str_duplicate: Don't share with a frozen shared string

This is a follow up for 3f9562015e651735bfc2fdd14e8f6963b673e22a.
Before this commit, it was possible to create a shared string which
shares with another shared string by passing a frozen shared string
to `str_duplicate`.

Such string looks like:

```
 --------                    -----------------
 | root | ------ owns -----> | root's buffer |
 --------                    -----------------
     ^                             ^   ^
 -----------                       |   |
 | shared1 | ------ references -----   |
 -----------                           |
     ^                                 |
 -----------                           |
 | shared2 | ------ references ---------
 -----------
```

This is bad news because `rb_fstring(shared2)` can make `shared1`
independent, which severs the reference from `shared1` to `root`:

```c
/* from fstr_update_callback() */
str = str_new_frozen(rb_cString, shared2);  /* can return shared1 */
if (STR_SHARED_P(str)) { /* shared1 is also a shared string */
    str_make_independent(str);  /* no frozen check */
}
```

If `shared1` was the only reference to `root`, then `root` can be
reclaimed by the GC, leaving `shared2` in a corrupted state:

```
 -----------                         --------------------
 | shared1 | -------- owns --------> | shared1's buffer |
 -----------                         --------------------
      ^
      |
 -----------                         -------------------------
 | shared2 | ------ references ----> | root's buffer (freed) |
 -----------                         -------------------------
```

Here is a reproduction script for the situation this commit fixes.

```ruby
a = ('a' * 24).strip.freeze.strip
-a
p a
4.times { GC.start }
p a
```

 - string.c (str_duplicate): always share with the root string when
   the original is a shared string.
 - test_rb_str_dup.rb: specifically test `rb_str_dup` to make
   sure it does not try to share with a shared string.

[Bug #15792]

Closes: https://github.com/ruby/ruby/pull/2159

ext/-test-/string/rb_str_dup.c

@@ -0,0 +1,35 @@
+#include "ruby.h"
+
+VALUE rb_str_dup(VALUE str);
+
+static VALUE
+bug_rb_str_dup(VALUE self, VALUE str)
+{
+    rb_check_type(str, T_STRING);
+    return rb_str_dup(str);
+}
+
+static VALUE
+bug_shared_string_p(VALUE self, VALUE str)
+{
+    rb_check_type(str, T_STRING);
+    return RB_FL_TEST(str, RUBY_ELTS_SHARED) && RB_FL_TEST(str, RSTRING_NOEMBED) ? Qtrue : Qfalse;
+}
+
+static VALUE
+bug_sharing_with_shared_p(VALUE self, VALUE str)
+{
+    rb_check_type(str, T_STRING);
+    if (bug_shared_string_p(self, str)) {
+        return bug_shared_string_p(self, RSTRING(str)->as.heap.aux.shared);
+    }
+    return Qfalse;
+}
+
+void
+Init_string_rb_str_dup(VALUE klass)
+{
+    rb_define_singleton_method(klass, "rb_str_dup", bug_rb_str_dup, 1);
+    rb_define_singleton_method(klass, "shared_string?", bug_shared_string_p, 1);
+    rb_define_singleton_method(klass, "sharing_with_shared?", bug_sharing_with_shared_p, 1);
+}

string.c

@@ -1504,15 +1504,13 @@ str_duplicate(VALUE klass, VALUE str)
     MEMCPY(RSTRING(dup)->as.ary, RSTRING(str)->as.ary,
 	   char, embed_size);
     if (flags & STR_NOEMBED) {
-	if (UNLIKELY(!(flags & FL_FREEZE))) {
-            if (FL_TEST_RAW(str, STR_SHARED)) {
-                str = RSTRING(str)->as.heap.aux.shared;
-            }
-            else {
-                str = str_new_frozen(klass, str);
-                FL_SET_RAW(str, flags & FL_TAINT);
-                flags = FL_TEST_RAW(str, flag_mask);
-            }
+        if (FL_TEST_RAW(str, STR_SHARED)) {
+            str = RSTRING(str)->as.heap.aux.shared;
+        }
+        else if (UNLIKELY(!(flags & FL_FREEZE))) {
+            str = str_new_frozen(klass, str);
+            FL_SET_RAW(str, flags & FL_TAINT);
+            flags = FL_TEST_RAW(str, flag_mask);
 	}
 	if (flags & STR_NOEMBED) {
 	    RB_OBJ_WRITE(dup, &RSTRING(dup)->as.heap.aux.shared, str);

test/-ext-/string/test_rb_str_dup.rb

@@ -0,0 +1,16 @@
+require 'test/unit'
+require '-test-/string'
+
+class Test_RbStrDup < Test::Unit::TestCase
+  def test_nested_shared_non_frozen
+    str = Bug::String.rb_str_dup(Bug::String.rb_str_dup("a" * 50))
+    assert_send([Bug::String, :shared_string?, str])
+    assert_not_send([Bug::String, :sharing_with_shared?, str], '[Bug #15792]')
+  end
+
+  def test_nested_shared_frozen
+    str = Bug::String.rb_str_dup(Bug::String.rb_str_dup("a" * 50).freeze)
+    assert_send([Bug::String, :shared_string?, str])
+    assert_not_send([Bug::String, :sharing_with_shared?, str], '[Bug #15792]')
+  end
+end