From: "Eregon (Benoit Daloze) via ruby-core" <ruby-core@ml.ruby-lang.org>
To: ruby-core@ml.ruby-lang.org
Cc: "Eregon (Benoit Daloze)" <noreply@ruby-lang.org>
Subject: [ruby-core:117010] [Ruby master Bug#20314] Simultaneous Timeout expires may raise an exception after the block
Date: Thu, 29 Feb 2024 09:31:02 +0000 (UTC) [thread overview]
Message-ID: <redmine.journal-107071.20240229093102.18@ruby-lang.org> (raw)
In-Reply-To: <redmine.issue-20314.20240229062526.18@ruby-lang.org>
Issue #20314 has been updated by Eregon (Benoit Daloze).
I'm not sure how Solution 3 would work.
`Thread#raise` would block until what?
Until the exception started to be raised/thrown on that thread? I think that would not fix that snippet.
It does not seem reasonable to wait until the exception has been rescued (or escapes the thread) because that could run arbitrary code via `ensure` which could take a long time (and block the caller of `Thread#raise` for a long time).
I wonder if we should always use `Timeout::ExitException` to "unwind" until we exit the corresponding block given to `Timeout.timeout`.
Unsure if that would help for this issue though.
Maybe there are other solutions too?
Between solutions 1 and 2, 2 seems better because it seems clearly broken code to `rescue B; sleep; end` in `Timeout.timeout(0.1, B)`.
In any case `sleep` in `ensure`/`rescue` is broken code as it can already hang (e.g. with a single `Timeout.timeout(0.1, SomeKlass)` around it, or without using Timeout).
----------------------------------------
Bug #20314: Simultaneous Timeout expires may raise an exception after the block
https://bugs.ruby-lang.org/issues/20314#change-107071
* Author: mame (Yusuke Endoh)
* Status: Open
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
Launchable reports `TestTimeout#test_nested_timeout` as a flaky test, and I reproduced it as follows.
```ruby
require "timeout"
class A < Exception
end
class B < Exception
end
begin
Timeout.timeout(0.1, A) do
Timeout.timeout(0.1, B) do
nil while true
end
end
rescue A, B
p $! #=> #<A: execution expired>
# Exception B is raised after the above call returns
#=> test.rb:16:in `p': execution expired (B)
p :end # not reach
end
```
This is because the timer thread performs two consecutive `Thread#raise` to the target thread.
I have discussed this with @ko1 and have come up with three solutions.
### Solution 1
When multiple nested Timeouts expire simultaneously, raise an exception for the outer-most Timeout and let the inner Timeouts expire without throwing an exception. In the above example, it would only raise A.
The problem with this approach is that if you are rescuing A in the inner block, it may never ends:
```ruby
Timeout.timeout(0.1, A) do
Timeout.timeout(0.1, B) do
begin
sleep
rescue A
sleep # The exception A is caught. The inner Timeout is already expired, so the code (may) never end.
end
end
end
```
Note that, if A and B did not occur at the same time, it would raise B. This is a race condition.
### Solution 2
When multiple nested Timeouts expire simultaneously, raise an exception for the inner-most Timeout and let the outer Timeouts wait until the inner-most Timeout returns. In the above example, it would raise either A or B, not both.
The problem with this approach is that if you are rescuing B in the inner block, it never ends:
```ruby
Timeout.timeout(0.1, A) do
Timeout.timeout(0.1, B) do
begin
sleep
rescue B
sleep # The outer Timeout waits for the inner timeout, and the inner Timeout never return. So this code never ends.
end
end
end
```
### Solution 3
Make thread interrupt queue one length. If the target thread has already been `Thread#raise(A)`, the new `Thread#raise(B)` blocks until the target thread processes A.
Since there will be no more simultaneous Thread#raise, there will be no more exceptions after the end of the block. The timeout timer thread should be changed in consideration that `Thread#raise` may block.
--
https://bugs.ruby-lang.org/
______________________________________________
ruby-core mailing list -- ruby-core@ml.ruby-lang.org
To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org
ruby-core info -- https://ml.ruby-lang.org/mailman3/postorius/lists/ruby-core.ml.ruby-lang.org/
next prev parent reply other threads:[~2024-02-29 9:31 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-29 6:25 [ruby-core:117003] " mame (Yusuke Endoh) via ruby-core
2024-02-29 9:31 ` Eregon (Benoit Daloze) via ruby-core [this message]
2024-03-04 17:38 ` [ruby-core:117053] " jeremyevans0 (Jeremy Evans) via ruby-core
2024-03-06 0:43 ` [ruby-core:117059] " mame (Yusuke Endoh) via ruby-core
2024-10-21 8:50 ` [ruby-core:119550] " Eregon (Benoit Daloze) via ruby-core
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=redmine.journal-107071.20240229093102.18@ruby-lang.org \
--to=ruby-core@ml.ruby-lang.org \
--cc=noreply@ruby-lang.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).