From 58e82206057f2a1644b69ac3631016009ae48590 Mon Sep 17 00:00:00 2001
From: Alan Wu <XrXr@users.noreply.github.com>
Date: Fri, 12 Feb 2021 22:45:08 -0500
Subject: [PATCH] Check for cyclic prepend before making origin

It's important to only make the origin when the prepend goes
through, as the precense of the origin informs whether to do an
origin backfill.

This plus 2d877327e fix [Bug #17590].
---
 class.c                  | 56 +++++++++++++++++++++++++---------------
 test/ruby/test_module.rb | 13 ++++++++++
 2 files changed, 48 insertions(+), 21 deletions(-)

diff --git a/class.c b/class.c
index 12a67d16bc..68cfbfb257 100644
--- a/class.c
+++ b/class.c
@@ -351,7 +351,7 @@ copy_tables(VALUE clone, VALUE orig)
     }
 }
 
-static void ensure_origin(VALUE klass);
+static bool ensure_origin(VALUE klass);
 
 /* :nodoc: */
 VALUE
@@ -1014,27 +1014,31 @@ clear_module_cache_i(ID id, VALUE val, void *data)
     return ID_TABLE_CONTINUE;
 }
 
+static bool
+module_in_super_chain(const VALUE klass, VALUE module)
+{
+    struct rb_id_table *const klass_m_tbl = RCLASS_M_TBL(RCLASS_ORIGIN(klass));
+    if (klass_m_tbl) {
+        while (module) {
+            if (klass_m_tbl == RCLASS_M_TBL(module))
+                return true;
+            module = RCLASS_SUPER(module);
+        }
+    }
+    return false;
+}
+
 static int
-include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
+do_include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super, bool check_cyclic)
 {
     VALUE p, iclass, origin_stack = 0;
     int method_changed = 0, constant_changed = 0, add_subclass;
     long origin_len;
     VALUE klass_origin = RCLASS_ORIGIN(klass);
-    struct rb_id_table *const klass_m_tbl = RCLASS_M_TBL(klass_origin);
     VALUE original_klass = klass;
 
-    if (klass_m_tbl) {
-        VALUE original_module = module;
-
-        while (module) {
-            if (klass_m_tbl == RCLASS_M_TBL(module))
-                return -1;
-            module = RCLASS_SUPER(module);
-        }
-
-        module = original_module;
-    }
+    if (check_cyclic && module_in_super_chain(klass, module))
+        return -1;
 
     while (module) {
         int c_seen = FALSE;
@@ -1129,6 +1133,12 @@ include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
     return method_changed;
 }
 
+static int
+include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
+{
+    return do_include_modules_at(klass, c, module, search_super, true);
+}
+
 static enum rb_id_table_iterator_result
 move_refined_method(ID key, VALUE value, void *data)
 {
@@ -1169,7 +1179,7 @@ cache_clear_refined_method(ID key, VALUE value, void *data)
     return ID_TABLE_CONTINUE;
 }
 
-static void
+static bool
 ensure_origin(VALUE klass)
 {
     VALUE origin = RCLASS_ORIGIN(klass);
@@ -1182,20 +1192,24 @@ ensure_origin(VALUE klass)
 	RCLASS_M_TBL_INIT(klass);
         rb_id_table_foreach(RCLASS_M_TBL(origin), cache_clear_refined_method, (void *)klass);
 	rb_id_table_foreach(RCLASS_M_TBL(origin), move_refined_method, (void *)klass);
+        return true;
     }
+    return false;
 }
 
 void
 rb_prepend_module(VALUE klass, VALUE module)
 {
-    int changed = 0;
-    bool klass_had_no_origin = RCLASS_ORIGIN(klass) == klass;
+    int changed;
+    bool klass_had_no_origin;
 
     ensure_includable(klass, module);
-    ensure_origin(klass);
-    changed = include_modules_at(klass, klass, module, FALSE);
-    if (changed < 0)
-	rb_raise(rb_eArgError, "cyclic prepend detected");
+    if (module_in_super_chain(klass, module))
+        rb_raise(rb_eArgError, "cyclic prepend detected");
+
+    klass_had_no_origin = ensure_origin(klass);
+    changed = do_include_modules_at(klass, klass, module, FALSE, false);
+    RUBY_ASSERT(changed >= 0); // already checked for cyclic prepend above
     if (changed) {
 	rb_vm_check_redefinition_by_prepend(klass);
     }
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index 0a5597fd6c..e5152b1012 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -485,6 +485,19 @@ class TestModule < Test::Unit::TestCase
     assert_equal([m], m.ancestors)
   end
 
+  def test_bug17590
+    m = Module.new
+    c = Class.new
+    c.prepend(m)
+    c.include(m)
+    m.prepend(m) rescue nil
+    m2 = Module.new
+    m2.prepend(m)
+    c.include(m2)
+
+    assert_equal([m, c, m2] + Object.ancestors, c.ancestors)
+  end
+
   def test_prepend_works_with_duped_classes
     m = Module.new
     a = Class.new do