Anonymous users getting same product items in cart - drupal-7

Question

I am using drupal commerce module, i have an issue with anonymous users, when users click on add to cart button, cart is showing extra items which are added by other anonymous users.
I am using custom code for add to cart using ajax. following is the code.
// Add a product to cart on ajax call.
function mymodule_custom_add_to_cart($product_id,$uid){
$line_item = commerce_product_line_item_new(commerce_product_load($product_id));
commerce_cart_product_add($uid, $line_item);
$order = commerce_cart_order_load($uid);
commerce_cart_order_refresh($order);
// loads data array from order object
$data = mymoudle_custom_cart_load_all_variables($order->order_id);
$jsonencoded = json_encode($data);
print $jsonencoded;
}
I dont know why all anonymous users are getting same products in there cart.
Please help me to find out the issue.
UPDATE 1:
I have changed the code for anonymous user as following, then i am getting an error: EntityMetadataWrapperException: Unable to get the data property data as the parent data structure is not set. in EntityStructureWrapper->getPropertyValue() (line 451 of /entity.wrapper.inc
CHANGED CODE
// Add a product to cart on ajax call.
function mymodule_custom_add_to_cart($product_id,$uid){
if($uid == 0){
$order_id = commerce_cart_order_id($uid);
if($order_id == false){
$order = commerce_cart_order_new(0, 'checkout_checkout');
} else {
$order = commerce_order_load($order_id);
}
// Load whatever product represents the item the customer will be
// paying for and create a line item for it.
$product = commerce_product_load($product_id);
$line_item = commerce_product_line_item_new($product, 1, $order->order_id);
commerce_line_item_save($line_item);
// Add the line item to the order using fago's rockin' wrapper.
$order_wrapper = entity_metadata_wrapper('commerce_order', $order);
$order_wrapper->commerce_line_items[] = $line_item;
// Save the order again to update its line item reference field.
commerce_order_save($order);
// loads data array from order object
$data = mymoudle_custom_cart_load_all_variables($order->order_id);
$jsonencoded = json_encode($data);
print $jsonencoded;
}else{
$line_item = commerce_product_line_item_new(commerce_product_load($product_id));
commerce_cart_product_add($uid, $line_item);
$order = commerce_cart_order_load($uid);
commerce_cart_order_refresh($order);
// loads data array from order object
$data = mymoudle_custom_cart_load_all_variables($order->order_id);
$jsonencoded = json_encode($data);
print $jsonencoded;
}
}
UPDATE 2: ISSUE RESOLVED.
Site is crawling by googlebot, which is clicking addtocart link and its adding an product to anonymous user account automatically, so i removed link for addtocart button and used commerce addtocart form.

Answer 1

All anonymous users have uid zero (see documentation). That is the reason why all the items added by different anonymous users are being added to the same shopping cart.