[Fuzi 0.1.1, iOS9, Swift 2] XMLElement.firstChild(...) crash: "libswiftCore.dylib`_swift_abortRetainUnowned:", "attempted to retain deallocated object".

[razvandub]

Input:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
    <SOAP-ENV:Header />
    <SOAP-ENV:Body>
        <sub:rr xmlns:sub="http://www.rr.com/schema">
            <sub:error>
                <sub:code>1234</sub:code>
            </sub:error>
        </sub:rr>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>"

Code:

...
guard let rootXML = try? XMLDocument(data: responseEnvelopeData).root else {
   ...
}

...

if let _ = rootXML?.firstChild(tag: "Body")?.firstChild(tag: "rr")?.firstChild(tag: "error")?.stringValue {
... 
}

...

Crash:

(lldb) bt

• thread Any chance u would provide a guide video or article? #15: tid = 0x6b9ffb, 0x0000000108b1899f libswiftCore.dylib`_swift_abortRetainUnowned + 15, queue = 'NSOperationQueue 0x7fcf91d2fb50 :: NSOperation 0x7fcf91e4f7b0 (QOS: LEGACY)', stop reason = EXC_BREAKPOINT (code=EXC_I386_BPT, subcode=0x0)
  • frame #0: 0x0000000108b1899f libswiftCore.dylib_swift_abortRetainUnowned + 15 frame #1: 0x0000000108b18986 libswiftCore.dylibswift_retainUnowned + 38
    frame Accessing parents children #2: 0x0000000105a2398b Fuzi`Fuzi.XMLElement.firstChild (tag="Body", ns=nil, self=0x00007fcf91e50290)(tag : Swift.String, inNamespace : Swift.Optional<Swift.String>) -> Swift.Optional<Fuzi.XMLElement> + 411 at Element.swift:81
    ...

[cezheng]

@razvandub Hello thanks for the feedback. The reason why this happens is that an XMLElement holds a weak reference to the document and in your code there is no strong reference to the document and it is released after you get the root element.

Changing your code to this will resolve your issue

...
guard let doc = try? XMLDocument(data: responseEnvelopeData) else {
   ...
}

...

if let _ = doc.root?.firstChild(tag: "Body")?.firstChild(tag: "rr")?.firstChild(tag: "error")?.stringValue {
... 
}

...

[razvandub]

Thanks @cezheng, I'll do that for now. Could you please update the readme to make this explicit since Fuzi's mem management is not intuitive and also because this was not the case with 0.1.0? Thanks.

[cezheng]

@razvandub Sorry about that. I was changing this in 0.1.1 to resolve a reference loop between root & document making the document instances never released in 0.1.0, but didn't realized that might be dangerous if users are not properly guided. I appreciate your feedback.

I will put some thoughts on how to make your code not crash first. If I can't come up with a way of doing that I will update the README

[razvandub]

No problem @cezheng, thanks.

[stepanhruda]

@cezheng Why do you use unowned rather than weak?

[cezheng]

@stepanhruda There're 2 reasons I don't use weak

• because weak can only be used on a mutable optional, and an XMLElement cannot function properly if the document is nil.
• when the document is deallocated, the underlying libxml2 document will be freed as well, i.e. the underlying libxml2 node of the document is also freed, still making code like this to crash.

So currently, the proper way to use this library is to keep a strong reference on the document rather than the element. If you have any thoughts on improving this, I will appreciate it if you could share your thoughts or even a PR