Implement the last missing Write Barriers and dsize

Ref: https://github.com/ffi/ffi/pull/991

Write barrier protected objects are allowed to be promoted to the old generation,
which means they only get marked on major GC.

The downside is that the RB_BJ_WRITE macro MUST be used to set references,
otherwise the referenced object may be garbaged collected.

This commit also implement a `dsize` function so that these instance report
a more relevant size in various memory profilers. It's not counting everything
because some types are opaque right now, so a larger refactoring would be needed.

3 files changed, 36 insertions, 9 deletions

diff --git a/ext/ffi_c/ArrayType.c b/ext/ffi_c/ArrayType.c
index f6e6fc4..1d766a4 100644
--- a/ext/ffi_c/ArrayType.c
+++ b/ext/ffi_c/ArrayType.c
@@ -35,16 +35,19 @@ static VALUE array_type_s_allocate(VALUE klass);
 static VALUE array_type_initialize(VALUE self, VALUE rbComponentType, VALUE rbLength);
 static void array_type_mark(void *);
 static void array_type_free(void *);
+static size_t array_type_memsize(const void *);
 
 const rb_data_type_t rbffi_array_type_data_type = { /* extern */
   .wrap_struct_name = "FFI::ArrayType",
   .function = {
       .dmark = array_type_mark,
       .dfree = array_type_free,
-      .dsize = NULL,
+      .dsize = array_type_memsize,
   },
   .parent = &rbffi_type_data_type,
-  .flags = RUBY_TYPED_FREE_IMMEDIATELY
+  // IMPORTANT: WB_PROTECTED objects must only use the RB_OBJ_WRITE()
+  // macro to update VALUE references, as to trigger write barriers.
+  .flags = RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED
 };
 
 
@@ -63,7 +66,7 @@ array_type_s_allocate(VALUE klass)
     array->base.ffiType->type = FFI_TYPE_STRUCT;
     array->base.ffiType->size = 0;
     array->base.ffiType->alignment = 0;
-    array->rbComponentType = Qnil;
+    RB_OBJ_WRITE(obj, &array->rbComponentType, Qnil);
 
     return obj;
 }
@@ -84,6 +87,15 @@ array_type_free(void *data)
     xfree(array);
 }
 
+static size_t
+array_type_memsize(const void *data)
+{
+    const ArrayType *array = (const ArrayType *)data;
+    size_t memsize = sizeof(ArrayType);
+    memsize += array->length * sizeof(*array->ffiTypes);
+    memsize += sizeof(*array->base.ffiType);
+    return memsize;
+}
 
 /*
  * call-seq: initialize(component_type, length)
@@ -101,7 +113,7 @@ array_type_initialize(VALUE self, VALUE rbComponentType, VALUE rbLength)
     TypedData_Get_Struct(self, ArrayType, &rbffi_array_type_data_type, array);
 
     array->length = NUM2UINT(rbLength);
-    array->rbComponentType = rbComponentType;
+    RB_OBJ_WRITE(self, &array->rbComponentType, rbComponentType);
     TypedData_Get_Struct(rbComponentType, Type, &rbffi_type_data_type, array->componentType);
 
     array->ffiTypes = xcalloc(array->length + 1, sizeof(*array->ffiTypes));
diff --git a/ext/ffi_c/DynamicLibrary.c b/ext/ffi_c/DynamicLibrary.c
index c69074a..2e0bf4b 100644
--- a/ext/ffi_c/DynamicLibrary.c
+++ b/ext/ffi_c/DynamicLibrary.c
@@ -56,7 +56,7 @@ typedef struct LibrarySymbol_ {
 
 static VALUE library_initialize(VALUE self, VALUE libname, VALUE libflags);
 static void library_free(void *);
-
+static size_t library_memsize(const void *);
 
 static VALUE symbol_allocate(VALUE klass);
 static VALUE symbol_new(VALUE library, void* address, VALUE name);
@@ -68,9 +68,11 @@ static const rb_data_type_t rbffi_library_data_type = {
     .function = {
         .dmark = NULL,
         .dfree = library_free,
-        .dsize = NULL,
+        .dsize = library_memsize,
     },
-    .flags = RUBY_TYPED_FREE_IMMEDIATELY
+    // IMPORTANT: WB_PROTECTED objects must only use the RB_OBJ_WRITE()
+    // macro to update VALUE references, as to trigger write barriers.
+    .flags = RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED
 };
 
 static const rb_data_type_t library_symbol_data_type = {
@@ -200,6 +202,12 @@ library_free(void *data)
     xfree(library);
 }
 
+static size_t
+library_memsize(const void *data)
+{
+    return sizeof(Library);
+}
+
 #if (defined(_WIN32) || defined(__WIN32__)) && !defined(__CYGWIN__)
 static void*
 dl_open(const char* name, int flags)
diff --git a/ext/ffi_c/LastError.c b/ext/ffi_c/LastError.c
index 4cef93b..d5ef132 100644
--- a/ext/ffi_c/LastError.c
+++ b/ext/ffi_c/LastError.c
@@ -91,14 +91,21 @@ thread_data_free(void *ptr)
 }
 
 #else
+static size_t
+thread_data_memsize(const void *data) {
+    return sizeof(ThreadData);
+}
+
 static const rb_data_type_t thread_data_data_type = {
     .wrap_struct_name = "FFI::ThreadData",
     .function = {
         .dmark = NULL,
         .dfree = RUBY_TYPED_DEFAULT_FREE,
-        .dsize = NULL,
+        .dsize = thread_data_memsize,
     },
-    .flags = RUBY_TYPED_FREE_IMMEDIATELY
+    // IMPORTANT: WB_PROTECTED objects must only use the RB_OBJ_WRITE()
+    // macro to update VALUE references, as to trigger write barriers.
+    .flags = RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED
 };
 
 static ID id_thread_data;