Thread#join no longer runs the deferred body (single-threaded hang fix); Process.detach reaps via a dedicated waiter

[sisshiki1969]

Summary

monoruby is single-threaded and emulates Thread by deferring the block and running it lazily. Thread#join previously ran that deferred body synchronously — which hangs the entire VM whenever a worker's body loops until another thread stops it.

This is the standard ruby/spec core/thread pattern:

t = Thread.new { loop { Thread.pass } }   # runs until killed
status = Status.new(t)                     # capture status
t.kill                                      # no-op in a single-threaded VM
t.join                                      # ← ran loop { Thread.pass } → HANG

There is no other thread to break the loop, so the join spins forever and takes the whole mspec run down with it. Root cause: pseudo-simulating multithreading in a runtime that has none.

Changes

1. Thread#join reports completion and returns self without executing the body.

• A thread whose result is actually needed is still run lazily by #value.
• Bodies driven cooperatively by Thread.pass (the Thread.pass until flag idiom) are unaffected.
• The class stays intact, so RubyGems / Bundler keep loading.
• This trades a hang for a plain failure on "assert a side effect after #join" specs — the correct outcome for a runtime with no real concurrency.

2. Process.detach returns a Thread::Waiter that still reaps on #join/#value.
Making #join a no-op regressed the Process.detach(pid).join reap-and-wait idiom (detach relied on join running its Process.wait2 body). Reaping one specific child is a terminating operation, so the waiter safely runs it synchronously — unlike an arbitrary user body. The waiter also carries the child pid (#pid / the :pid thread-local) and tolerates a missing child (Errno::ECHILD → nil), matching CRuby. Open3's wait_thr.value is unaffected.

3. Process.wait/waitpid/wait2 raise the errno-mapped Errno::E* (a SystemCallError) instead of a bare RuntimeError.
So Process.waitpid on an already-reaped child raises Errno::ECHILD ("No child processes") as CRuby does — and the waiter's rescue SystemCallError catches it.

Impact

• core/thread: now completes 42 of 45 files (213 examples run) instead of deadlocking the runner. The join-driven hangs (status_spec, stop_spec, kill-then-join) are gone.
• core/process/detach_spec: 2/9 → 6/9 passing (0 errors).
• No regression to the wait/waitpid specs (their remaining failures are unrelated: waitpid isn't defined as a literal alias of wait, and Process.waitall is missing).

The three core/thread files that still hang — alive_spec, raise_spec, report_on_exception_spec — spawn bodies with an internal busy-wait (Thread.pass until flag, where flag is only set by the main thread after the body is spawned). These are unsolvable without real concurrency or deadlock detection and would need to be excluded or addressed in a separate follow-up.

Tests

• builtins::class::tests::thread_value_runs_block_lazily: #value runs the body; #join returns the thread itself and does not run the body.
• builtins::process::tests::process_spawn_detach: extended to cover #pid / :pid, reap-on-join, and the Errno::ECHILD mapping.
• All existing Thread / Process unit + integration tests pass; Open3, detach-value, and value-collection verified manually.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.50%. Comparing base (8753ca6) to head (0ebd32d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
monoruby/src/builtins/process.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #898   +/-   ##
=======================================
  Coverage   90.50%   90.50%           
=======================================
  Files         189      189           
  Lines      123227   123253   +26     
=======================================
+ Hits       111521   111547   +26     
  Misses      11706    11706           

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