[ruby-core:106258] [Ruby master Bug#18154] String#initialize leaks memory for STR_NOFREE strings

Issue #18154 has been updated by usa (Usaku NAKAMURA).

Backport changed from 2.6: REQUIRED, 2.7: REQUIRED, 3.0: DONE to 2.6: REQUIRED, 2.7: DONE, 3.0: DONE

ruby_2_7 d55426f800546cbc3b333ae7ab98c1893f710612 merged revision(s) 5d815542815fe8b939239750bba7f8f0b79c97d6.

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Bug #18154: String#initialize leaks memory for STR_NOFREE strings
https://bugs.ruby-lang.org/issues/18154#change-94878

* Author: peterzhu2118 (Peter Zhu)
* Status: Closed
* Priority: Normal
* Backport: 2.6: REQUIRED, 2.7: DONE, 3.0: DONE
----------------------------------------
# GitHub PR: https://github.com/ruby/ruby/pull/4814

There is a memory leak in calling the constructor on a string that is marked `STR_NOFREE` (e.g. a string created from a C string literal). The script below reproduces the memory leak. This is reproducible on all maintained Rubies (2.6.8, 2.7.4, 3.0.2, master) on Ubuntu 20.04.

We create a string marked `STR_NOFREE` with `0.to_s`. `to_s` for Fixnum has a [special optimization](https://github.com/ruby/ruby/blob/26153667f91f0c883f6af6b61fac2c0df5312b45/numeric.c#L3393) for the value `0` (it directly converts it to a C string literal). When we call `String#initialize` with a capacity it creates a buffer using `malloc` but does not unset the `STR_NOFREE` flag. This causes the buffer to be permanently leaked.

```ruby
100.times do
  1000.times do
    # 0.to_s is a special case that creates a string from a C string literal.
    # https://github.com/ruby/ruby/blob/26153667f91f0c883f6af6b61fac2c0df5312b45/numeric.c#L3393
    # C string literals are always marked STR_NOFREE.
    str = 0.to_s
    # Call String#initialize again to create a buffer with a capacity of 10000
    # characters.
    str.send(:initialize, capacity: 10000)
  end

  # Output the Resident Set Size (memory usage, in KB) of the current Ruby process.
  puts `ps -o rss= -p #{$$}`
end
```

We can see the leak through the following graph of the Resident Set Size (RSS) comparing the branch vs. master (at commit 26153667f91f0c883f6af6b61fac2c0df5312b45).

![](https://user-images.githubusercontent.com/15860699/132392215-9686259e-8c76-4fc9-9b63-427b89f8df2c.png)




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