Add ArrayDesc destructor to avoid possible stack overflow (#982)

mlx/array.cpp

@@ -192,6 +192,36 @@ array::ArrayDesc::ArrayDesc(
   init();
 }
 
+array::ArrayDesc::~ArrayDesc() {
+  // When an array description is destroyed it will delete a bunch of arrays
+  // that may also destory their corresponding descriptions and so on and so
+  // forth.
+  //
+  // This calls recursively the destructor and can result in stack overflow, we
+  // instead put them in a vector and destroy them one at a time resulting in a
+  // max stack depth of 2.
+  std::vector<std::shared_ptr<ArrayDesc>> for_deletion;
+
+  for (array& a : inputs) {
+    if (a.array_desc_.use_count() == 1) {
+      for_deletion.push_back(std::move(a.array_desc_));
+    }
+  }
+
+  while (!for_deletion.empty()) {
+    // top is going to be deleted at the end of the block *after* the arrays
+    // with inputs have been moved into the vector
+    auto top = std::move(for_deletion.back());
+    for_deletion.pop_back();
+
+    for (array& a : top->inputs) {
+      if (a.array_desc_.use_count() == 1) {
+        for_deletion.push_back(std::move(a.array_desc_));
+      }
+    }
+  }
+}
+
 array::ArrayIterator::ArrayIterator(const array& arr, int idx)
     : arr(arr), idx(idx) {
   if (arr.ndim() == 0) {

mlx/array.h

@@ -404,6 +404,8 @@ class array {
         std::shared_ptr<Primitive> primitive,
         std::vector<array> inputs);
 
+    ~ArrayDesc();
+
    private:
     // Initialize size, strides, and other metadata
     void init();