contain a list of values (#201)

The `Rules.match` function now receives an optional `allow_duplicate_metadata=True` argument, which changes the structure of `Match.meta`. By default `Match.meta` is a dictionary with metadata names and their corresponding values, if a metadata name appears duplicated in a rule, the last value will be used. For example, consider the following rule:

```yara
rule demo {
   meta: 
     foo = "foo #1"
     foo = "foo #2"
     bar = "bar"
   condition:
      false
}
```

In that case `Match.meta` would be `{"foo": "foo #2", "bar": "bar"}` by default (`allow_duplicate_metadata=False`), but with `allow_duplicate_metadata=True` it would be: `{"foo": ["foo #1", "foo #2"], "bar": ["bar"]}`. 

tests.py

@@ -752,6 +752,28 @@ class TestYara(unittest.TestCase):
         self.assertTrue(meta['b'] == 'ñ')
         self.assertTrue(meta['c'] == 'ñ')
 
+    # This test is similar to testScanMeta but it tests for displaying multiple values in the meta data generated
+    # when a Match object is created (upon request).
+    def testDuplicateMeta(self):
+        r = yara.compile(source="""
+        rule test {
+            meta:
+                a = 1
+                a = 2
+                b = 3
+            condition:
+                true
+        }
+        """)
+
+        # Default behaviour should produce a simple KV map and should use the 'latest' metadata value per field
+        meta = r.match(data="dummy")[0].meta
+        self.assertTrue(meta['a'] == 2 and meta['b'] == 3)
+
+        # `allow_duplicate_metadata` flag should reveal all metadata values per field as a list
+        meta = r.match(data="dummy", allow_duplicate_metadata=True)[0].meta
+        self.assertTrue(meta['a'] == [1, 2] and meta['b'] == [3])
+
     def testFilesize(self):
 
         self.assertTrueRules([

yara-python.c

@@ -432,6 +432,7 @@ typedef struct _CALLBACK_DATA
   PyObject* warnings_callback;
   PyObject* console_callback;
   int which;
+  bool allow_duplicate_metadata;
 
 } CALLBACK_DATA;
 
@@ -885,7 +886,6 @@ _exit:
 #define CALLBACK_NON_MATCHES 0x02
 #define CALLBACK_ALL CALLBACK_MATCHES | CALLBACK_NON_MATCHES
 
-
 int yara_callback(
     YR_SCAN_CONTEXT* context,
     int message,
@@ -987,8 +987,24 @@ int yara_callback(
     else
       object = PY_STRING(meta->string);
 
-    PyDict_SetItemString(meta_list, meta->identifier, object);
-    Py_DECREF(object);
+    if (((CALLBACK_DATA*) user_data)->allow_duplicate_metadata){
+      // Check if we already have an array under this key
+      PyObject* existing_item = PyDict_GetItemString(meta_list, meta->identifier);
+      // Append object to existing list
+      if (existing_item)
+        PyList_Append(existing_item, object);
+      else{
+        //Otherwise, instantiate array and append object as first item
+        PyObject* new_list = PyList_New(0);
+        PyList_Append(new_list, object);
+        PyDict_SetItemString(meta_list, meta->identifier, new_list);
+        Py_DECREF(new_list);
+      }
+    }
+    else{
+      PyDict_SetItemString(meta_list, meta->identifier, object);
+      Py_DECREF(object);
+    }
   }
 
   yr_rule_strings_foreach(rule, string)
@@ -1594,8 +1610,9 @@ static PyObject* Rules_next(
       else
         object = PY_STRING(meta->string);
 
-      PyDict_SetItemString(meta_list, meta->identifier, object);
-      Py_DECREF(object);
+        PyDict_SetItemString(meta_list, meta->identifier, object);
+        Py_DECREF(object);
+
     }
 
     rule->global = PyBool_FromLong(rules->iter_current_rule->flags & RULE_FLAGS_GLOBAL);
@@ -1623,7 +1640,7 @@ static PyObject* Rules_match(
       "filepath", "pid", "data", "externals",
       "callback", "fast", "timeout", "modules_data",
       "modules_callback", "which_callbacks", "warnings_callback",
-      "console_callback", NULL
+      "console_callback", "allow_duplicate_metadata", NULL
       };
 
   char* filepath = NULL;
@@ -1648,11 +1665,12 @@ static PyObject* Rules_match(
   callback_data.warnings_callback = NULL;
   callback_data.console_callback = NULL;
   callback_data.which = CALLBACK_ALL;
+  callback_data.allow_duplicate_metadata = false;
 
   if (PyArg_ParseTupleAndKeywords(
         args,
         keywords,
-        "|sis*OOOiOOiOO",
+        "|sis*OOOiOOiOOb",
         kwlist,
         &filepath,
         &pid,
@@ -1665,7 +1683,8 @@ static PyObject* Rules_match(
         &callback_data.modules_callback,
         &callback_data.which,
         &callback_data.warnings_callback,
-        &callback_data.console_callback))
+        &callback_data.console_callback,
+        &callback_data.allow_duplicate_metadata))
   {
     if (filepath == NULL && data.buf == NULL && pid == -1)
     {
@@ -1729,6 +1748,9 @@ static PyObject* Rules_match(
       }
     }
 
+    if (callback_data.allow_duplicate_metadata == NULL)
+      callback_data.allow_duplicate_metadata = false;
+
     if (yr_scanner_create(object->rules, &scanner) != 0)
     {
       return PyErr_Format(