Fixed get_active_producers for c API passing buffer byte length and added...

Fixed get_active_producers for c API passing buffer byte length and added returning the number of bytes written, so user knows how many bytes were populated.

contracts/eoslib/chain.h

@@ -21,19 +21,19 @@
 
 extern "C" {
    /**
-    * @brief Return the set of active producers
-    * @details Return the set of active producers
-    *
-    * @param producers - location to store the active producers
+    * Return the set of active producers
+    * @param producers - a pointer to an buffer of account_names
+    * @param datalen - byte length of buffer
+    * @return the number of bytes actually populated
     *
     *  Example:
     *  @code
     *  account_name producers[21];
-    *  get_active_producers(producers, sizeof(account_name)*21);
+    *  uint32_t bytes_populated = get_active_producers(producers, sizeof(account_name)*21);
     *  @endcode
     */
 
-   void get_active_producers( account_name* producers, uint32_t datalen );
+   uint32_t get_active_producers( account_name* producers, uint32_t datalen );
 
    ///@ } chaincapi
 }

libraries/chain/apply_context.cpp

@@ -224,15 +224,16 @@ const contracts::table_id_object& apply_context::find_or_create_table( name scop
    });
 }
 
-void apply_context::get_active_producers(account_name* producers, size_t datalen) {
+size_t apply_context::get_active_producers(account_name* producers, size_t len) {
    const auto& gpo = controller.get_global_properties();
    int index = 0;
    for(const auto& producer : gpo.active_producers.producers)
    {
       *(producers + index) = producer.producer_name;
-      if (++index >= datalen)
+      if (++index >= len)
          break;
    }
+   return index;
 }
 
 } } /// eosio::chain

libraries/chain/include/eosio/chain/apply_context.hpp

@@ -94,7 +94,7 @@ class apply_context {
       bool                     all_authorizations_used()const;
       vector<permission_level> unused_authorizations()const;
 
-      void get_active_producers(account_name* producers, size_t len);
+      size_t get_active_producers(account_name* producers, size_t len);
 
       const chain_controller&       controller;
       const chainbase::database&    db;  ///< database where state is stored

libraries/chain/wasm_interface.cpp

@@ -448,8 +448,9 @@ class producer_api : public context_aware_api {
    public:
       using context_aware_api::context_aware_api;
 
-      void get_active_producers(array_ptr<chain::account_name> producers, size_t datalen) {
-         context.get_active_producers(producers, datalen);
+      int get_active_producers(array_ptr<chain::account_name> producers, size_t datalen) {
+         auto actual_num_prod = context.get_active_producers(producers, datalen / sizeof(chain::account_name));
+         return actual_num_prod * sizeof(chain::account_name);
       }
 };
 
@@ -742,7 +743,7 @@ class transaction_api : public context_aware_api {
 };
 
 REGISTER_INTRINSICS(producer_api,
-   (get_active_producers,      void(int, int))
+   (get_active_producers,      int(int, int))
 );
 
 REGISTER_INTRINSICS(crypto_api,