Lazily create a linked list of context items (#1833)

This approach has the benefit that it does not require any *a priori*
initialization, and that each context pointer is identified by the way
in which it was created. Additionally, retrieving the context pointer
now requires that the entity responsible for creating/destroying it
(the manager) be given. Since managers are stored in global static
const structures, they should not normally be visible across source
files, and thus there should be no danger that a context item will be
retrieved by the wrong manager and thus cast into the wrong data type.

Since the items are stored in a linked list, their number will be
limited to exactly as many as are needed for a given context, with the
caveat that storing too many on a context will cause slow retrieval.

Thanks @mhdawson for the idea!

Fixes https://github.com/jerryscript-project/jerryscript/issues/1845

JerryScript-DCO-1.0-Signed-off-by: Gabriel Schulhof gabriel.schulhof@intel.com

jerry-core/include/jerryscript-core.h

@@ -195,15 +195,16 @@ typedef jerry_value_t (*jerry_vm_exec_stop_callback_t) (void *user_p);
 typedef bool (*jerry_object_property_foreach_t) (const jerry_value_t property_name,
                                                  const jerry_value_t property_value,
                                                  void *user_data_p);
-/**
- * Function type for user context allocation
- */
-typedef void *(*jerry_user_context_init_t) (void);
 
 /**
- * Function type for user context deallocation
+ * User context item manager
  */
-typedef void (*jerry_user_context_deinit_t) (void *user_context_p);
+typedef struct
+{
+  void (*init_cb) (void *); /**< callback responsible for initializing a context item, or NULL to zero out the memory */
+  void (*deinit_cb) (void *); /**< callback responsible for deinitializing a context item */
+  size_t bytes_needed; /**< number of bytes to allocate for this manager */
+} jerry_context_data_manager_t;
 
 /**
  * Function type for allocating buffer for JerryScript instance.
@@ -227,15 +228,12 @@ typedef struct jerry_instance_t jerry_instance_t;
  * General engine functions.
  */
 void jerry_init (jerry_init_flag_t flags);
-void jerry_init_with_user_context (jerry_init_flag_t flags,
-                                   jerry_user_context_init_t init_cb,
-                                   jerry_user_context_deinit_t deinit_cb);
 void jerry_cleanup (void);
 void jerry_register_magic_strings (const jerry_char_ptr_t *ex_str_items_p, uint32_t count,
                                    const jerry_length_t *str_lengths_p);
 void jerry_get_memory_limits (size_t *out_data_bss_brk_limit_p, size_t *out_stack_limit_p);
 void jerry_gc (void);
-void *jerry_get_user_context (void);
+void *jerry_get_context_data (const jerry_context_data_manager_t *manager_p);
 
 /**
  * Parser and executor functions.