Exception in `ensure` stops threads from exiting
When the Ruby process is gracefully exiting, if a thread has an exception during an
ensure block it appears the Ruby process forgets it's attempting to exit and will keep running forever. Since there's still an alive thread,
rb_thread_terminate_all doesn't finish (since
vm_living_thread_num(vm) > 1 is still true), and the Ruby process never exits until you
kill -9 it.
I was able to cause this going back as far as MRI 2.0.0, but didn't have a 1.9.3 install to double check with. Repo case:
Thread.new do loop do puts "Loop start" begin begin sleep ensure raise end rescue => e p e end end end sleep 1 exit
Will result in a two
Loop start messages.
When running GDB on the process, we see Ruby is waiting on
sleep_forever which is expected, but you can see the main thread is stuck on
(gdb) t a a bt Thread 3 (Thread 0x7f956f36e700 (LWP 401088)): #0 0x00000033d90df113 in poll () from /lib64/libc.so.6 #1 0x00007f9575863775 in timer_thread_sleep (p=0x7f957524c008) at thread_pthread.c:1460 #2 thread_timer (p=0x7f957524c008) at thread_pthread.c:1568 #3 0x00000033d9407aa1 in start_thread () from /lib64/libpthread.so.0 #4 0x00000033d90e893d in clone () from /lib64/libc.so.6 Thread 2 (Thread 0x7f956f363700 (LWP 401117)): #0 0x00000033d940b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007f957585d959 in native_cond_wait (cond=<value optimized out>, mutex=<value optimized out>) at thread_pthread.c:343 #2 0x00007f95758675b8 in native_sleep (th=0x7f956c491800, timeout_tv=0x0) at thread_pthread.c:1147 #3 0x00007f95758682f2 in sleep_forever () at thread.c:1083 #4 rb_thread_sleep_forever () at thread.c:1157 #5 0x00007f95757e1ca5 in rb_f_sleep (argc=0, argv=0x7f956bce6a78) at process.c:4393 #6 0x00007f9575890bda in vm_call_cfunc_with_frame (th=0x7f956c491800, reg_cfp=0x7f956bde6940, calling=<value optimized out>, ci=0x7f956e98c910, cc=<value optimized out>) at vm_insnhelper.c:1752 #7 vm_call_cfunc (th=0x7f956c491800, reg_cfp=0x7f956bde6940, calling=<value optimized out>, ci=0x7f956e98c910, cc=<value optimized out>) at vm_insnhelper.c:1847 #8 0x00007f957589f12b in vm_exec_core (th=<value optimized out>, initial=<value optimized out>) at insns.def:1066 #9 0x00007f95758a470b in vm_exec (th=0x7f956c491800) at vm.c:1727 #10 0x00007f95758abc04 in invoke_block () at vm.c:969 #11 invoke_iseq_block_from_c () at vm.c:1014 #12 invoke_block_from_c_splattable () at vm.c:1032 #13 vm_yield () at vm.c:1074 #14 rb_yield_0 () at vm_eval.c:1010 #15 loop_i () at vm_eval.c:1088 #16 0x00007f9575755954 in rb_rescue2 (b_proc=0x7f95758ab8b0 <loop_i>, data1=0, r_proc=0x7f957588b2a0 <loop_stop>, data2=0) at eval.c:838 #17 0x00007f9575890bda in vm_call_cfunc_with_frame (th=0x7f956c491800, reg_cfp=0x7f956bde69a0, calling=<value optimized out>, ci=0x7f956c5090f0, cc=<value optimized out>) at vm_insnhelper.c:1752 #18 vm_call_cfunc (th=0x7f956c491800, reg_cfp=0x7f956bde69a0, calling=<value optimized out>, ci=0x7f956c5090f0, cc=<value optimized out>) at vm_insnhelper.c:1847 #19 0x00007f95758a726b in vm_call_method (th=0x7f956c491800, cfp=0x7f956bde69a0, calling=<value optimized out>, ci=<value optimized out>, cc=<value optimized out>) at vm_insnhelper.c:2295 #20 0x00007f957589f97c in vm_exec_core (th=<value optimized out>, initial=<value optimized out>) at insns.def:967 #21 0x00007f95758a470b in vm_exec (th=0x7f956c491800) at vm.c:1727 #22 0x00007f95758a5771 in invoke_block (th=0x7f956c491800, captured=<value optimized out>, self=140279789266160, argc=<value optimized out>, argv=<value optimized out>, passed_block_handler=<value optimized out>, cref=0x0, splattable=0, is_lambda=0) at vm.c:969 #23 invoke_iseq_block_from_c (th=0x7f956c491800, captured=<value optimized out>, self=140279789266160, argc=<value optimized out>, argv=<value optimized out>, passed_block_handler=<value optimized out>, cref=0x0, splattable=0, is_lambda=0) at vm.c:1014 #24 0x00007f95758a581f in invoke_block_from_c_unsplattable (th=<value optimized out>, block=<value optimized out>, self=<value optimized out>, argc=<value optimized out>, argv=<value optimized out>, passed_block_handler=<value optimized out>, is_lambda=<value optimized out>) at vm.c:1101 #25 0x00007f95758a595a in vm_invoke_proc (th=0x7f956c491800, proc=0x7f956e04da50, self=140279789266160, argc=0, argv=0x7f956c547c18, passed_block_handler=0) at vm.c:1126 #26 0x00007f9575864589 in thread_do_start (th=0x7f956c491800, stack_start=0x7f956f364000) at thread.c:577 #27 thread_start_func_2 (th=0x7f956c491800, stack_start=0x7f956f364000) at thread.c:619 #28 0x00007f95758649b6 in thread_start_func_1 (th_ptr=0x7f956c491800) at thread_pthread.c:887 #29 0x00000033d9407aa1 in start_thread () from /lib64/libpthread.so.0 #30 0x00000033d90e893d in clone () from /lib64/libc.so.6 Thread 1 (Thread 0x7f95756ab760 (LWP 401080)): #0 0x00000033d940b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007f957585d959 in native_cond_wait (cond=<value optimized out>, mutex=<value optimized out>) at thread_pthread.c:343 #2 0x00007f95758675b8 in native_sleep (th=0x7f9575229400, timeout_tv=0x0) at thread_pthread.c:1147 #3 0x00007f9575867e34 in rb_thread_terminate_all () at thread.c:494 #4 0x00007f9575758466 in ruby_cleanup (ex=6) at eval.c:186 #5 0x00007f9575758725 in ruby_run_node (n=0x7f956c54c6b8) at eval.c:300 #6 0x00000000004008eb in main (argc=2, argv=0x7fffccd8e7c8) at main.c:36 (gdb)
Looking at the thread state shows that the
rb_threadptr_to_kill executed properly, because
to_kill is properly set, but the
errinfo was nilled out:
(gdb) p ruby_current_thread $1 = (rb_thread_t *) 0x7f8cd06eb800 (gdb) p ruby_current_thread->to_kill $2 = 1 (gdb) p ruby_current_thread->status $3 = THREAD_STOPPED (gdb) p ruby_current_thread->errinfo $4 = 8
I'm happy to contribute a patch, but not quite sure what the appropriate fix for this would be. It looks like the issue is an exception in
ensure is causing the
errinfo to be overwritten so the thread doesn't think it should exit, and the right fix is to also check if
to_kill is set.
Updated by zanker (Zachary Anker) over 2 years ago
ko1 (Koichi Sasada) wrote:
Sorry for long absent.
The point is "when should we check
It is not clear.
This is a issue that Ctrl-C doesn't work on it (and we need to use
I think modify Ctrl-C issue is enough on this ticket.
No problem! The problem with Ctrl-C is that will fix it for user facing scripts, but not background processes. The example we ran into with this, was using Puma with forked child processes. The Puma master process was sent a
kill -int, which then sent it to the child processes, but not all of them exited properly due to this bug.
To clarify the
to_kill part, what is happening internally is:
- Ruby VM receives a signal telling it to exit
terminate_allis called which enqueues the
- Eventually hits
errinfo = INT2FIX(TAG_FATAL)and
to_kill = 1on the thread struct
- Exception is caught (see example in the initial report), which causes
errinfoto be nilled out
- Ruby VM doesn't try and finish exiting the thread since the
errinfoflag is now nilled out
It seems like the Ruby VM shouldn't allow you to rescue exceptions in such a way that it can block a thread from exiting.
Updated by wjordan (Will Jordan) 2 months ago
This bug still seems to be affecting all more recent versions of Ruby up to and including 2.7.1.
I encountered this in the
newrelic_rpm gem which was intermittently hanging on shutdown in a short-lived script. It turns out that if Ruby exits while a thread is in the middle of receiving a gzip-encoded request via
Net::HTTP, this call to
#finish in an ensure block raises a
Zlib::BufError exception, triggering this bug.
I've attached a minimal repro of this issue being triggered by gzip decoding in
Net::HTTP, hope this is helpful.
Updated by Eregon (Benoit Daloze) 2 months ago
In the original example, it gets stuck on the second call to
Basically the "kill Thread" exception (which cannot be rescue-d directly) was thrown after Ctrl+C to the Thread.
ensure raises another (RuntimeError) exception, and that replaces the "kill Thread" exception.
I'm not sure what we could do in the VM to fix that.
raise during shutdown should always raise a "kill Thread" exception and ignore the actual exception passed?
Or the "kill Thread" exception could be set as an internal cause, and when
rescue-ing the RuntimeError, that "kill Thread" exception would be re-raised at the end of the
rescue block? (sounds fragile)
Retrying from the main thread to kill other threads is not great, because how to know how long we should wait until the retry?
I would argue it's at least partly the program's bug, because it swallows an exception and keeps going ignoring it.
For instance, if some other error (e.g. a SyntaxError, an OutOfMemoryError, a NoMethodError if sleep was misspelled, etc) happened in such a program, it would not terminate the program and probably go unnoticed.
Updated by Eregon (Benoit Daloze) 2 months ago
Regarding the http/gzip example, it seems unfortunate but it's probably the same for any exception raised in
ensure: that exception might replace a more "critical" exception such as "kill Thread" exception/NoMemoryError/SignalException/etc.
And that in itself can be useful, i.e., it allows to find what failed in that
In the example, the thread goes to infinite sleep after
rescue, so it seems somewhat expected that program can hang though.
https://github.com/ruby/ruby/commit/6811973d1355ba40ee4b3fc5a43ed65b67aac9b7 by naruse (Yui NARUSE), which attemps to re-raise the original exception, but that doesn't work for "kill Thread" exceptions.
https://github.com/ruby/ruby/commit/d7bb66df2667040518186eb72928dedb4b7de6b9 which has to workaround because "kill Thread" exceptions set
I think the only good fix for this case is to make
inflate_body_io.finish not raise or have an alternative method doing similar cleanup but not
ensure is problematic as seen in this issue.
One thought is we could always re-raised the exception entering the
ensure block when leaving it.
But that would then completely ignore exceptions raised inside
ensure, which might or not be wanted.
For this specific case, I think this is a simple fix, assuming we don't care (i.e., won't be shown anywhere, needs
--debug) about the exception of
finish if there is another exception:
begin yield inflate_body_io success = true ensure begin inflate_body_io.finish rescue => err # Ignore #finish's error if there is an exception from yield raise err if success end end
Then the script terminates as expected.