feat: support wildcard namespaces in xpath queries

This is almost as fast as a standard child-axis search, and much
faster than the builtin or using local-name():

  //span
       18.923k (± 8.9%) i/s -     93.906k in   5.010792s
  //*[local-name()='span']
        1.849k (± 2.8%) i/s -      9.261k in   5.011560s
  //*[nokogiri-builtin:local-name-is('span')]
        3.191k (± 2.4%) i/s -     16.150k in   5.064798s
  //*:span
       18.016k (± 4.6%) i/s -     89.900k in   5.003444s

  Comparison:
  //span:
      18922.5 i/s
  //*:span:
      18016.5 i/s - same-ish: difference falls within error
  //*[nokogiri-builtin:local-name-is('span')]:
       3190.6 i/s - 5.93x  (± 0.00) slower
  //*[local-name()='span']:
       1849.4 i/s - 10.23x  (± 0.00) slower

lib/nokogiri/css/xpath_visitor.rb

@@ -7,6 +7,8 @@ module CSS
     # class allows for changing some of the behaviors related to builtin xpath functions and quirks
     # of HTML5.
     class XPathVisitor
+      WILDCARD_NAMESPACES = Nokogiri.libxml2_patches.include?("0009-allow-wildcard-namespaces.patch") # :nodoc:
+
       # Enum to direct XPathVisitor when to use Nokogiri builtin XPath functions.
       module BuiltinsConfig
         # Never use Nokogiri builtin functions, always generate vanilla XPath 1.0 queries. This is
@@ -254,7 +256,11 @@ def visit_element_name(node)
           # HTML5 has namespaces that should be ignored in CSS queries
           # https://github.com/sparklemotion/nokogiri/issues/2376
           if @builtins == BuiltinsConfig::ALWAYS || (@builtins == BuiltinsConfig::OPTIMAL && Nokogiri.uses_libxml?)
-            "*[nokogiri-builtin:local-name-is('#{node.value.first}')]"
+            if WILDCARD_NAMESPACES
+              "*:#{node.value.first}"
+            else
+              "*[nokogiri-builtin:local-name-is('#{node.value.first}')]"
+            end
           else
             "*[local-name()='#{node.value.first}']"
           end

patches/libxml2/0009-allow-wildcard-namespaces.patch

@@ -0,0 +1,77 @@
+From 96e31fa0a94daa42aae174f2eb3f42e1ba7d5720 Mon Sep 17 00:00:00 2001
+From: Mike Dalessio <mike.dalessio@gmail.com>
+Date: Fri, 24 Dec 2021 19:08:01 -0500
+Subject: [PATCH] attempt to hack in wildcard namespaces to xpath
+
+I'm not confident this is a bulletproof patch.
+---
+ xpath.c | 24 ++++++++++++++++++------
+ 1 file changed, 18 insertions(+), 6 deletions(-)
+
+diff --git a/xpath.c b/xpath.c
+index 1aa2f1a..dd28d4d 100644
+--- a/xpath.c
++++ b/xpath.c
+@@ -146,6 +146,9 @@
+ #define XPATH_MAX_RECURSION_DEPTH 5000
+ #endif
+
++#define WILDCARD_PREFIX "*"
++#define IS_WILDCARD_PREFIX(p) xmlStrEqual((xmlChar*)WILDCARD_PREFIX, prefix)
++
+ /*
+  * TODO:
+  * There are a few spots where some tests are done which depend upon ascii
+@@ -11073,12 +11076,15 @@ xmlXPathCompNodeTest(xmlXPathParserContextPtr ctxt, xmlXPathTestVal *test,
+     SKIP_BLANKS;
+
+     if ((name == NULL) && (CUR == '*')) {
+-	/*
+-	 * All elements
+-	 */
+ 	NEXT;
+-	*test = NODE_TEST_ALL;
+-	return(NULL);
++	if (CUR != ':') {
++	    /*
++	     * All elements
++	     */
++	    *test = NODE_TEST_ALL;
++	    return(NULL);
++	}
++	name = xmlCharStrdup(WILDCARD_PREFIX);
+     }
+
+     if (name == NULL)
+@@ -11327,6 +11333,10 @@ xmlXPathCompStep(xmlXPathParserContextPtr ctxt) {
+ 	}
+ #endif
+ 	if (CUR == '*') {
++	    if (NXT(1) == ':') {
++		NEXT;
++		name = xmlCharStrdup(WILDCARD_PREFIX);
++	    }
+ 	    axis = AXIS_CHILD;
+ 	} else {
+ 	    if (name == NULL)
+@@ -12030,7 +12040,7 @@ xmlXPathNodeCollectAndTest(xmlXPathParserContextPtr ctxt,
+     /*
+     * Setup namespaces.
+     */
+-    if (prefix != NULL) {
++    if (prefix != NULL && !IS_WILDCARD_PREFIX(prefix)) {
+         URI = xmlXPathNsLookup(xpctxt, prefix);
+         if (URI == NULL) {
+ 	    xmlXPathReleaseObject(xpctxt, obj);
+@@ -12369,6 +12379,8 @@ xmlXPathNodeCollectAndTest(xmlXPathParserContextPtr ctxt,
+ 				    {
+ 					XP_TEST_HIT
+                                     }
++                                } else if (IS_WILDCARD_PREFIX(prefix)) {
++                                    XP_TEST_HIT
+                                 } else {
+                                     if ((cur->ns != NULL) &&
+                                         (xmlStrEqual(URI, cur->ns->href)))
+-- 
+2.31.0
+

test/css/test_xpath_visitor.rb

@@ -565,6 +565,34 @@ def visit_pseudo_class_aaron(node)
       end
     end
 
+    #
+    #  HTML5 documents have namespaces, and gumbo attaches namespaces to the relevant elements; but
+    #  CSS selectors should not require namespaces. See #2376 for the discussion around this design
+    #  decision, along with some of the relevant benchmarks and call stack analyses.
+    #
+    #  (HTML5 today is only supported by CRuby/gumbo/libxml2 and so we'll ignore JRuby support for
+    #  now.)
+    #
+    #  The way to implement this CSS search using standard XPath 1.0 queries is to check for a match
+    #  with `local-name()`. However, this is about ~10x slower than a standard search along the
+    #  `child` axis.
+    #
+    #  I've written a builtin function in C named `nokogiri-builtin:local-name-is()` which is a bit
+    #  faster, but still ~7x slower than a standard search.
+    #
+    #  Finally, I've patched libxml2 to support wildcard namespaces, and this is ~1.1x slower but
+    #  only available with the packaged libxml2.
+    #
+    #  In any case, the logic for the html5 builtins here goes:
+    #
+    #    if ALWAYS or (OPTIMAL and libxml2)
+    #      if we've patched libxml2 with wildcard support
+    #        use wildard namespacing
+    #      else
+    #        use `nokogiri-builtin:local-name-is()`
+    #    else
+    #      use `local-name()`
+    #
     describe "doctype:html5" do
       let(:visitor) do
         Nokogiri::CSS::XPathVisitor.new(
@@ -575,8 +603,13 @@ def visit_pseudo_class_aaron(node)
 
       describe "builtins:always" do
         let(:builtins) { Nokogiri::CSS::XPathVisitor::BuiltinsConfig::ALWAYS }
+
         it "matches on the element's local-name, ignoring namespaces" do
-          assert_xpath("//*[nokogiri-builtin:local-name-is('foo')]", parser.parse("foo"))
+          if Nokogiri.libxml2_patches.include?("0009-allow-wildcard-namespaces.patch")
+            assert_xpath("//*:foo", parser.parse("foo"))
+          else
+            assert_xpath("//*[nokogiri-builtin:local-name-is('foo')]", parser.parse("foo"))
+          end
         end
 
         it "avoids the wildcard when using namespaces" do
@@ -595,7 +628,11 @@ def visit_pseudo_class_aaron(node)
         let(:builtins) { Nokogiri::CSS::XPathVisitor::BuiltinsConfig::OPTIMAL }
         it "matches on the element's local-name, ignoring namespaces" do
           if Nokogiri.uses_libxml?
-            assert_xpath("//*[nokogiri-builtin:local-name-is('foo')]", parser.parse("foo"))
+            if Nokogiri.libxml2_patches.include?("0009-allow-wildcard-namespaces.patch")
+              assert_xpath("//*:foo", parser.parse("foo"))
+            else
+              assert_xpath("//*[nokogiri-builtin:local-name-is('foo')]", parser.parse("foo"))
+            end
           else
             assert_xpath("//*[local-name()='foo']", parser.parse("foo"))
           end

test/xml/test_xpath.rb

@@ -699,6 +699,27 @@ def collision(nodes)
           end
         end
       end
+
+      describe "XPath wildcard namespaces" do
+        let(:xml) { <<~XML }
+          <root xmlns:ns1="http://nokogiri.org/ns1" xmlns:ns2="http://nokogiri.org/ns2">
+            <ns1:child>ns1 child</ns1:child>
+            <ns2:child>ns2 child</ns2:child>
+            <child>plain child</child>
+          </root>
+        XML
+
+        let(:doc) { Nokogiri::XML::Document.parse(xml) }
+
+        it "allows namespace wildcards" do
+          skip_unless_libxml2_patch("0009-allow-wildcard-namespaces.patch")
+
+          assert_equal(1, doc.xpath("//n:child", { "n" => "http://nokogiri.org/ns1" }).length)
+          assert_equal(3, doc.xpath("//*:child").length)
+          assert_equal(1, doc.xpath("//self::n:child", { "n" => "http://nokogiri.org/ns1" }).length)
+          assert_equal(3, doc.xpath("//self::*:child").length)
+        end
+      end
     end
   end
 end