[ruby-core:121015] [Ruby master Bug#21096] `Process.fork` hangs up on QEMU when called multiple times.

Issue #21096 has been updated by midnight (Sarun R).

Subject changed from Ruby hangs up when compiling for bytecode on AArch64 emulated by QEMU to `Process.fork` hangs up on QEMU when called multiple times.

Hello, I made progress somehow and the issue has been isolated.
Here is the minimal reproduction code without projects, 3rd party gems, or external dependencies apart from MRI on Linux/QEMU.

~~~Ruby
$stdout.sync = true
pid_list = 4.times.map do
  Process.fork do
    puts 'success!'
    exit!(true)
  end
end

pid_list.each do |pid|
  Process.wait2(pid)
end
~~~

On QEMU, you'll get only one `success!` printed and the Ruby interpreter hangs.

----------------------------------------
Bug #21096: `Process.fork` hangs up on QEMU when called multiple times.
https://bugs.ruby-lang.org/issues/21096#change-111923

* Author: midnight (Sarun R)
* Status: Open
* ruby -v: ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Hello,

We are experiencing issues when using [Bootsnap](https://github.com/Shopify/bootsnap) for production container image building, specifically when running `bundle exec bootsnap precompile --gemfile` on an emulated ARM64 environment on AMD64 hosts.

Here are more details:
* Bootsnap is a Ruby gem for bytecode caching, which speeds up loading times. It achieves this by calling `RubyVM::InstructionSequence.compile_file` and `RubyVM::InstructionSequence.load_from_binary`.
* When running `bundle exec bootsnap precompile --gemfile` in the environment described (using QEMU to emulate the AArch64 instruction set), the process can compile and generate some bytecode but eventually hangs.
* The hang seems random, occurring at different points during the process.
* According to [a Rails GitHub issue](https://github.com/rails/rails/issues/54039), the issue affects both `Docker` and `containerd`, as well as native Linux and macOS environments.
* From [a Bootsnap GitHub issue](https://github.com/Shopify/bootsnap/issues/495), the problem likely doesn't occur in Ruby `3.2.6` but appears in later versions.
* So far, the common factors observed are:
  * Emulated ARM64 on AMD64 CPUs (likely using QEMU)
  * Newer Ruby versions (e.g., 3.3 and 3.4)

As a user, I don't have the expertise to debug the issue but am willing to gather more information if provided with sufficient instructions.



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