Description

This fixes a crash happening in Frame::read caused by stale previous StackChunk entries persisting across thread iterations during stack sampling.

Root Cause

When the Sampling Thread samples more than one Thread, it uses the same global StackChunk for each Thread's stack chain. StackChunk::update_with_depth recursively copies the linked list of _PyStackChunk's.
However, when a stack chunk has no previous chunk, we would not clear the old previous pointer. This left stale StackChunk entries from previously-sampled threads in the chain.

When a subsequent Thread's frame address happened to fall within the remote address range of a stale chunk's origin, StackChunk::resolve would return a pointer into the stale local buffer. The stale data contained garbage field values, which would result in invalid accesses.

This is the same crash signature as #16519 (which fixed a race condition on copied_size) and #16631 (which added full-frame bounds checking). The stale previous chain was an additional vector for the same class of bug.

This is the crash we would see:

#0 0x00007fa8a3507fc6 Frame::read
#1 0x00007fa8a3508114 unwind_frame
#2 0x00007fa8a3509c68 ThreadInfo::unwind
#3 0x00007fa8a3509da2 ThreadInfo::sample
#4 0x00007fa8a350a02e std::_Function_handler<void (_ts*, ThreadInfo&), Datadog::Sampler::sampling_thread(unsigned long)::{lambda(InterpreterInfo&)#1}::operator()(InterpreterInfo&) const::{lambda(_ts*, ThreadInfo&)#1}>::_M_invoke
#5 0x00007fa8a350a2c0 for_each_thread
#6 0x00007fa8a350a382 std::_Function_handler<void (InterpreterInfo&), Datadog::Sampler::sampling_thread(unsigned long)::{lambda(InterpreterInfo&)#1}>::_M_invoke
#7 0x00007fa8a35072f9 for_each_interp
#8 0x00007fa8a350a6e7 Datadog::Sampler::sampling_thread
#9 0x00007fa8a350a853 call_sampling_thread