[swift-corelibs-dev] Encountering "Constant strings cannot be deallocated" in Linux foundation

Wed Nov 29 22:31:59 CST 2017

I think that perhaps we have a problem with the retain count flags in the definition of CFSTR. Previously the pinned reference would prevent the deallocation. This may have gotten clobbered with the latest CF import.

Sent from my iPhone

> On Nov 29, 2017, at 7:55 PM, Brandon Williams via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
> 
> Hey Tony,
> 
> We were able to fix our crashes by avoiding our uses of `URLQueryItem`s. In one place I had to completely remove the use of `URLQueryItem`:
> 
> https://github.com/pointfreeco/swift-web/pull/79/files#diff-293b73a926ba418bd9511ef735fc947cL147
> 
> And in another my colleague Stephen Celis discovered that duplicate keys in a query string can cause this to crash:
> 
> https://github.com/pointfreeco/swift-web/pull/80
> 
> He was able to come up with a minimal test case to demonstrate this:
> 
> https://github.com/pointfreeco/swift-web/commit/cfa9b519a829f1c1627620521326f0877c2d023a
> 
> And you can see the failure in this Travis CI report:
> 
> https://travis-ci.org/pointfreeco/swift-web/builds/309316418
> 
> At the bottom you’ll find:
> 
> ```fatal error: Constant strings cannot be deallocated: file Foundation/NSCFString.swift, line 118```
> 
> So definitely seems to be in that area!
> 
> Now, as far as rebuilding swift-corelibs-foundation, I’m down to try but I dont really know much about how to do that. With some instructions I could give it a shot.
> 
> Thanks for the help!
> 
> 
> 
> 
>> On Wed, Nov 29, 2017 at 5:59 PM Tony Parker <anthony.parker at apple.com> wrote:
>> I see a couple of places that are suspicious there:
>> 
>> Description:
>> 
>> https://github.com/apple/swift-corelibs-foundation/blob/master/CoreFoundation/URL.subproj/CFURLComponents.c#L66
>> 
>> And the nameString / valueString constants in the copy query items function:
>> 
>> https://github.com/apple/swift-corelibs-foundation/blob/master/CoreFoundation/URL.subproj/CFURLComponents.c#L1057
>> https://github.com/apple/swift-corelibs-foundation/blob/master/CoreFoundation/URL.subproj/CFURLComponents.c#L1079
>> https://github.com/apple/swift-corelibs-foundation/blob/master/CoreFoundation/URL.subproj/CFURLComponents.c#L1104
>> https://github.com/apple/swift-corelibs-foundation/blob/master/CoreFoundation/URL.subproj/CFURLComponents.c#L1134
>> https://github.com/apple/swift-corelibs-foundation/blob/master/CoreFoundation/URL.subproj/CFURLComponents.c#L1158
>> 
>> Do you have the ability to re-build swift-corelibs-foundation and run a test? I suspect if you replace those with CFRetain(CFSTR(…)) then it will fix your crash.
>> 
>> - Tony
>> 
>> 
>>> On Nov 29, 2017, at 2:06 PM, Brandon Williams <mbw234 at gmail.com> wrote:
>>> 
>>> Thanks for the info!
>>> 
>>> FWIW, most of the places I have encountered this so far have made use of `URLComponents`, and have even been able to eliminate the crash by getting rid of that code in a few places. The other JIRA bug on this topic also mentions URLComponents in their repro case. Seems to be the culprit.
>>> 
>>> Also worth noting that in the above cases dealing with URLComponents the crash only happens in DEBUG compilations, not RELEASE.
>>> 
>>> However, I do have another one of these crashes that _does_ happen on RELEASE builds that I haven’t yet been able to reduce.
>>> 
>>> 
>>>> On Wed, Nov 29, 2017 at 4:43 PM Tony Parker <anthony.parker at apple.com> wrote:
>>>> Hi Brandon,
>>>> 
>>>> This is probably a bug in the CoreFoundation C sources part of swift-corelibs-foundation. Unlike Darwin platforms, we can’t make the CFSTR(“”) macro produce a CFStringRef that cannot be overreleased. Probably there is a constant string returned from CF function, then the Swift runtime assumes it can release it, and bam we wind up with an overrelease.
>>>> 
>>>> We’ll have to track down which function is getting called, which probably just requires stepping through that test case with a debugger…
>>>> 
>>>> - Tony
>>>>> On Nov 29, 2017, at 1:34 PM, Brandon Williams via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
>>>>> 
>>>> 
>>>>> Hello all!
>>>>> 
>>>>> We’ve been encountering this runtime error quite a bit recently, and we have no idea why. We’ve filed a bug and there’s one other on JIRA related to this, but both without any comments:
>>>>> 
>>>>> https://bugs.swift.org/browse/SR-6422
>>>>> https://bugs.swift.org/browse/SR-6398
>>>>> 
>>>>> We’re curious if others are aware of this and or has more info? Is it a red herring for some other problem?
>>>>> 
>>>>> Any info would be helpful!
>>>> 
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>>>>> swift-corelibs-dev at swift.org
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>> 
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