tenderjit: Test suite occasionally fails, with a segfault at 0x0

I’ve noticed, while running the test suite for a large amount of sequential runs, that TJ sometimes segfaults.

The failure is very nondeterministic. Sometimes it takes a few runs, sometimes it doesn’t happen in a hundred runs. It does not depend on the test seed.

Since my last two PRs were quite sensitive, I’ve checked if the issue was present before they were merged, and I can confirm that the issue was already present.

Below there are some sample failures; I think the only pointers (haha) they give is that, since the segfault address is 0, this should be either a null pointer, or I think more likely, a misaligned stack.

A very long run on Mac may confirm if this is Linux-only, or cross-platform. But unfortunately, since even 100 runs don’t guarantee a failure, it may hard to reproduce.

Sample 1:

......S...........SS......SSS.S....S...S.SS....S.S..S../home/saverio/code/fisk-dev/lib/fisk.rb:847: [BUG] Segmentation fault at 0x0000000000000000
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0040 p:---- s:0223 e:000222 CFUNC  :zip
c:0039 p:0023 s:0218 e:000217 BLOCK  /home/saverio/code/fisk-dev/lib/fisk.rb:847 [FINISH]
c:0038 p:---- s:0214 e:000213 IFUNC 
c:0037 p:---- s:0211 e:000210 CFUNC  :each
c:0036 p:---- s:0208 e:000207 CFUNC  :find_all
c:0035 p:0007 s:0204 e:000203 METHOD /home/saverio/code/fisk-dev/lib/fisk.rb:845
c:0034 p:0022 s:0192 e:000191 METHOD /home/saverio/code/fisk-dev/lib/fisk/instructions.rb:1400
c:0033 p:0014 s:0187 e:000186 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/runtime.rb:198
c:0032 p:0049 s:0181 e:000180 BLOCK  /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:2312
c:0031 p:0038 s:0177 e:000176 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:3001
c:0030 p:0011 s:0171 e:000170 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:2307
c:0029 p:0360 s:0166 e:000165 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:130
c:0028 p:0137 s:0154 e:000153 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:84
c:0027 p:0148 s:0149 e:000148 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit.rb:558
c:0026 p:0009 s:0140 e:000139 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:557 [FINISH]
Segmentation fault

Sample 2:

.S..S.............S.SS.S.S....SSS...SS...................SS.........SS......./home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:45: [BUG] Segmentation fault at 0x0000000000000000
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0029 p:0294 s:0163 e:000162 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:45 [FINISH]
c:0028 p:---- s:0155 e:000154 CFUNC  :new
c:0027 p:0125 s:0149 e:000148 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit.rb:552
c:0026 p:0009 s:0140 e:000139 METHOD /home/saverio/code/tenderjit-dev/lib/tenderjit/iseq_compiler.rb:557 [FINISH]
Segmentation fault