[swift-corelibs-dev] Memory leak with String.range(options: .regularExpression) [SR-3536]

[Alex Blewitt]

On 8 Feb 2017, at 16:51, Tony Parker via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
> 
> Hi Nethra,
> 
>> On Feb 7, 2017, at 11:44 PM, Nethra Ravindran via swift-corelibs-dev <swift-corelibs-dev at swift.org <mailto:swift-corelibs-dev at swift.org>> wrote:
>> 
>> Hi everyone,
>> 
>> I am working on https://bugs.swift.org/browse/SR-3536 <https://bugs.swift.org/browse/SR-3536>
>> There is a memory leak when searching for the substring of a string using regular expression.
>> 
>> 
>> import Foundation
>> 
>> let myString = "Foo"
>> for _ in 1...10000 {
>>   let _ = myString.range(of: "bar", options: .regularExpression)
>> }
>> 
>> 
>> From the above test case i could see that over a period of time, around 60 Mb of memory was leaked. 
>> 
>> I see in String.range we eventually call NSString._createRegexForPattern. Here we maintain a mapping between NSString and NSRegularExpression object in NSCache<NSString, NSRegularExpression>. All the entries in the cache are maintained in a dictionary ( _entries ) which takes the UnsafeRawPointer as the key, which seems to be the address of the NSString Object and NSCachedEntry as value.
>> 
>> Though the pattern is of type String, it is stored in the NSCache as NSString. And since we are storing the NSCachedEntry objects in a dictionary indexed by the address (UnsafeRawPointer) of the NSString object, there is a new cache entry created for each iteration ( in the test case ) though the pattern string remains the same.
>> 
>> Can someone guide me about how to go about resolving this issue.
>> 
> 
> Looks like you’ve done most of the analysis, so you’re already pretty much there. =)
> 
> Is there some other way we could be caching the results here?

There's a 'cache.countLimit = 10' set on the cache:

https://github.com/apple/swift-corelibs-foundation/blob/16657160c2c441a58ea01bf7baa90607a0b395f7/Foundation/NSString.swift#L109 <https://github.com/apple/swift-corelibs-foundation/blob/16657160c2c441a58ea01bf7baa90607a0b395f7/Foundation/NSString.swift#L109>

Shouldn't it start discarding some of the previous entries after it hits the first 10?

Alex
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