A simple mutation counter works in the single-threaded case
for i in x {
print(i)
if middle_of_x { x.add(y) } // bang!
}
The add while enumerating should change the mutation counter, and the implicit
getting of next value for the for-loop should see the current mutation counter
doesn't equal the initial cached value of the mutation counter.
In the multi-threaded case, the mutation counter needs to be thread-safe,
otherwise comparing its current value to a cached value could fail from timing
issues.
// thread one
for i in x { print(i) }
// thread two
x.add(y) // bang?
> On Jan 8, 2018, at 2:59 PM, Greg Parker via swift-users
> <[email protected]> wrote:
>
>>
>> On Jan 8, 2018, at 12:51 PM, Jordan Rose via swift-users
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>>> On Jan 8, 2018, at 11:47, Fadi Botros <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>
>>> 1st: How to invoke the thread sanitizer?
>>
>> Check out this article on developer.apple.com <http://developer.apple.com/>:
>> https://developer.apple.com/documentation/code_diagnostics/thread_sanitizer/enabling_the_thread_sanitizer
>>
>> <https://developer.apple.com/documentation/code_diagnostics/thread_sanitizer/enabling_the_thread_sanitizer>
>>> 2nd: I think making something like ConcurrentModificationException of Java
>>> should be a little better experience (if it will not make, or make a small
>>> performance penalty, also if it makes performance penalty, it would be
>>> activated only when you make a debug version or non-optimized version)
>>
>> I don't exactly disagree, but my understanding is that the work to do such a
>> thing is nearly equivalent to having thread sanitizer on all the time. But
>> maybe there's a simpler model that we could still turn on in debug builds.
>> Can you file a bug report requesting this now that bugs.swift.org
>> <http://bugs.swift.org/> is back up?
>
> Something like ConcurrentModificationException ought to be cheaper than the
> thread sanitizer. The thread sanitizer works hard to detect every concurrency
> error. ConcurrentModificationException typically does something simple and
> cheap that catches errors sometimes but makes no attempt to be exhaustive.
>
> Objective-C's fast enumeration protocol includes a mechanism like this. The
> collection can have a simple mutation counter, and the enumerator captures
> the counter's value at the start and checks that the value is unchanged as
> the enumeration proceeds.
>
>
> --
> Greg Parker [email protected] <mailto:[email protected]> Runtime
> Wrangler
>
>
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