From ec8e5f5aa64e2a54cf1e303f2b012c98e8d521ba Mon Sep 17 00:00:00 2001
From: Luke Gruber <luke.gru@gmail.com>
Date: Sat, 22 Jun 2019 18:37:08 -0400
Subject: [PATCH] array.c: always check frozenness in Array#unshift. Fixes [Bug
 #15952]

Closes: https://github.com/ruby/ruby/pull/2251
---
 array.c                 |  3 ++-
 test/ruby/test_array.rb | 11 +++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/array.c b/array.c
index 8501074ebe..2d68b608c0 100644
--- a/array.c
+++ b/array.c
@@ -1373,6 +1373,8 @@ ary_ensure_room_for_unshift(VALUE ary, int argc)
 	rb_raise(rb_eIndexError, "index %ld too big", new_len);
     }
 
+    rb_ary_modify(ary);
+
     if (ARY_SHARED_P(ary)) {
 	VALUE shared = ARY_SHARED(ary);
 	capa = RARRAY_LEN(shared);
@@ -1383,7 +1385,6 @@ ary_ensure_room_for_unshift(VALUE ary, int argc)
 	}
     }
 
-    rb_ary_modify(ary);
     capa = ARY_CAPA(ary);
     if (capa - (capa >> 6) <= new_len) {
 	ary_double_capa(ary, new_len);
diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 88d61b79e0..b1778d191c 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -1958,6 +1958,17 @@ class TestArray < Test::Unit::TestCase
     assert_equal(@cls[@cls[1,2], nil, 'dog', 'cat'], a.unshift(@cls[1, 2]))
   end
 
+  def test_unshift_frozen
+    bug15952 = '[Bug #15952]'
+    assert_raise(FrozenError, bug15952) do
+      a = [1] * 100
+      b = a[4..-1]
+      a.replace([1])
+      b.freeze
+      b.unshift("a")
+    end
+  end
+
   def test_OR # '|'
     assert_equal(@cls[],  @cls[]  | @cls[])
     assert_equal(@cls[1], @cls[1] | @cls[])