From: "nonster (Nony Dutton) via ruby-core" <ruby-core@...>
Date: 2025-12-01T13:55:21+00:00
Subject: [ruby-core:123963] [Ruby Bug#21716] YJIT panic: "all PendingBranchRefs should be unique" in net/http response reading (infinite unwind loop)

Issue #21716 has been updated by nonster (Nony Dutton).


alanwu (Alan Wu) wrote in #note-2:
> The rust unwinder seems to be working fine, and the infinite loop seems to be from ruby's own DWARF parser and unwinder in addr2line.c. Can you confirm that before the repeated stack trace lines there is a line that says "C level backtrace information"?

Yep, here's more context:

```
ruby: YJIT has panicked. More info to follow...
thread '<unnamed>' panicked at 'all PendingBranchRefs should be unique when ready to construct a Block', ./yjit/src/core.rs:2403:27
stack backtrace:
0: rust_begin_unwind
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/panicking.rs:578:5
1: core::panicking::panic_fmt
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/panicking.rs:67:14
2: core::panicking::panic_display
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/panicking.rs:150:5
3: core::panicking::panic_str
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/panicking.rs:134:5
4: core::option::expect_failed
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/option.rs:2025:5
5: core::option::Option<T>::expect
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/option.rs:913:21
6: yjit::core::<impl yjit::codegen::JITState>::into_block::{{closure}}
at /workspace/ruby-3.4.7/yjit/src/core.rs:2403:27
7: core::iter::adapters::map::map_try_fold::{{closure}}
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/adapters/map.rs:91:28
8: core::iter::traits::iterator::Iterator::try_fold
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/traits/iterator.rs:2304:21
9: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/adapters/map.rs:117:9
10: <I as alloc::vec::in_place_collect::SpecInPlaceCollect<T,I>>::collect_in_place
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/alloc/src/vec/in_place_collect.rs:257:13
11: alloc::vec::in_place_collect::<impl alloc::vec::spec_from_iter::SpecFromIter<T,I> for alloc::vec::Vec<T>>::from_iter
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/alloc/src/vec/in_place_collect.rs:181:19
12: <alloc::vec::Vec<T> as core::iter::traits::collect::FromIterator<T>>::from_iter
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/alloc/src/vec/mod.rs:2712:9
13: core::iter::traits::iterator::Iterator::collect
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/traits/iterator.rs:1896:9
14: <alloc::boxed::Box<[I]> as core::iter::traits::collect::FromIterator<I>>::from_iter
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/alloc/src/boxed.rs:2001:9
15: core::iter::traits::iterator::Iterator::collect
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/traits/iterator.rs:1896:9
16: yjit::core::<impl yjit::codegen::JITState>::into_block
at /workspace/ruby-3.4.7/yjit/src/core.rs:2405:16
17: yjit::codegen::gen_single_block
at /workspace/ruby-3.4.7/yjit/src/codegen.rs:1457:8
18: yjit::core::gen_block_series_body
at /workspace/ruby-3.4.7/yjit/src/core.rs:3088:23
19: yjit::core::gen_block_series
at /workspace/ruby-3.4.7/yjit/src/core.rs:3066:18
20: yjit::core::branch_stub_hit_body
at /workspace/ruby-3.4.7/yjit/src/core.rs:3604:17
21: yjit::core::branch_stub_hit::{{closure}}::{{closure}}
at /workspace/ruby-3.4.7/yjit/src/core.rs:3493:36
22: yjit::stats::with_compile_time
at /workspace/ruby-3.4.7/yjit/src/stats.rs:1084:15
23: yjit::core::branch_stub_hit::{{closure}}
at /workspace/ruby-3.4.7/yjit/src/core.rs:3493:13
24: std::panicking::try::do_call
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/panicking.rs:485:40
25: std::panicking::try
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/panicking.rs:449:19
26: std::panic::catch_unwind
at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/panic.rs:140:14
27: yjit::cruby::with_vm_lock
at /workspace/ruby-3.4.7/yjit/src/cruby.rs:688:21
28: yjit::core::branch_stub_hit
at /workspace/ruby-3.4.7/yjit/src/core.rs:3492:9
29: <unknown>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:604: [BUG] YJIT: panicked at 'all PendingBranchRefs should be unique when ready to construct a Block', ./yjit/src/core.rs:2403:27
ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +YJIT +PRISM [aarch64-linux]

'-- Control frame information -----------------------------------------------
c:0047 p:0049 s:0248 e:000247 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:604
c:0046 p:0098 s:0243 e:000242 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:588
c:0045 p:0004 s:0235 e:000234 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:593
c:0044 p:0062 s:0230 e:000229 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:363
c:0043 p:0003 s:0222 e:000221 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:401
c:0042 p:0025 s:0218 e:000217 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:321
c:0041 p:0029 s:0212 e:000211 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http.rb:2430
c:0040 p:0050 s:0204 e:000203 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http.rb:2384
c:0039 p:0033 s:0196 e:000195 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry/net/http.rb:44
c:0038 p:0029 s:0189 e:000188 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry/hub.rb:138 [FINISH]
c:0037 p:0022 s:0179 e:000178 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry-ruby.rb:536
c:0036 p:0052 s:0173 e:000172 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry/net/http.rb:37
c:0035 p:0006 s:0166 e:000165 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/net.rb:78
c:0034 p:0025 s:0162 e:000161 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http.rb:1632
c:0033 p:0023 s:0158 e:000157 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/unix_socket.rb:43
c:0032 p:0066 s:0152 e:000151 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/net.rb:77
c:0031 p:0018 s:0145 e:000144 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/net.rb:44
c:0030 p:0024 s:0140 e:000139 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/api/instance.rb:47
c:0029 p:0005 s:0135 e:000134 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:65
c:0028 p:0020 s:0131 e:000130 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/api/endpoint.rb:24 [FINISH]
c:0027 p:0055 s:0126 e:000125 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:98
c:0026 p:0029 s:0118 e:000117 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:47
c:0025 p:0028 s:0112 e:000111 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:64
c:0024 p:0006 s:0107 e:000106 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:34
c:0023 p:0014 s:0102 e:000101 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/client.rb:27
c:0022 p:0006 s:0092 e:000091 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:33
c:0021 p:0022 s:0087 e:000086 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:148 [FINISH]
c:0020 p:---- s:0081 e:000080 IFUNC
c:0019 p:---- s:0078 e:000077 IFUNC
c:0018 p:---- s:0075 e:000074 CFUNC :<<
c:0017 p:---- s:0072 E:001440 IFUNC
c:0016 p:---- s:0069 E:0013e0 CFUNC :each
c:0015 p:---- s:0066 e:000065 CFUNC :each
c:0014 p:---- s:0063 e:000062 CFUNC :each
c:0013 p:---- s:0060 E:0013c0 IFUNC
c:0012 p:---- s:0057 E:0013a0 CFUNC :each
c:0011 p:---- s:0054 e:000053 CFUNC :each
c:0010 p:---- s:0051 E:001380 CFUNC :to_a
c:0009 p:0038 s:0047 E:0009e0 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168
c:0008 p:0015 s:0039 e:000038 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/writer.rb:90
c:0007 p:0007 s:0032 e:000031 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/writer.rb:160 [FINISH]
c:0006 p:0051 s:0027 e:000023 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:55
c:0005 p:0004 s:0018 e:000017 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:114 [FINISH]
c:0004 p:0017 s:0015 e:000014 METHOD <internal:kernel>:168
c:0003 p:0004 s:0010 e:000009 METHOD /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:113
c:0002 p:0003 s:0006 e:000005 BLOCK /var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:73 [FINISH]
c:0001 p:---- s:0003 e:000002 DUMMY [FINISH]

'-- Ruby level backtrace information ----------------------------------------
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:73:in 'block (2 levels) in start'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:113:in 'perform'
<internal:kernel>:168:in 'loop'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:114:in 'block in perform'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/workers.rb:55:in 'callback_traces'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/writer.rb:160:in 'block in start_worker'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/writer.rb:90:in 'send_spans'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in 'send_traces'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in 'to_a'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in 'each'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in 'each'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in 'each'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in 'each'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in 'each'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:168:in '<<'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/traces.rb:148:in 'block in send_traces'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:33:in 'send_traces_payload'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/client.rb:27:in 'send_request'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:34:in 'block in send_traces_payload'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:64:in 'send_traces'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:47:in 'send_traces'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:98:in 'call'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/api/endpoint.rb:24:in 'call'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/tracing/transport/http/traces.rb:65:in 'block in send_traces'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/api/instance.rb:47:in 'call'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/net.rb:44:in 'call'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/net.rb:77:in 'post'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/unix_socket.rb:43:in 'open'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http.rb:1632:in 'start'
/var/zendesk/bundle/ruby/3.4.0/gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/net.rb:78:in 'block in post'
/var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry/net/http.rb:37:in 'request'
/var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry-ruby.rb:536:in 'with_child_span'
/var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry/hub.rb:138:in 'with_child_span'
/var/zendesk/bundle/ruby/3.4.0/gems/sentry-ruby-5.26.0/lib/sentry/net/http.rb:44:in 'block in request'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http.rb:2384:in 'request'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http.rb:2430:in 'transport_request'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:321:in 'reading_body'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:401:in 'body'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:363:in 'read_body'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:593:in 'read_body_0'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:588:in 'inflater'
/var/zendesk/bundle/ruby/3.4.0/gems/net-http-0.6.0/lib/net/http/response.rb:604:in 'block in read_body_0'

'-- Threading information ---------------------------------------------------
Total ractor count: 1
Ruby thread count for this ractor: 9

'-- C level backtrace information -------------------------------------------
/usr/lib/libruby.so.3.4(fprintf+0x0) [0xf0c8b9f94d80] /workspace/ruby-3.4.7/vm_dump.c:823
/usr/lib/libruby.so.3.4(try_fold<alloc::vec::into_iter::IntoIter<alloc::rc::Rc<yjit::core::PendingBranch>, alloc::alloc::Global>, alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, core::iter::adapters::map::map_try_fold::{closure_env#0}<alloc::rc::Rc<yjit::core::PendingBranch>, core::ptr::non_null::NonNull<yjit::core::Branch>, alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, core::result::Result<alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, !>, yjit::core::{impl#19}::into_block::{closure_env#0}, alloc::vec::in_place_collect::write_in_place_with_drop::{closure_env#0}<core::ptr::non_null::NonNull<yjit::core::Branch>>>, core::result::Result<alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, !>>) /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/traits/iterator.rs:2303
/usr/lib/libruby.so.3.4(try_fold<alloc::vec::into_iter::IntoIter<alloc::rc::Rc<yjit::core::PendingBranch>, alloc::alloc::Global>, alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, core::iter::adapters::map::map_try_fold::{closure_env#0}<alloc::rc::Rc<yjit::core::PendingBranch>, core::ptr::non_null::NonNull<yjit::core::Branch>, alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, core::result::Result<alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, !>, yjit::core::{impl#19}::into_block::{closure_env#0}, alloc::vec::in_place_collect::write_in_place_with_drop::{closure_env#0}<core::ptr::non_null::NonNull<yjit::core::Branch>>>, core::result::Result<alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, !>>) /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/traits/iterator.rs:2303
/usr/lib/libruby.so.3.4(try_fold<alloc::vec::into_iter::IntoIter<alloc::rc::Rc<yjit::core::PendingBranch>, alloc::alloc::Global>, alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, core::iter::adapters::map::map_try_fold::{closure_env#0}<alloc::rc::Rc<yjit::core::PendingBranch>, core::ptr::non_null::NonNull<yjit::core::Branch>, alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, core::result::Result<alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, !>, yjit::core::{impl#19}::into_block::{closure_env#0}, alloc::vec::in_place_collect::write_in_place_with_drop::{closure_env#0}<core::ptr::non_null::NonNull<yjit::core::Branch>>>, core::result::Result<alloc::vec::in_place_drop::InPlaceDrop<core::ptr::non_null::NonNull<yjit::core::Branch>>, !>>) /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/traits/iterator.rs:2303

# Continues looping from here.
``

----------------------------------------
Bug #21716: YJIT panic: "all PendingBranchRefs should be unique" in net/http response reading (infinite unwind loop)
https://bugs.ruby-lang.org/issues/21716#change-115363

* Author: angelim@angelim.com.br (Alexandre Angelim)
* Status: Open
* ruby -v: ruby 3.4.7
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
We encountered a YJIT panic in production that caused the process to enter an infinite logging loop for several hours. The panic occurs within `yjit/src/core.rs` when handling branch references.

The trigger appears to be in `net/http`, specifically when reading a response body (possibly involving chunked encoding or zlib) inside a Datadog trace transport flush.

Key Observations:

The Panic: YJIT fails with all PendingBranchRefs should be unique when ready to construct a Block.

The Loop: After the panic, the process fails to exit and instead logs `rust_begin_unwind` and `try_fold` iterator errors repeatedly, filling logs for hours.

System Configuration
Ruby Version: ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +YJIT +PRISM [aarch64-linux]

Environment: Linux aarch64 (AWS Graviton/ARM)

Reproduction Context
While we cannot provide a standalone reproduction script yet, the crash happens intermittently during a Net::HTTP request initiated by datadog-ruby (v2.17.0) to flush traces to a Unix socket, which is wrapped by sentry-ruby (v5.26.0).

Ruby Location: `/gems/net-http-0.6.0/lib/net/http/response.rb:604`

Rust Location: `./yjit/src/core.rs:2403:27`

Backtrace & Logs

```
ruby: YJIT has panicked. More info to follow...
thread '<unnamed>' panicked at 'all PendingBranchRefs should be unique when ready to construct a Block', ./yjit/src/core.rs:2403:27

stack backtrace:
   0: rust_begin_unwind
      at /rustc/.../library/std/src/panicking.rs:578:5
   ...
   28: yjit::core::branch_stub_hit
      at /workspace/ruby-3.4.7/yjit/src/core.rs:3492:9

/gems/net-http-0.6.0/lib/net/http/response.rb:604: [BUG] YJIT: panicked at 'all PendingBranchRefs should be unique when ready to construct a Block', ./yjit/src/core.rs:2403:27 

-- Control frame information -----------------------------------------------
c:0047 p:0049 s:0248 e:000247 BLOCK  /gems/net-http-0.6.0/lib/net/http/response.rb:604
c:0046 p:0098 s:0243 e:000242 METHOD /gems/net-http-0.6.0/lib/net/http/response.rb:588
c:0045 p:0004 s:0235 e:000234 METHOD /gems/net-http-0.6.0/lib/net/http/response.rb:593
...
c:0039 p:0033 s:0196 e:000195 BLOCK  /gems/sentry-ruby-5.26.0/lib/sentry/net/http.rb:44
...
c:0033 p:0023 s:0158 e:000157 METHOD /gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/unix_socket.rb:43
c:0032 p:0066 s:0152 e:000151 METHOD /gems/datadog-2.17.0/lib/datadog/core/transport/http/adapters/net.rb:77
```

The stack suggests the crash happens during `branch_stub_hit` resolution. The presence of sentry-ruby and datadog suggests complex interaction with Net::HTTP patching.

The infinite loop of logs following the crash looks like this (repeated):

```
/usr/lib/libruby.so.3.4(try_fold<alloc::vec::into_iter::IntoIter<alloc::rc::Rc<yjit::core::PendingBranch> ...
```

This suggests the panic handler itself is tripping over the corrupted `PendingBranch` state. We encountered a YJIT panic in production that caused the process to enter an infinite logging loop for several hours. The panic occurs within `yjit/src/core.rs` when handling branch references.

The trigger appears to be in `net/http`, specifically when reading a response body (possibly involving chunked encoding or zlib) inside a Datadog trace transport flush.





-- 
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