merge revision(s) 3f9562015e651735bfc2fdd14e8f6963b673e22a,c06ddfee878524168e4af07443217ed2f8d0954b,3b3b4a44e57dfe03ce3913009d69a33d6f6100be: [Backport #15792]

	Get rid of indirect sharing

	* string.c (str_duplicate): share the root shared string if the
	  original string is already sharing, so that all shared strings
	  refer the root shared string directly.  indirect sharing can
	  cause a dangling pointer.

	[Bug #15792]

	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

	Update dependencies


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67731 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

2 files changed, 25 insertions, 0 deletions

diff --git a/test/-ext-/string/test_rb_str_dup.rb b/test/-ext-/string/test_rb_str_dup.rb
new file mode 100644
index 0000000000..49b6af9598
--- /dev/null
+++ b/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
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 1fa5126931..d50380aaf6 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -2972,6 +2972,15 @@ CODE
   end
 =end
 
+  def test_nesting_shared
+    a = ('a' * 24).encode(Encoding::ASCII).gsub('x', '')
+    hash = {}
+    hash[a] = true
+    assert_equal(('a' * 24), a)
+    4.times { GC.start }
+    assert_equal(('a' * 24), a, '[Bug #15792]')
+  end
+
   def test_shared_force_encoding
     s = "\u{3066}\u{3059}\u{3068}".gsub(//, '')
     h = {}