Free XML/HTML Parser Context after Use

[toddsundsted]

Parser contexts created with xmlNewParserCtxt() and htmlNewParserCtxt() are never freed. This PR adds missing cleanup calls (xmlFreeParserCtxt() and htmlFreeParserCtxt()) in four methods in src/xml.cr. I believe this is a regression introduced in PR #15906.

The following spec demonstrates the problem. I had considered adding this to the suite of tests, but decided against it because it is non-deterministic. Running this against a pre-1.17.x version succeeds. Running this against the version with this PR in place succeeds. Running this against 1.17.x to 1.18.x results in over 1GB of memory allocated but not freed. (More critically, if you adjust the number of iterations, the memory allocated scales linearly with iterations and never plateaus.)

require "spec"
require "xml"

describe "XML memory management" do
  it "does not leak parser contexts" do
    xml = "<root><item>test</item></root>"

    GC.collect
    baseline_rss = get_rss_kb

    1_000_000.times { XML.parse(xml) }

    GC.collect
    final_rss = get_rss_kb
    growth = final_rss - baseline_rss

    (growth < 50_000).should be_true
  end
end

private def get_rss_kb : Int64
  {% if flag?(:darwin) %}
    `ps -o rss= -p #{Process.pid}`.strip.to_i64
  {% else %}
    File.read("/proc/self/status").lines.each do |line|
      return line.split[1].to_i64 if line.starts_with?("VmRSS:")
    end
    0_i64
  {% end %}
end

[ysbaddaden]

Indeed 🤦

Polish: maybe there could be a pair #parse_xml(&) and #parse_html(&) methods. For example:

private def parse_xml(&)
  ctxt = LibXML.xmlNewParserCtxt
  begin
    from_ptr(ctxt) { yield ctxt }
  ensure
    LibXML.xmlFreeParserCtxt(ctxt)
  end
end

parse_xml do |ctxt|
  LibXML.xmlCtxtReadMemory(ctxt, ...)
end

[toddsundsted]

Indeed 🤦

Polish: maybe there could be a pair #parse_xml(&) and #parse_html(&) methods. For example:

private def parse_xml(&)
  ctxt = LibXML.xmlNewParserCtxt
  begin
    from_ptr(ctxt) { yield ctxt }
  ensure
    LibXML.xmlFreeParserCtxt(ctxt)
  end
end

parse_xml do |ctxt|
  LibXML.xmlCtxtReadMemory(ctxt, ...)
end

addressed in 36c769a

[Sija]

@toddsundsted Why was that closed? :(

[straight-shoota]

Seems like a clerical error. 😆
The same source branch is reused in #16412 which autoclosed this PR.