only reset extents if they are unusable

this allows callers of Buildings::setSize() to "pre-initialize" the
extents to declare non-rectangular structures. this allows quickfort to
create non-rectangular stockpiles, farm plots, zones, etc. the extents
are still reset as before if the size of the building doesn't match the
caller's expectations.

this commit also fixes a memory leak when setSize() allocates memory for
extents, but the memory is not deallocated if the building is ultimately
invalid for some reason.

docs/Lua API.rst

@@ -1679,7 +1679,11 @@ Low-level building creation functions:
   Returns *false* if the building cannot be placed, or *true, width,
   height, rect_area, true_area*. Returned width and height are the
   final values used by the building; true_area is less than rect_area
-  if any tiles were removed from designation.
+  if any tiles were removed from designation. You can specify a non-rectangular
+  designation for building types that support extents by setting the
+  ``room.extents`` bitmap before calling this function. The extents will be
+  reset, however, if the size returned by this function doesn't match the
+  input size parameter.
 
 * ``dfhack.buildings.constructAbstract(building)``
 
@@ -1768,7 +1772,9 @@ Among them are:
 
   - ``fields = { ... }``
 
-    Initializes fields of the building object after creation with ``df.assign``.
+    Initializes fields of the building object after creation with
+    ``df.assign``. If ``room.extents`` is assigned this way and this function
+    returns with error, the memory allocated for the extents is freed.
 
   - ``width = ..., height = ..., direction = ...``
 

library/include/modules/Buildings.h

@@ -162,6 +162,8 @@ DFHACK_EXPORT bool hasSupport(df::coord pos, df::coord2d size);
 
 /**
  * Sets the size of the building, using size and direction as guidance.
+ * Any custom extents set for the building will be cleared if the size passed in
+ * as guidance cannot be applied to the building.
  * Returns true if the building can be created at its position, using that size.
  */
 DFHACK_EXPORT bool setSize(df::building *bld, df::coord2d size, int direction = 0);

library/lua/dfhack/buildings.lua

@@ -485,6 +485,11 @@ function buildings.constructBuilding(info)
     local to_delete = instance
     return dfhack.with_finalize(
         function()
+            -- ensure we don't leak extents created by Buildings::checkFreeTiles
+            if to_delete and to_delete.room.extents then
+                df.delete(to_delete.room.extents)
+                to_delete.room.extents = nil
+            end
             df.delete(to_delete)
         end,
         function()

library/modules/Buildings.cpp

@@ -758,18 +758,19 @@ bool Buildings::setSize(df::building *bld, df::coord2d size, int direction)
     CHECK_NULL_POINTER(bld);
     CHECK_INVALID_ARGUMENT(bld->id == -1);
 
-    // Delete old extents
-    if (bld->room.extents)
-    {
-        delete[] bld->room.extents;
-        bld->room.extents = NULL;
-    }
-
     // Compute correct size and apply it
+    df::coord2d old_size = size;
     df::coord2d center;
     getCorrectSize(size, center, bld->getType(), bld->getSubtype(),
                    bld->getCustomType(), direction);
 
+    // Delete old extents if size has changed.
+    if (old_size != size && bld->room.extents)
+    {
+        delete[] bld->room.extents;
+        bld->room.extents = NULL;
+    }
+
     bld->x2 = bld->x1 + size.x - 1;
     bld->y2 = bld->y1 + size.y - 1;
     bld->centerx = bld->x1 + center.x;