Freeciv

The Unit_Value_Shield_Pct feature and merging recycle_unit with help_build_wonder was great for empowering ruleset designers!!!! Why? Because having the effect Unit_Value_Shield_Pct tied to certain requirements or legal actionenablers, gives ruleset control over when/how/where/what kind of shield value is legal and how much.
That's why those are great features!

However, it was discovered an unintended exploit that nullifies almost all possible benefits of this feature. :-{ Specifically, pcity->disbanded_shields will count toward any production type you change to later in
the turn. Therefore, any actionenabler restraints for BuildingGenus or even being allowed to recycle, are rendered
useless! There is a way for the player to always get maximum bonus and legally recycle, with an exploit. Here is the exploit: 1) change building product temporarily to a legal one or the one that gets the most bonus or least penalty, 2) then recycle the unit, 3) Then change it back to desired product target. Voila! Legal recycling for max bonus into whatever production type you want!

For this new feature to have any fair use and not just be a backdoor for exploiters/cheaters, we have to re-think
how pcity->disbanded_shields gets counted and credited.

Here is the TERRIBLE hack fix we have to use now, that makes whatever product type you disband into, a commitment for those shields to be dedicated to that product. Any change loses the disband shields completely, even if you switch back.. This is a terrible. So if it's better than before, and we're forced to use this, then you know it's broken before. I hope this code isn't released with the hack or with this code staying as how it is, because both would be terrible.

Let's please empower the great potential in these features!

/**********************************************************************//**
Help build the current production in a city. {
const char *work;
const char *prod;
const char *action_name;
const char *info_emoji;
int shields;

/* Sanity check: The actor still exists. */
  fc_assert_ret_val(pplayer, FALSE);
  fc_assert_ret_val(punit, FALSE);

/* Sanity check: The target city still exists. */
  fc_assert_ret_val(pcity_dest, FALSE);

shields = unit_shield_value(punit, unit_type_get(punit), paction);

bool full_contributor = (shields >= unit_type_get(punit)->build_cost) 
                          ? true : false;
  bool three_quarters = three_quarters = 
                          (!full_contributor
                          && shields >= (float)unit_type_get(punit)->build_cost*70/100)
                          ? true : false;

if (action_has_result(paction, ACTION_HELP_WONDER)) {
    /* Add the caravan shields */
    pcity_dest->shield_stock += shields;

/* Will be punished for changing production to something that can't
 * receive "Help Wonder" help. /
    fc_assert(city_production_gets_caravan_shields(
                  &pcity_dest->production));
    pcity_dest->caravan_shields += shields;
  } else {
    fc_assert(action_has_result(paction, ACTION_RECYCLE_UNIT));
    / Add the shields from recycling the unit to the city's current
 * production. */
    pcity_dest->shield_stock += shields;

// This had to change, otherwise there is no control at all over what 
    // production types can get a Unit_Shield_Value_Pct penalty/bonus, since
    // you can just change production to allowed/bonus type, disband, then
    // change back again:
    // ...
    /* If we change production later at this turn. No penalty is added. */
    //pcity_dest->disbanded_shields += shields;
    // ...
    // Shield type for disbanding is credited to whatever output was being made (see above),
    // and is lost forever if production type is changed.
  }
[...no changes in function from here ...]