non-thread-safe XPathFactory reused in XpathDirective.FACTORY

[rgevaerd]

I believe this reuse of non-thread-safe object by xembly is causing zerocracy/farm#1167 (comment):

java.lang.ArrayIndexOutOfBoundsException: 8
	at java.util.ArrayList.add(ArrayList.java:459)
	at net.sf.saxon.functions.FunctionLibraryList.addFunctionLibrary(FunctionLibraryList.java:44)
	at net.sf.saxon.Configuration.getBuiltInExtensionLibraryList(Configuration.java:1388)
	at net.sf.saxon.sxpath.AbstractStaticContext.setDefaultFunctionLibrary(AbstractStaticContext.java:127)
	at net.sf.saxon.xpath.JAXPXPathStaticContext.<init>(JAXPXPathStaticContext.java:56)
	at net.sf.saxon.xpath.XPathEvaluator.<init>(XPathEvaluator.java:71)
	at net.sf.saxon.xpath.XPathFactoryImpl.newXPath(XPathFactoryImpl.java:203)
	at org.xembly.XpathDirective.traditional(XpathDirective.java:138)
	at org.xembly.XpathDirective.exec(XpathDirective.java:98)
	at org.xembly.Xembler.apply(Xembler.java:155)
	at org.xembly.Xembler.dom(Xembler.java:210)
	at org.xembly.Xembler.xml(Xembler.java:256)
	at org.xembly.Xembler.xmlQuietly(Xembler.java:221)
	at com.zerocracy.claims.ClaimOut.postTo(ClaimOut.java:127)
	at com.zerocracy.pm.FootprintTest.lambda$addsInThreads$0(FootprintTest.java:88)
	at com.zerocracy.RunsInThreads.lambda$matchesSafely$0(RunsInThreads.java:90)
	at com.jcabi.log.VerboseCallable.call(VerboseCallable.java:173)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at com.jcabi.log.VerboseThreads$Wrap.run(VerboseThreads.java:221)
	at java.lang.Thread.run(Thread.java:745)

As you can see in XPathFactory documentation (https://docs.oracle.com/javase/8/docs/api/javax/xml/xpath/XPathFactory.html), it is not thread-safe and must never be used by more than one thread concurrently.
Xembly stores in the static final attribute XpathDirective.FACTORY an instance of such object and reuses it in many operations. Because of this, any multi-threaded application are subject to suffer misbehavior when using Xembly.
The error above is one example of such misbehavior that may happen when using Xembly in more than one thread. In this case this exception occurs on a race condition when calling Xembler.xmlQuietly on different threads, even with the threads using their own Xembler instance (because xembly internally will reuse the same XPathFactory causing the error).

[0crat]

@carlosmiranda/z please, pay attention to this issue

[0crat]

@rgevaerd/z this project will fix the problem faster if you donate a few dollars to it; just click here and pay via Stripe, it's very fast, convenient and appreciated; thanks a lot!

[carlosmiranda]

@rgevaerd can you revise the description a bit with some more details? What is the problematic code and why?

[rgevaerd]

@carlosmiranda added more details to the description.

[carlosmiranda]

@0crat in

[0crat]

@0crat in (here)

@carlosmiranda Job #82 is now in scope, role is DEV

[0crat]

Bug was reported, see §29: +15 point(s) just awarded to @rgevaerd/z

[0crat]

@carlosmiranda/z everybody who has role DEV is banned at #82; I won't be able to assign anyone automatically; consider assigning someone manually (as in §19), or invite more people (as in §51), or remove the job from the scope (as in §14)

[0crat]

The job #82 assigned to @carlosmiranda/z, here is why; the budget is 30 minutes, see §4; please, read §8 and §9; if the task is not clear, read this and this; there will be a monetary reward for this job

[0crat]

Manual assignment of issues is discouraged, see §19: -5 point(s) just awarded to @carlosmiranda/z

[carlosmiranda]

@rgevaerd should be fixed in #83, I am working on releasing a patch.

[carlosmiranda]

@rgevaerd Released as 0.22.1, see https://github.com/yegor256/xembly/releases. Please check and close if all is in order.

[rgevaerd]

@carlosmiranda Thanks

[0crat]

Job was finished in 21 hours, bonus for fast delivery is possible (see §36)

[0crat]

Order was finished: +35 point(s) just awarded to @carlosmiranda/z

[0crat]

The job #82 is now out of scope