Do not fail sampling completely if a single thread dies before sampling

[I-Al-Istannen]

The flow when sampling a process starts by

1. Enumerate all threads by reading /proc/pid/task/<tid>.
2. For each found thread, check if it is currently active by reading its state file in /proc/<tid>/stat.

During this time, we did not suspend the process yet. Therefore, it might happen that a thread dies precisely in the milliseconds between the enumeration readdir and the stat for active checking. In those cases, py-spy bails out completely, skipping all threads. If a process creates a lot of threads and tears them down (e.g. through using some native dependency with a saner threading model, such as rust with polars), this case can be hit a lot.

I do not see why we would want to throw out a full sample, just because a single thread died prematurely. Therefore, I propose silently ignoring stat errors for threads. The .threads() method will re-enumerate and find the thread lacking, if it is called in later parts of the code.

Doing this also exposed benfred/remoteprocess#106.

Even without this patch, sampling might fail with EPERM, but that is an expected quirk of ptrace: https://elixir.bootlin.com/linux/v6.15.3/source/kernel/ptrace.c#L458 if the process is currently exiting, ptrace returns EPERM instead of ESRCH.

[I-Al-Istannen]

I have a stress test lying around with:

import threading

import os

print(os.getpid())

def _dummy():
    #time.sleep(0.1)
    pass

for index in range(0, 100_000):
    if index % 1000 == 0:
        print("Iteration: ", index)
    ts = []
    for i in range(0, 10):
        t = threading.Thread(target=_dummy)
        ts.append(t)
        t.start()
    for t in ts:
        t.join()

print("HEY")

If you record this with py-spy top --pid <pid> it will have a quite high error rate of ~50% in my tests, without this patch.

ETA: With this patch and my remoteprocess change, it is ~1%.