Update get_active_producers for gh#2132

contracts/eosiolib/chain.h

@@ -23,7 +23,7 @@ extern "C" {
    /**
     * Return the set of active producers
     * @param producers - a pointer to an buffer of account_names
-    * @param datalen - byte length of buffer
+    * @param datalen - byte length of buffer, 0 to report required size
     * @return the number of bytes actually populated
     *
     *  Example:

contracts/identity/identity.hpp

@@ -249,7 +249,8 @@ namespace identity {
 
          static bool is_trusted( account_name acnt ) {
             account_name active_producers[21];
-            auto count = get_active_producers( active_producers, sizeof(active_producers) );
+            auto active_prod_size = get_active_producers( active_producers, sizeof(active_producers) );
+            auto count = active_prod_size / sizeof(account_name);
             for( size_t i = 0; i < count; ++i ) {
                if( active_producers[i] == acnt )
                   return true;

libraries/chain/wasm_interface.cpp

@@ -676,12 +676,17 @@ class producer_api : public context_aware_api {
    public:
       using context_aware_api::context_aware_api;
 
-      int get_active_producers(array_ptr<chain::account_name> producers, size_t datalen) {
+      int get_active_producers(array_ptr<chain::account_name> producers, size_t buffer_size) {
          auto active_producers = context.get_active_producers();
+
          size_t len = active_producers.size();
-         size_t cpy_len = std::min(datalen, len);
-         memcpy(producers, active_producers.data(), cpy_len * sizeof(chain::account_name));
-         return len;
+         auto s = len * sizeof(chain::account_name);
+         if( buffer_size == 0 ) return s;
+
+         auto copy_size = std::min( buffer_size, s );
+         memcpy( producers, active_producers.data(), copy_size );
+
+         return copy_size;
       }
 };