[Bug #20307] Fix Hash#update to make frozen copy of string keys

hash.c

@@ -3905,19 +3905,10 @@ rb_hash_invert(VALUE hash)
     return h;
 }
 
-static int
-rb_hash_update_callback(st_data_t *key, st_data_t *value, struct update_arg *arg, int existing)
-{
-    *value = arg->arg;
-    return ST_CONTINUE;
-}
-
-NOINSERT_UPDATE_CALLBACK(rb_hash_update_callback)
-
 static int
 rb_hash_update_i(VALUE key, VALUE value, VALUE hash)
 {
-    RHASH_UPDATE(hash, key, rb_hash_update_callback, value);
+    rb_hash_aset(hash, key, value);
     return ST_CONTINUE;
 }
 
@@ -3929,6 +3920,9 @@ rb_hash_update_block_callback(st_data_t *key, st_data_t *value, struct update_ar
     if (existing) {
         newvalue = (st_data_t)rb_yield_values(3, (VALUE)*key, (VALUE)*value, (VALUE)newvalue);
     }
+    else if (RHASH_STRING_KEY_P(arg->hash, *key) && !RB_OBJ_FROZEN(*key)) {
+        *key = rb_hash_key_str(*key);
+    }
     *value = newvalue;
     return ST_CONTINUE;
 }

test/ruby/test_hash.rb

@@ -1268,6 +1268,15 @@ def test_update5
     assert_equal(@cls[a: 10, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8, i: 9, j: 10], h)
   end
 
+  def test_update_on_identhash
+    key = +'a'
+    i = @cls[].compare_by_identity
+    i[key] = 0
+    h = @cls[].update(i)
+    key.upcase!
+    assert_equal(0, h.fetch('a'))
+  end
+
   def test_merge
     h1 = @cls[1=>2, 3=>4]
     h2 = {1=>3, 5=>7}
@@ -1290,21 +1299,21 @@ def test_merge_on_identhash
     expected[7] = 8
     h2 = h.merge(7=>8)
     assert_equal(expected, h2)
-    assert_equal(true, h2.compare_by_identity?)
+    assert_predicate(h2, :compare_by_identity?)
     h2 = h.merge({})
     assert_equal(h, h2)
-    assert_equal(true, h2.compare_by_identity?)
+    assert_predicate(h2, :compare_by_identity?)
 
     h = @cls[]
     h.compare_by_identity
     h1 = @cls[7=>8]
     h1.compare_by_identity
     h2 = h.merge(7=>8)
     assert_equal(h1, h2)
-    assert_equal(true, h2.compare_by_identity?)
+    assert_predicate(h2, :compare_by_identity?)
     h2 = h.merge({})
     assert_equal(h, h2)
-    assert_equal(true, h2.compare_by_identity?)
+    assert_predicate(h2, :compare_by_identity?)
   end
 
   def test_merge!