I am concerned about the lack of mutual exlusion in code listing
14.2 “Chat server using TCPServer and threads” on page 430 in “The
Well-Grounded Rubyist” book. That code goes something like this:
chatters = []
while …
Thread.new(…) do |c|
chatters.each { … }
chatters.push c
chatters.each { … }
chatters.delete c
chatters.each { … }
end
end
Multiple threads are spawned and each of them accesses the shared
resource (the chatters array) directly without mutual exclusion.
What happens when a thread inside chatters.each() gets descheduled
and another thread proceeds to add or delete an object to chatters?
Shouldn’t there be a Mutex#synchronize call wrapping all access to
the chatters array?
What happens when a thread inside chatters.each() gets descheduled
and another thread proceeds to add or delete an object to chatters?
Shouldn’t there be a Mutex#synchronize call wrapping all access to
the chatters array?
Probably. You’re right that the code as it stands runs the risk of
modifying an array while iterating over it – which, though it
probably wouldn’t do any serious damage in this case, is (I think)
undefined as to its results in Ruby, and in any case is something I
don’t (consciously advocate doing.
David
–
David A. Black, Senior Developer, Cyrus Innovation Inc.
THE Ruby training with Black/Brown/McAnally
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Won’t do any “serious damage”? Since when is producing the wrong
answer not “serious” in programming?
You would be much better off making a frank admission the mistake. We
all know cognitive dissonance can be disorienting, but that’s no
excuse to indulge it.
advocate doing.