Giampaolo Rodola' <[email protected]> added the comment:
To further elaborate on the creation time solution, the idea in pseudo-code is
the following:
class Popen:
def __init__(self, ...):
self._execute_child(...)
try:
self._ctime = get_create_time(self.pid)
except ProcessLookupError:
self._ctime = None
def send_signal(self, sig):
if self._ctime is not None:
if self._ctime != get_create_time(self.pid):
raise ProecssLookupError("PID has been reused")
os.kill(self.pid, sig)
Technically there is still a race condition between _execute_child() and
get_create_time(), but:
1) the time between the two calls is supposed to be extremely short
2) an OS should not reuse a PID that fast, precisely in order to minimize the
chances of such a race condition to occur
3) as a general rule, on most systems PIDs are supposed to be assigned
sequentially and wrap around. E.g. on Linux:
~$ for i in $(seq 20); do ps; done | grep ps
3199 pts/0 00:00:00 ps
3200 pts/0 00:00:00 ps
3201 pts/0 00:00:00 ps
3202 pts/0 00:00:00 ps
3203 pts/0 00:00:00 ps
3204 pts/0 00:00:00 ps
3205 pts/0 00:00:00 ps
3207 pts/0 00:00:00 ps
3208 pts/0 00:00:00 ps
3209 pts/0 00:00:00 ps
3210 pts/0 00:00:00 ps
3213 pts/0 00:00:00 ps
3214 pts/0 00:00:00 ps
3215 pts/0 00:00:00 ps
3216 pts/0 00:00:00 ps
3217 pts/0 00:00:00 ps
3218 pts/0 00:00:00 ps
3219 pts/0 00:00:00 ps
3221 pts/0 00:00:00 ps
3223 pts/0 00:00:00 ps
As for Windows, the termination is based on the process handle instead of the
PID, so I assume that means Windows is safe in this regard. There was a prior
discussion about this actually:
https://bugs.python.org/issue36067#msg336243
Getting process creation time is relatively easy, even though
platform-dependent (and should be done in C).
pidfd API would be nicer to use, but it seems it's not portable and relatively
recent.
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<https://bugs.python.org/issue38630>
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