Make pinned list a linked list instead of hardcoded array.

src/wren_compiler.c

@@ -1542,7 +1542,8 @@ ObjFn* wrenCompile(WrenVM* vm, const char* source)
   Compiler compiler;
   initCompiler(&compiler, &parser, NULL, 0);
 
-  pinObj(vm, (Obj*)compiler.fn);
+  PinnedObj pinned;
+  pinObj(vm, (Obj*)compiler.fn, &pinned);
 
   for (;;)
   {
@@ -1563,7 +1564,7 @@ ObjFn* wrenCompile(WrenVM* vm, const char* source)
 
   emit(&compiler, CODE_END);
 
-  unpinObj(vm, (Obj*)compiler.fn);
+  unpinObj(vm);
 
   return parser.hasError ? NULL : compiler.fn;
 }

src/wren_value.c

@@ -53,12 +53,13 @@ ObjClass* wrenNewClass(WrenVM* vm, ObjClass* superclass, int numFields)
   ObjClass* metaclass = newClass(vm, NULL, vm->classClass, 0);
 
   // Make sure it isn't collected when we allocate the metaclass.
-  pinObj(vm, (Obj*)metaclass);
+  PinnedObj pinned;
+  pinObj(vm, (Obj*)metaclass, &pinned);
 
   ObjClass* classObj = newClass(vm, metaclass, superclass, numFields);
   classObj->numFields = numFields;
 
-  unpinObj(vm, (Obj*)metaclass);
+  unpinObj(vm);
 
   return classObj;
 }

src/wren_vm.c

@@ -23,7 +23,10 @@ WrenVM* wrenNewVM(WrenReallocateFn reallocateFn)
 
   // TODO(bob): Make this configurable.
   vm->nextGC = 1024 * 1024 * 10;
+  vm->first = NULL;
 
+  vm->pinned = NULL;
+  
   // Clear out the global variables. This ensures they are NULL before being
   // initialized in case we do a garbage collection before one gets initialized.
   for (int i = 0; i < MAX_SYMBOLS; i++)
@@ -284,9 +287,11 @@ static void collectGarbage(WrenVM* vm)
   }
 
   // Pinned objects.
-  for (int j = 0; j < vm->numPinned; j++)
+  PinnedObj* pinned = vm->pinned;
+  while (pinned != NULL)
   {
-    markObj(vm->pinned[j]);
+    markObj(pinned->obj);
+    pinned = pinned->previous;
   }
 
   // Stack functions.
@@ -1086,14 +1091,14 @@ void callFunction(Fiber* fiber, ObjFn* fn, int numArgs)
   fiber->numFrames++;
 }
 
-void pinObj(WrenVM* vm, Obj* obj)
+void pinObj(WrenVM* vm, Obj* obj, PinnedObj* pinned)
 {
-  ASSERT(vm->numPinned < MAX_PINNED - 1, "Too many pinned objects.");
-  vm->pinned[vm->numPinned++] = obj;
+  pinned->obj = obj;
+  pinned->previous = vm->pinned;
+  vm->pinned = pinned;
 }
 
-void unpinObj(WrenVM* vm, Obj* obj)
+void unpinObj(WrenVM* vm)
 {
-  ASSERT(vm->pinned[vm->numPinned - 1] == obj, "Unpinning out of stack order.");
-  vm->numPinned--;
+  vm->pinned = vm->pinned->previous;
 }

src/wren_vm.h

@@ -7,7 +7,6 @@
 // TODO(bob): Make these externally controllable.
 #define STACK_SIZE 1024
 #define MAX_CALL_FRAMES 256
-#define MAX_PINNED 16
 
 typedef enum
 {
@@ -117,6 +116,23 @@ typedef struct
   int count;
 } SymbolTable;
 
+// A pinned object is an Obj that has been temporarily made an explicit GC root.
+// This is for temporary or new objects that are not otherwise reachable but
+// should not be collected.
+//
+// They are organized as linked list of these objects stored on the stack. The
+// WrenVM has a pointer to the head of the list and walks it if a collection
+// occurs. This implies that pinned objects need to have stack semantics: only
+// the most recently pinned object can be unpinned.
+typedef struct sPinnedObj
+{
+  // The pinned object.
+  Obj* obj;
+
+  // The next pinned object.
+  struct sPinnedObj* previous;
+} PinnedObj;
+
 struct WrenVM
 {
   SymbolTable methods;
@@ -151,13 +167,9 @@ struct WrenVM
   // The first object in the linked list of all currently allocated objects.
   Obj* first;
 
-  // How many pinned objects are in the stack.
-  int numPinned;
-
-  // The stack "pinned" objects. These are temporary explicit GC roots so that
-  // the collector can find them before they are reachable through a normal
-  // root.
-  Obj* pinned[MAX_PINNED];
+  // The head of the list of pinned objects. Will be `NULL` if nothing is
+  // pinned.
+  PinnedObj* pinned;
 };
 
 typedef struct
@@ -221,10 +233,11 @@ Value interpret(WrenVM* vm, ObjFn* fn);
 // arguments (including the receiver) to be on the top of the stack already.
 void callFunction(Fiber* fiber, ObjFn* fn, int numArgs);
 
-// Mark [obj] as a GC root so that it doesn't get collected.
-void pinObj(WrenVM* vm, Obj* obj);
+// Mark [obj] as a GC root so that it doesn't get collected. Initializes
+// [pinned], which must be then passed to [unpinObj].
+void pinObj(WrenVM* vm, Obj* obj, PinnedObj* pinned);
 
-// Unmark [obj] as a GC root so that it doesn't get collected.
-void unpinObj(WrenVM* vm, Obj* obj);
+// Remove the most recently pinned object from the list of pinned GC roots.
+void unpinObj(WrenVM* vm);
 
 #endif