make compile with clang21 #3
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kvahed
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Jan 22, 2026
In case of two nested back-to-back signals (such as what happens in NestedSignal test) we could end up erroneously using the frame pointer from ucontext_t twice, leading to premature backtrace termination. In the situation where this happens, the call stack looks like #0 <unwinder frames> #1 SigUsr2Handler #2 __kernel_rt_sigreturn #3 raise abseil#4 SigUsr1Handler abseil#5 __kernel_rt_sigreturn abseil#6 raise abseil#7 RaiseSignal ... When unwinding from #2, we get the fp value from the ucontext (as we should). However, because raise does not modify the fp and because SigUsr1Handler is also a signal handler, when we try to unwind from abseil#4 (#3 is skipped), NextStackFrame ends up looking at the ucontext fp again, and comparing it with the previous (identical) FP value. Non-strict equality accepts this as a valid frame, but the unwinder later bails out due to a zero-sized frame. Using a strict equality causes NextStackFrame to reject the ucontext fp and use the FP from FP chain instead. This causes us to skip a few more frames, but at least we continue to unwind instead of giving up. In this case, the computed backtrace skips functions #3, abseil#4 and abseil#6. PiperOrigin-RevId: 804308754 Change-Id: I5d43e6bea80e4abff1075ada03782ae11c599161
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This could be used to make our fork of abseil-cpp compile with clang21