Document for JRuby users the best practices for thread safety (doc.xpath is not thread-safe on JRuby)

[kares]

somehow, I got the impression that xpath is thread-safe (even under JRuby)

... or at least fixable (with the xpath-support cache) when its just an expr (no binds/fn-handler passed)

but was proven wrong as I added a test :

      def test_concurrent_xpath
        # skip("no need to run under MRI") unless Nokogiri.jruby?
        doc = Nokogiri::XML(File.open(XPATH_FILE))

        threads = []; errors = []
        4.times do |i|
          threads << Thread.start do
            begin
              sleep 0.01 * i
              assert_equal 11, doc.xpath('.//category').size
              assert_equal 1165, doc.xpath('//module').size
              assert_equal 10353, doc.xpath('//module/param').size
              assert_equal 14, doc.xpath('//param[@name="href"]').size
            rescue Exception => ex
              errors << ex
            end
          end
        end
        threads.each(&:join)
        raise errors.first if errors.any?
      end

it either reports incorrect results or in a better case fails with an error such as :

Nokogiri::XML::TestXPath#test_concurrent_xpath:
Java::JavaLang::ArrayIndexOutOfBoundsException: -1
    org.apache.xml.utils.IntStack.push(IntStack.java:86)
    org.apache.xpath.XPathContext.pushCurrentNode(XPathContext.java:793)
    org.apache.xpath.axes.PredicatedNodeTest.acceptNode(PredicatedNodeTest.java:468)
    org.apache.xpath.axes.DescendantIterator.nextNode(DescendantIterator.java:222)
    org.apache.xpath.axes.NodeSequence.nextNode(NodeSequence.java:335)
    org.apache.xpath.axes.NodeSequence.runTo(NodeSequence.java:494)
    org.apache.xml.dtm.ref.DTMNodeList.<init>(DTMNodeList.java:81)
    org.apache.xpath.objects.XNodeSet.nodelist(XNodeSet.java:346)
    nokogiri.XmlXpathContext.tryGetNodeSet(XmlXpathContext.java:190)
    nokogiri.XmlXpathContext.node_set(XmlXpathContext.java:163)
    nokogiri.XmlXpathContext.evaluate(XmlXpathContext.java:130)

this is caused by re-using the same cached xpath object between threads.
on the other hand, maybe there never was an official guarantee regarding thread-safety.

[flavorjones]

@kares has submitted a fix for this in sparklemotion/nokogiri#1597

[flavorjones]

Ah, my mistake, sparklemotion/nokogiri#1597 does not contain a fix for this. Ignore my previous comment.

[flavorjones]

I've pushed a branch, 1596_kares_xpath-not-threadsafe, with this failing test in it.

[kares]

@flavorjones thanks, was more interested in your input on this (if you have any or know anyone who does).

somehow I assumed its meant to be (that doc.xpath is "officially") thread-safe, but of course with the caching done as is I was proven wrong (doing sparklemotion/nokogiri#1597 I assumed that its only the binding cases that are problematic but its the whole underlying cached internal XPath structure). was thinking about introducing a system property to switch to a thread-local cache as it would make sense in some scenarios (but probably not to have it on by default) ?

basically current status is: doc.xpath not thread-safe due the caching, without the caching its (terribly slow but) thread-safe. so the switch might have several states:

• -Dnokogiri.xpath.cache=true (default)
• -Dnokogiri.xpath.cache=thread
• -Dnokogiri.xpath.cache=false

[flavorjones]

@jvshahid Any thoughts on this?

[flavorjones]

I've rebased the branch 1596_kares_xpath-not-threadsafe just to try to keep this issue up to date. Still not sure what the right thing to do is.

[jvshahid]

was thinking about introducing a system property to switch to a thread-local cache as it would make sense in some scenarios (but probably not to have it on by default) ?

Why not ? I don't the idea of introducing more flags that the user has to discover and configure properly. I think Nokogiri should DTRT and have a thread local cache.

[jvshahid]

I researched this a little bit more. The underlying libraries aren't thread safe. Java DOM's specification doesn't require implementation and Xerces certainly is indeed not thread safe according to their FAQ 1. I think the right thing to do is to document this fact somewhere on the docs site, i.e. best practices for concurrent usage of Nokogiri.

[flavorjones]

Updated title to indicate the plan is to document thread-safety limitations, and am deleting the branch with the failing test.

[kares]

I don't the idea of introducing more flags that the user has to discover and configure properly. I think Nokogiri should DTRT and have a thread local cache.

👍 should be the default despite it having a negative performance impact,
some 'power' users might want to change to a non-thread-safe cache option or maybe not ...