java: allow non-const maps to be passed as null

This allows e.g. skipping caching on the new signature hash call.

Author: jgriffiths

src/swig_java/src/com/blockstream/test/test_elements_tx.java

@@ -47,11 +47,23 @@ public void test_elements_taproot_sighash() {
 
         final String expected = "2c478ce6d5637e0ea8be37a53090e0955b6c501773fccf6738520cfcc5442150";
 
+        // Compute the signature hash
         Wally.tx_get_input_signature_hash(tx, input_idx, scripts, assets, values,
             tapleaf_script, key_version, codesep_pos, annex, genesis, sighash, sighash_type,
             cache, bytes_out
         );
-
+        assert_eq(expected, h(bytes_out), "different sighash");
+        // Compute again to ensure the cache works as expected
+        Wally.tx_get_input_signature_hash(tx, input_idx, scripts, assets, values,
+            tapleaf_script, key_version, codesep_pos, annex, genesis, sighash, sighash_type,
+            cache, bytes_out
+        );
+        assert_eq(expected, h(bytes_out), "different sighash");
+        // Compute with the cache disabled (passed as null)
+        Wally.tx_get_input_signature_hash(tx, input_idx, scripts, assets, values,
+            tapleaf_script, key_version, codesep_pos, annex, genesis, sighash, sighash_type,
+            null, bytes_out
+        );
         assert_eq(expected, h(bytes_out), "different sighash");
 
         Wally.map_free(cache);

src/swig_java/swig.i

@@ -393,9 +393,13 @@ static jobjectArray create_jstringArray(JNIEnv *jenv, char **p, size_t len) {
 %typemap(jtype) const struct NAME * "Object"
 %typemap(jni) const struct NAME * "jobject"
 %typemap (in) struct NAME * {
-    $1 = (struct NAME *)get_obj_or_throw(jenv, $input, ID, "NAME");
-    if (!$1)
-        return $null;
+    if (strcmp("NAME", "wally_map") == 0)
+        $1 = (struct NAME *)get_obj(jenv, $input, ID);
+    else {
+        $1 = (struct NAME *)get_obj_or_throw(jenv, $input, ID, "NAME");
+        if (!$1)
+            return $null;
+    }
 }
 %typemap(jtype) struct NAME * "Object"
 %typemap(jni) struct NAME * "jobject"