Support multi-process debugging: sync breakpoints and coordinate instances #1172
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st0012 wants to merge 2 commits intoruby:masterfrom
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Support multi-process debugging: sync breakpoints and coordinate instances #1172st0012 wants to merge 2 commits intoruby:masterfrom
st0012 wants to merge 2 commits intoruby:masterfrom
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…ances Two fixes for debugging multi-process Ruby applications: 1. Breakpoint synchronization across forked processes (fixes ruby#714): Store serialized breakpoint specs in a shared JSON tempfile alongside the existing flock tempfile. Publish on subsession leave, check on subsession enter, and in the socket reader retry paths for both DAP and console protocols. Breakpoints define to_sync_data for serialization. Only LineBreakpoint and CatchBreakpoint are synced. 2. Coordination of independent debugger instances: When parallel test runners fork workers before the debugger loads, each worker gets its own SESSION with no coordination. Add a well-known lock file keyed by process group ID (/tmp/ruby-debug-{uid}-pgrp-{getpgrp}.lock) that all sibling instances discover automatically. On enter_subsession, acquire the lock (blocking flock) so only one process enters the debugger at a time. While blocked, no prompt is shown and IRB/Reline never reads STDIN.
❌ 2/707 Tests Failed/home/runner/work/debug/debug/test/protocol/hover_raw_dap_test.rb#test_hover_works_correctly/home/runner/work/debug/debug/test/protocol/hover_raw_dap_test.rb#test_1641198331 |
Tests for breakpoint sync (fork_bp_sync_test.rb): - Breakpoint set/deleted after fork syncs to child - Multiple children receive synced breakpoints - Catch breakpoint syncs to child - Late-forked child catches up - Stress test with binding.break Tests for well-known lock (wk_lock_test.rb): - Single-process debugging unaffected - fork_mode: :both uses ProcessGroup not well-known lock - Independent workers serialized by well-known lock
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Problem 1: Breakpoints not shared between forked processes (#714)
Closes #714
Summary
When a Ruby app forks workers (Puma, Unicorn), breakpoints set in the debugger only fire in some workers. Users must toggle breakpoints multiple times to get them to register, and hits are inconsistent.
Cause
In
fork_mode: :both(the default), afterfork()each process gets an independent copy of@bps(the breakpoints hash). The existingProcessGroupflock only serializes which process talks to the debugger client — it never synchronizes breakpoint state. When VSCode sendssetBreakpointsor a user typesbreak, only the process currently holding the flock receives the update. Other processes never learn about it.Solution
Store serialized breakpoint specs in a shared JSON tempfile alongside the existing flock tempfile. Publish on subsession leave (before lock release), check on subsession enter (after lock acquire). Also sync in the DAP
recv_requestretry path and consoleprocesscan't-read path. Newly forked children sync via the fork child hook.to_sync_datafor serialization. OnlyLineBreakpointandCatchBreakpointare synced (as descriptors — path, line, condition).WatchIVarBreakpointis skipped sinceobject_idis process-local.File.rename).Tests
6 tests in
test/console/fork_bp_sync_test.rb:Problem 2: Multiple debugger instances competing for STDIN
Summary
When running tests with parallel process workers (parallel_tests, ci-queue, Rails parallelize), all workers that hit a
debuggerstatement enter the debug prompt simultaneously. Output is clobbered, input is swallowed by random processes, and the debugger is unusable.Cause
Parallel test runners fork workers before the debugger loads. Each worker independently creates its own
SESSIONandProcessGroupwhen it first hitsdebugger. There is no sharedProcessGroup, no flock coordination — every worker thinks it's the only debugger instance. They all callwait_command_loopand compete for the same STDIN (inherited from the parent via fork).Solution
Add a well-known lock file that all sibling debugger instances discover automatically, keyed by process group ID:
/tmp/ruby-debug-{uid}-pgrp-{getpgrp}.lock.On
enter_subsession, acquire the lock with blockingflock(LOCK_EX). If another debugger is active, the process waits until it finishes, then enters the debugger normally. Users debug each process in sequence by typingc.Process.getpgrpgroups all processes spawned from the same command — works across nested forksflock(), no prompt is shown and IRB/Reline never reads STDIN — no input competitionMultiProcessGroupis active (fork_mode: :bothalready handles coordination)Tests
3 tests in
test/console/wk_lock_test.rb:fork_mode: :bothuses existingProcessGroup, not the well-known lock