[ruby-core:127111] [Ruby Bug#22439] Segfault in `Thread::Backtrace::Location#syntax_tree` (via `Exception#detailed_message` / error_highlight) when called from multiple Ractors

14 Oct 2026

      Issue #22439 has been updated by mame (Yusuke Endoh).

Thank you for the report!

I could reproduce the SEGV only with GCC 16 (GCC 13 and 15 did not reproduce it). Are you also using GCC 16 to build Ruby?

This ticket contained two different problems, so I have split it into:

* #22440: SEGV when referencing an autoload constant from multiple Ractors (`Ruby::Prism` is autoloaded, which caused the crash here)
* #22442: the lazy loading of error_highlight etc. is skipped in threads and Ractors (which is why the snippet was not shown)

If possible, I'd appreciate it if you could test with both patches attached to those tickets applied together.

----------------------------------------
Bug #22439: Segfault in `Thread::Backtrace::Location#syntax_tree` (via `Exception#detailed_message` / error_highlight) when called from multiple Ractors
https://bugs.ruby-lang.org/issues/22439#change-119511

* Author: amalrik (amalrik maia)
* Status: Open
* ruby -v: ruby 4.1.0dev (2026-10-14T14:33:24Z master b709bdb3e3) +PRISM [x86_64-linux]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
Calling `Exception#detailed_message` concurrently from two or more Ractors crashes the VM with
`[BUG] Segmentation fault` on 4.1.0-preview1 and current master. The same script works on 4.0.6
and 4.0.7.

### Reproduction

No gems are involved; error_highlight is loaded by default.

```ruby
n = Integer(ARGV[0] || 2)
rs = Array.new(n) do |i|
  Ractor.new(i) do |id|
    500.times do |k|
      begin
        raise ArgumentError, "falha #{id}-#{k}"
      rescue => e
        e.detailed_message(highlight: false)
      end
    end
    :ok
  end
end
p rs.map(&:value)
```

```
$ ruby repro.rb
<internal:ast>:374: [BUG] Segmentation fault at 0x0000000000000030
```

| Version                      | Result                                                       |
| ---------------------------- | ------------------------------------------------------------ |
| 4.0.6, 4.0.7                 | `[:ok, :ok]` (8/8 runs)                                      |
| 4.1.0-preview1 (2d08b5f42f)  | segfault in 4/4 runs with 2 Ractors and 8/8 with 8 Ractors; no crash with 1 Ractor |
| 4.1.0dev master (b709bdb3e3) | segfault 6/6                                                 |

### Backtrace (master)

```
<internal:ast>:374: [BUG] Segmentation fault at 0x0000000000000030
-- Ruby level backtrace information ----------------------------------------
repro.rb:8:in 'detailed_message'
.../lib/ruby/4.1.0+5/error_highlight/core_ext.rb:43:in 'detailed_message'
.../lib/ruby/4.1.0+5/error_highlight/core_ext.rb:34:in 'generate_snippet'
.../lib/ruby/4.1.0+5/error_highlight/base.rb:57:in 'spot'
.../lib/ruby/4.1.0+5/error_highlight/base.rb:77:in 'find_node_from_loc'
<internal:ast>:478:in 'syntax_tree'
<internal:ast>:374:in 'syntax_tree'

-- C level backtrace information -------------------------------------------
rb_vm_bugreport
rb_ec_ractor_ptr / rb_current_execution_context / rb_current_ractor_raw / rb_current_ractor
vm_ic_update
rb_vm_opt_getconstant_path
vm_exec_core
rb_vm_exec
thread_start_func_2
```

The full output is attached (`crash-output-master.txt`).

### Additional observations

1. **The crash is reachable without error_highlight.** Calling
   `e.backtrace_locations.first.syntax_tree` inside `rescue` in 2 Ractors gives the same crash and
   stack. It crashed 1/3 runs on master and 0/3 on preview1, so the `detailed_message` script above
   is the more reliable reproducer.

2. **Some things that look similar do not crash.**

   - `caller_locations(0, 1).first.syntax_tree` in 2 Ractors works.
   - `Ruby::Prism.parse("1 + 1")` in 2 Ractors works.

   So the problem seems specific to backtrace locations taken from an exception, while the
   constant cache in `ast.rb` is updated from a non-main Ractor.

3. **With a single Ractor there is no crash, but error_highlight output regresses.** Inside the
   Ractor, `detailed_message` returns only the message, without the snippet:

   ```ruby
   r = Ractor.new do
     begin
       nil.upcase
     rescue => e
       e.detailed_message(highlight: false)
     end
   end
   puts r.value
   # 4.0.7:  undefined method 'upcase' for nil (NoMethodError)\n\n    nil.upcase\n       ^^^^^^^
   # 4.1:    undefined method 'upcase' for nil (NoMethodError)
   ```

   On 4.1 the snippet is still shown in the main Ractor.

4. **This is easy to hit in practice.** Any library that logs exceptions with `detailed_message`
   from inside Ractors can crash the process. I found it with the `async` gem, whose task-failure
   logging (via the `console` gem) calls `detailed_message`, running one reactor per Ractor. On
   4.0.6 that same setup hangs instead of crashing; I have not diagnosed the 4.0.6 hang.

### Related (but different) issues

- #22386: segfault when a Ractor helper thread reports an exception during teardown. It was
  fixed before preview1, and this still reproduces on preview1 and master.
- #21183: error_highlight raising `Ractor::IsolationError` in non-main Ractors (fixed in 2025).

### Environment and context

- Linux x86_64 (kernel 7.2, AMD Ryzen 7 5700U, 8C/16T). Also seen on Ubuntu 24.04, AMD EPYC
  Genoa (KVM).
  - These scripts come out of a benchmark study comparing threads, Ractors and Fibers on Ruby 4.0 and
    4.1. The code, raw results and other Ractor findings (I/O concurrency cap on 4.0.x, socket hangs
    on 4.0.x) are public at: https://github.com/amalrik/ruby-ractors-study. The reproducers for this issue are in
    `upstream/ruby-4.1-detailed-message-ractors/`

-- 
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