[ruby/fiddle] Add a "pinning" reference (#44)

* Add a "pinning" reference

A `Fiddle::Pinned` objects will prevent the objects they point to from
moving.  This is useful in the case where you need to pass a reference
to a C extension that keeps the address in a global and needs the
address to be stable.

For example:

```ruby
class Foo
  A = "hi" # this is an embedded string

  some_c_function A # A might move!
end
```

If `A` moves, then the underlying string buffer may also move.
`Fiddle::Pinned` will prevent the object from moving:

```ruby
class Foo
  A = "hi" # this is an embedded string

  A_pinner = Fiddle::Pinned.new(A) # :nodoc:

  some_c_function A # A can't move because of `Fiddle::Pinned`
end
```

This is a similar strategy to what Graal uses:

  https://www.graalvm.org/sdk/javadoc/org/graalvm/nativeimage/PinnedObject.html#getObject--

* rename global to match exception name

* Introduce generic Fiddle::Error and rearrange error classes

Fiddle::Error is the generic exception base class for Fiddle exceptions.
This commit introduces the class and rearranges Fiddle exceptions to
inherit from it.

https://github.com/ruby/fiddle/commit/ac52d00223

1 files changed, 3 insertions, 3 deletions

diff --git a/ext/fiddle/pointer.c b/ext/fiddle/pointer.c
index 7c60da477a..b531befd6e 100644
--- a/ext/fiddle/pointer.c
+++ b/ext/fiddle/pointer.c
@@ -561,7 +561,7 @@ rb_fiddle_ptr_aref(int argc, VALUE argv[], VALUE self)
     struct ptr_data *data;
 
     TypedData_Get_Struct(self, struct ptr_data, &fiddle_ptr_data_type, data);
-    if (!data->ptr) rb_raise(rb_eFiddleError, "NULL pointer dereference");
+    if (!data->ptr) rb_raise(rb_eFiddleDLError, "NULL pointer dereference");
     switch( rb_scan_args(argc, argv, "11", &arg0, &arg1) ){
       case 1:
 	offset = NUM2ULONG(arg0);
@@ -599,7 +599,7 @@ rb_fiddle_ptr_aset(int argc, VALUE argv[], VALUE self)
     struct ptr_data *data;
 
     TypedData_Get_Struct(self, struct ptr_data, &fiddle_ptr_data_type, data);
-    if (!data->ptr) rb_raise(rb_eFiddleError, "NULL pointer dereference");
+    if (!data->ptr) rb_raise(rb_eFiddleDLError, "NULL pointer dereference");
     switch( rb_scan_args(argc, argv, "21", &arg0, &arg1, &arg2) ){
       case 2:
 	offset = NUM2ULONG(arg0);
@@ -680,7 +680,7 @@ rb_fiddle_ptr_s_to_ptr(VALUE self, VALUE val)
 	    wrap = 0;
 	}
 	else{
-	    rb_raise(rb_eFiddleError, "to_ptr should return a Fiddle::Pointer object");
+	    rb_raise(rb_eFiddleDLError, "to_ptr should return a Fiddle::Pointer object");
 	}
     }
     else{