Check if pidlock process is still running

[maleadt]

I somehow interrupted a precompilation process, resulting in a stale pidlock somewhere in my depot. This causes precompilation to hang, which for all intents and purposes looks like a plain hang (IIUC JuliaLang/Pkg.jl#3519 wouldn't help here, because this is triggered by using Package in code, not by Pkg.precompile()).

Setting JULIA_DEBUG shows that its stuck waiting for a pidlock, as does the backtrace when interrupting:

┌ Debug: Waiting for another process (pid: 5719) to finish precompiling CUDA [052768ef-5323-5732-b1bb-66c8b64840ba]
└ @ Base loading.jl:2831
^C
[7169] signal (2): Interrupt
in expression starting at /home/tim/Julia/pkg/CUDA/wip.jl:1
epoll_wait at /usr/lib/libc.so.6 (unknown line)
uv__io_poll at /workspace/srcdir/libuv/src/unix/epoll.c:236
uv_run at /workspace/srcdir/libuv/src/unix/core.c:400
ijl_task_get_next at /home/tim/Julia/src/julia/src/partr.c:436
unknown function (ip: 0x7f59f774c93e)
unknown function (ip: 0x7f59f75d646c)
unknown function (ip: 0x7f59f77a27c5)
unknown function (ip: 0x7f59f704af20)
unknown function (ip: 0x7f59f6fa0c03)
watch_file at /home/tim/Julia/src/julia/build/dev/usr/share/julia/stdlib/v1.10/FileWatching/src/FileWatching.jl:778 [inlined]
JuliaLang/Pkg.jl#13 at /home/tim/Julia/src/julia/build/dev/usr/share/julia/stdlib/v1.10/FileWatching/src/pidfile.jl:257
unknown function (ip: 0x7f5a0d07daf9)
_jl_invoke at /home/tim/Julia/src/julia/src/gf.c:2870 [inlined]
ijl_apply_generic at /home/tim/Julia/src/julia/src/gf.c:3071
jl_apply at /home/tim/Julia/src/julia/src/julia.h:1963 [inlined]
start_task at /home/tim/Julia/src/julia/src/task.c:1238
unknown function (ip: (nil))
Allocations: 2836 (Pool: 2827; Big: 9); GC: 0

Crucially, process 5719 just doesn't exist anymore. Instead of waiting for the pidlock to go stale, which seems like ages (it's been almost 10 minutes), shouldn't we first check if the process even exists and if not break the lock?

[IanButterworth]

The lock should be broken if the process doesn't exist anymore but I forgot to set stale_age which disabled all that.

julia/stdlib/FileWatching/src/pidfile.jl

Lines 173 to 184 in a41892f

	function isvalidpid(hostname::AbstractString, pid::Cuint)
	# can't inspect remote hosts
	(hostname == "" || hostname == gethostname()) || return true
	# pid < 0 is never valid (must be a parser error or different OS),
	# and would have a completely different meaning when passed to kill
	!iswindows() && pid > typemax(Cint) && return false
	# (similarly for pid 0)
	pid == 0 && return false
	# see if the process id exists by querying kill without sending a signal
	# and checking if it returned ESRCH (no such process)
	return ccall(:uv_kill, Cint, (Cuint, Cint), pid, 0) != UV_ESRCH
	end

Fixed in #50214