Lazily create a linked list of context items (jerryscript-project#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 jerryscript-project#1845

JerryScript-DCO-1.0-Signed-off-by: Gabriel Schulhof [email protected]

Author: Gabriel "_|Nix|_" Schulhof

docs/02.API-REFERENCE.md

@@ -101,6 +101,25 @@ created by API functions has the error flag set.
 typedef uint32_t jerry_value_t;
 ```
 
+## jerry_context_data_manager_t
+
+**Summary**
+
+Structure that defines how a context data item will be initialized and deinitialized. JerryScript zeroes out the memory
+for the item by default, and if the `init_cb` field is not NULL, it will be called with the pointer to the memory  as
+an additional custom initializer.
+
+**Prototype**
+
+```c
+typedef struct
+{
+  void (*init_cb) (void *); /**< callback responsible for initializing a context item, or NULL */
+  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;
+```
+
 ## jerry_property_descriptor_t
 
 **Summary**
@@ -264,9 +283,7 @@ typedef jerry_value_t (*jerry_vm_exec_stop_callback_t) (void *user_p);
 **Summary**
 
 Initializes the JerryScript engine, making it possible to run JavaScript code and perform operations
-on JavaScript values. See also [jerry_init_with_user_context](#jerry_init_with_user_context) if you
-wish to initialize the JerryScript engine in such a way that its context contains a custom pointer
-which you can later retrieve using [jerry_get_user_context](#jerry_get_user_context).
+on JavaScript values.
 
 **Prototype**
 
@@ -299,91 +316,13 @@ jerry_init (jerry_init_flag_t flags)
 **See also**
 
 - [jerry_cleanup](#jerry_cleanup)
-- [jerry_init_with_user_context](#jerry_init_with_user_context)
-
-
-## jerry_init_with_user_context
-
-**Summary**
-
-Calls [jerry_init](#jerry_init) to initialize the JerryScript engine, thereby making it possible
-to run JavaScript code and perform operations on JavaScript values. In addition to the first
-parameter this function accepts two more parameters with which it allows the caller to store a
-`void *` pointer inside the context being initialized with `jerry_init ()`. The function calls the
-callback given in its `init_cb` parameter to allocate the memory for the pointer and it stores the
-function pointer given in the `deinit_cb` parameter along with the pointer so that it may be called
-to free the stored pointer when `jerry_cleanup ()` is later called to dispose of the context.
-
-**Prototype**
-
-```c
-void
-jerry_init_with_user_context (jerry_init_flag_t flags,
-                              jerry_user_context_init_cb init_cb,
-                              jerry_user_context_deinit_cb deinit_cb);
-```
-
-`flags` - combination of various engine configuration flags:
-
-- `JERRY_INIT_EMPTY` - no flags, just initialize in default configuration.
-- `JERRY_INIT_SHOW_OPCODES` - print compiled byte-code.
-- `JERRY_INIT_SHOW_REGEXP_OPCODES` - print compiled regexp byte-code.
-- `JERRY_INIT_MEM_STATS` - dump memory statistics.
-- `JERRY_INIT_MEM_STATS_SEPARATE` - dump memory statistics and reset peak values after parse.
-- `JERRY_INIT_DEBUGGER` - enable all features required by debugging.
-
-`init_cb` - a function pointer that will be called to allocate the custom pointer.
-
-`deinit_cb` - a function pointer that will be called when the custom pointer must be freed.
-
-**Example**
-
-```c
-void *
-init_user_context (void)
-{
-  void *return_value;
-
-  /* allocate and initialize return_value */
-
-  return return_value;
-} /* init_user_context */
-
-void
-free_user_context (void *context)
-{
-
-  /* free the value allocated above */
-
-} /* free_user_context */
-
-{
-  /* init_user_context () will be called before the call below returns */
-  jerry_init_with_user_context (JERRY_INIT_SHOW_OPCODES | JERRY_INIT_SHOW_REGEXP_OPCODES,
-                                init_user_context,
-                                free_user_context);
-
-  /* ... */
-
-  /* free_user_context () will be called before the call below returns */
-  jerry_cleanup ();
-}
-```
-
-**See also**
-
-- [jerry_cleanup](#jerry_cleanup)
-- [jerry_get_user_context](#jerry_get_user_context)
 
 
 ## jerry_cleanup
 
 **Summary**
 
-Finish JavaScript engine execution, freeing memory and JavaScript values. If the context was
-initialized with `jerry_init_with_user_context ()` and a `deinit_cb` was provided, then it will
-be called to free the memory at the custom pointer which was associated with the context being
-cleaned up.
+Finish JavaScript engine execution, freeing memory and JavaScript values.
 
 *Note*: JavaScript values, received from engine, will be inaccessible after the cleanup.
 
@@ -397,37 +336,80 @@ jerry_cleanup (void);
 **See also**
 
 - [jerry_init](#jerry_init)
-- [jerry_init_with_user_context](#jerry_init_with_user_context)
 
 
-## jerry_get_user_context
+## jerry_get_context_data
 
 **Summary**
 
-Retrieve the pointer stored within the current context.
+Retrieve a pointer to the item stored within the current context by the given manager.
+
+*Note*: Since internally the pointer to a manager's context data item is linked to the next such pointer in a linked
+        list, it is inadvisable to invoke too many different managers, because doing so will increase the time it takes
+        to retrieve a manager's context data item, degrading performance. For example, try to keep the number of
+        managers below five.
 
 **Prototype**
 
 ```c
 void *
-jerry_get_user_context (void);
+jerry_get_context_data (const jerry_context_data_manager *manager_p);
 ```
 
-- return value: the pointer that was assigned during `jerry_init_with_user_context ()`
+- `manager_p`: the manager of this context data item.
+- return value: the item created by `manager_p` when `jerry_get_context_data ()` was first called, or a new item created
+  by `manager_p`, which will be stored for future identical calls to `jerry_get_context_data ()`, and which will be
+  deinitialized using the `deinit_cb` callback provided by `manager_p` when the context will be destroyed.
 
 **Example**
 
 ```c
+typedef struct
 {
-  /* ... */
-  my_context *custom_data = (my_context *) jerry_get_user_context ();
-  /* ... */
+  int my_data1;
+  double my_data2;
+  char *my_data3;
+} my_context_data_t;
+
+/* Define how context items will be initialized. */
+static void
+my_context_data_new (void *user_data_p)
+{
+  my_context_data_t *my_data_p = (my_context_data_t *) user_data_p;
+
+  /*
+   * Initialize my_data_p. JerryScript will store it on the current context and return it whenever
+   * jerry_get_context_data () is called with a pointer to my_manager as defined below.
+   */
 }
-```
 
-**See also**
-- [jerry_init_with_user_context](#jerry_init_with_user_context)
-- [jerry_cleanup](#jerry_cleanup)
+/* Define how context items will be deinitialized */
+static void
+my_context_data_free (void *user_data_p)
+{
+  my_context_data_t *my_data_p = ((my_context_data_t *) user_data_p);
+
+  /* Perform any necessary cleanup on my_data. JerryScript will free the pointer after this function completes. */
+}
+
+/* Wrap the creation and destruction functions into a manager */
+static const jerry_context_data_manager_t my_manager =
+{
+  .init_cb = my_context_data_new,
+  .deinit_cb = my_context_data_free,
+  .bytes_needed = sizeof (my_context_data_t)
+};
+
+/*
+ * Then, in some function in your code, you can retrieve an item of type my_context_data_t from the currently active
+ * context such that JerryScript will create and store such an item if one was not previously created
+ */
+void someplace_in_the_code (void)
+{
+  my_context_data_t *my_data = (my_context_data_t *) jerry_get_context_data (&my_manager);
+  /* Perform useful things using the data found in my_data */
+}
+```
 
 
 ## jerry_register_magic_strings

jerry-core/api/jerry.c

@@ -166,22 +166,6 @@ jerry_init (jerry_init_flag_t flags) /**< combination of Jerry flags */
 #endif /* JERRY_DEBUGGER */
 } /* jerry_init */
 
-/**
- * Initialize Jerry engine with custom user context.
- */
-void
-jerry_init_with_user_context (jerry_init_flag_t flags,  /**< combination of Jerry flags */
-                              jerry_user_context_init_t init_cb, /**< callback to call to create the user context or
-                                                                   *  NULL, in which case no user context will be
-                                                                   *  created */
-                              jerry_user_context_deinit_t deinit_cb) /**< callback to call to free the user context or
-                                                                       *  NULL if it does not need to be freed */
-{
-  jerry_init (flags);
-  JERRY_CONTEXT (user_context_p) = (init_cb ? init_cb () : NULL);
-  JERRY_CONTEXT (user_context_deinit_cb) = deinit_cb;
-} /* jerry_init_with_user_context */
-
 /**
  * Terminate Jerry engine
  */
@@ -190,6 +174,15 @@ jerry_cleanup (void)
 {
   jerry_assert_api_available ();
 
+  for (jerry_context_data_header_t *this_p = JERRY_CONTEXT (context_data_p), *next_p = NULL;
+       this_p != NULL;
+       this_p = next_p)
+  {
+    next_p = this_p->next_p;
+    this_p->manager_p->deinit_cb (JERRY_CONTEXT_DATA_HEADER_USER_DATA (this_p));
+    jmem_heap_free_block (this_p, sizeof (jerry_context_data_header_t) + this_p->manager_p->bytes_needed);
+  }
+
   ecma_finalize ();
 
 #ifdef JERRY_DEBUGGER
@@ -201,23 +194,44 @@ jerry_cleanup (void)
 
   jmem_finalize ();
   jerry_make_api_unavailable ();
-
-  if (JERRY_CONTEXT (user_context_deinit_cb))
-  {
-    JERRY_CONTEXT (user_context_deinit_cb) (JERRY_CONTEXT (user_context_p));
-  }
 } /* jerry_cleanup */
 
 /**
- * Retrieve user context.
+ * Retrieve a context data item, or create a new one.
  *
- * @return the user-provided context-specific pointer
+ * @param manager_p pointer to the manager whose context data item should be returned.
+ *
+ * @return a pointer to the user-provided context-specific data item for the given manager, creating such a pointer if
+ * none was found.
  */
 void *
-jerry_get_user_context (void)
+jerry_get_context_data (const jerry_context_data_manager_t *manager_p)
 {
-  return JERRY_CONTEXT (user_context_p);
-} /* jerry_get_user_context */
+  void *ret = NULL;
+  jerry_context_data_header_t *item_p;
+
+  for (item_p = JERRY_CONTEXT (context_data_p); item_p != NULL; item_p = item_p->next_p)
+  {
+    if (item_p->manager_p == manager_p)
+    {
+      return JERRY_CONTEXT_DATA_HEADER_USER_DATA (item_p);
+    }
+  }
+
+  item_p = jmem_heap_alloc_block (sizeof (jerry_context_data_header_t) + manager_p->bytes_needed);
+  item_p->manager_p = manager_p;
+  item_p->next_p = JERRY_CONTEXT (context_data_p);
+  JERRY_CONTEXT (context_data_p) = item_p;
+  ret = JERRY_CONTEXT_DATA_HEADER_USER_DATA (item_p);
+
+  memset (ret, 0, manager_p->bytes_needed);
+  if (manager_p->init_cb)
+  {
+    manager_p->init_cb (ret);
+  }
+
+  return ret;
+} /* jerry_get_context_data */
 
 /**
  * Register external magic string array

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.

jerry-core/jcontext/jcontext.h

@@ -26,6 +26,7 @@
 #include "jmem.h"
 #include "re-bytecode.h"
 #include "vm-defines.h"
+#include "jerryscript.h"
 
 /** \addtogroup context Context
  * @{
@@ -36,6 +37,18 @@
  */
 #define JERRY_CONTEXT_FIRST_MEMBER ecma_builtin_objects
 
+/**
+ * User context item
+ */
+typedef struct jerry_context_data_header
+{
+  struct jerry_context_data_header *next_p; /**< pointer to next context item */
+  const jerry_context_data_manager_t *manager_p; /**< manager responsible for deleting this item */
+} jerry_context_data_header_t;
+
+#define JERRY_CONTEXT_DATA_HEADER_USER_DATA(item_p) \
+  ((uint8_t *) (item_p + 1))
+
 /**
  * JerryScript context
  *
@@ -63,8 +76,7 @@ typedef struct
   ecma_lit_storage_item_t *number_list_first_p; /**< first item of the literal number list */
   ecma_object_t *ecma_global_lex_env_p; /**< global lexical environment */
   vm_frame_ctx_t *vm_top_context_p; /**< top (current) interpreter context */
-  void *user_context_p; /**< user-provided context-specific pointer */
-  ecma_user_context_deinit_t user_context_deinit_cb; /**< user-provided deleter for context-specific pointer */
+  jerry_context_data_header_t *context_data_p; /**< linked list of user-provided context-specific pointers */
   size_t ecma_gc_objects_number; /**< number of currently allocated objects */
   size_t ecma_gc_new_objects; /**< number of newly allocated objects since last GC session */
   size_t jmem_heap_allocated_size; /**< size of allocated regions */