Fix compaction during ary_make_partial

The ary_make_shared call may allocate, which can trigger a GC
compaction. This can cause the array to be embedded because it has a
length of 0.

array.c

@@ -1204,7 +1204,10 @@ ary_make_partial(VALUE ary, VALUE klass, long offset, long len)
     else {
         VALUE shared = ary_make_shared(ary);
 
-        assert(!ARY_EMBED_P(result));
+        /* The ary_make_shared call may allocate, which can trigger a GC
+         * compaction. This can cause the array to be embedded because it has
+         * a length of 0. */
+        FL_UNSET_EMBED(result);
 
         ARY_SET_PTR(result, RARRAY_CONST_PTR(ary));
         ARY_SET_LEN(result, RARRAY_LEN(ary));

test/ruby/test_array.rb

@@ -1693,6 +1693,10 @@ class TestArray < Test::Unit::TestCase
     assert_equal([100], a.slice(-1, 1_000_000_000))
   end
 
+  def test_slice_gc_compact_stress
+    EnvUtil.under_gc_compact_stress { assert_equal([1, 2, 3, 4, 5], (0..10).to_a[1, 5]) }
+  end
+
   def test_slice!
     a = @cls[1, 2, 3, 4, 5]
     assert_equal(3, a.slice!(2))

tool/lib/envutil.rb

@@ -245,6 +245,15 @@ module EnvUtil
   end
   module_function :under_gc_stress
 
+  def under_gc_compact_stress(&block)
+    auto_compact = GC.auto_compact
+    GC.auto_compact = true
+    under_gc_stress(&block)
+  ensure
+    GC.auto_compact = auto_compact
+  end
+  module_function :under_gc_compact_stress
+
   def with_default_external(enc)
     suppress_warning { Encoding.default_external = enc }
     yield