io_buffer: Reimplement dcompact for IO::Buffer

The `source` field in IO::Buffer can have a String or an IO::Buffer
object, if not nil.

- When the `source` is a String object. The `base` field points to the
  memory location of the String content, which can be embedded in
  RSTRING, and in that case, GC compaction can move the memory region
  along with the String object.

  Thus, IO::Buffer needs to pin the `source` object to prevent `base`
  pointer from becoming invalid.

- When the `source` is an IO::Buffer, then `base` is a pointer to a
  malloced or mmapped memory region, managed by the source IO::Buffer.
  In this case, we don't need to pin the source IO::Buffer object,
  since the referred memory region won't get moved by GC.

Closes: [Bug #21210]

io_buffer.c

@@ -273,10 +273,18 @@ io_buffer_free(struct rb_io_buffer *buffer)
 }
 
 static void
-rb_io_buffer_type_mark(void *_buffer)
+rb_io_buffer_type_mark_and_move(void *_buffer)
 {
     struct rb_io_buffer *buffer = _buffer;
-    rb_gc_mark(buffer->source);
+    if (buffer->source != Qnil) {
+        if (RB_TYPE_P(buffer->source, T_STRING)) {
+            // The `source` String has to be pinned, because the `base` may point to the embedded String content,
+            // which can be otherwise moved by GC compaction.
+            rb_gc_mark(buffer->source);
+        } else {
+            rb_gc_mark_and_move(&buffer->source);
+        }
+    }
 }
 
 static void
@@ -303,9 +311,10 @@ rb_io_buffer_type_size(const void *_buffer)
 static const rb_data_type_t rb_io_buffer_type = {
     .wrap_struct_name = "IO::Buffer",
     .function = {
-        .dmark = rb_io_buffer_type_mark,
+        .dmark = rb_io_buffer_type_mark_and_move,
         .dfree = rb_io_buffer_type_free,
         .dsize = rb_io_buffer_type_size,
+        .dcompact = rb_io_buffer_type_mark_and_move,
     },
     .data = NULL,
     .flags = RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_EMBEDDABLE,

test/ruby/test_io_buffer.rb

@@ -693,4 +693,17 @@ class TestIOBuffer < Test::Unit::TestCase
     buf.set_string('a', 0, 0)
     assert_predicate buf, :empty?
   end
+
+  # https://bugs.ruby-lang.org/issues/21210
+  def test_bug_21210
+    omit "compaction is not supported on this platform" unless GC.respond_to?(:compact)
+
+    str = +"hello"
+    buf = IO::Buffer.for(str)
+    assert_predicate buf, :valid?
+
+    GC.verify_compaction_references(expand_heap: true, toward: :empty)
+
+    assert_predicate buf, :valid?
+  end
 end