pcall function runs another LUA function in protected mode, this means
that any error will be caught by this function and will not stop the LUA
execution. The script kill mechanism uses error to stop the running script.
Scripts that uses pcall can catch the error raise by the script kill mechanism,
this will cause a script like this to be unkillable:
local f = function()
while 1 do
redis.call('ping')
end
end
while 1 do
pcall(f)
end
The fix is, when we want to kill the script, we set the hook function to be invoked
after each line. This will promise that the execution will get another
error before it is able to enter the pcall function again.
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