[xml/sgml-pkgs] Bug#994765: xmlParseEntityDecl: entity xhtml-qname-extra.mod not terminated

[Torrance, Douglas]

Package: libxml2
Version: 2.9.12+dfsg-3
Severity: normal
Control: affects -1 src:macaulay2
X-Debbugs-Cc: dtorrance at piedmont.edu

Beginning with the upload of 2.9.12 to sid, the build of the Macaulay2 package
began failing when validating its html documentation.  For example, from [1,2]:

/usr/bin/make -C M2 validate-html
make[2]: Entering directory '/<<BUILDDIR>>/macaulay2-1.18.0.1+git202109031258/M2'
-- validating all html and xhtml files in /<<BUILDDIR>>/macaulay2-1.18.0.1+git202109031258/M2/usr-dist/common/share/doc/Macaulay2
validating: BGG/html/_direct__Image__Complex.html
*** invalid HTML: /<<BUILDDIR>>/macaulay2-1.18.0.1+git202109031258/M2/usr-dist/common/share/doc/Macaulay2/BGG/html/_direct__Image__Complex.html
error: line 338: xmlParseEntityDecl: entity xhtml-qname-extra.mod not terminated

...

validating: AlgebraicSplines/html/index.html
*** invalid HTML: /<<BUILDDIR>>/macaulay2-1.18.0.1+git202109031258/M2/usr-dist/common/share/doc/Macaulay2/AlgebraicSplines/html/index.html
error: line 338: xmlParseEntityDecl: entity xhtml-qname-extra.mod not terminated

9328 HTML files checked; 9328 invalid
make[2]: *** [GNUmakefile:302: validate-html] Error 1

A bit more information is given by running xmllint on one of the affected files:

$  xmllint --noout --loaddtd /usr/share/doc/Macaulay2/Macaulay2Doc/html/_ideal.html 
file:///usr/share/xml/w3c-sgml-lib/schema/dtd/WD-XHTMLplusMathMLplusSVG-20020809/xhtml-math-svg.dtd:338: parser error : xmlParseEntityDecl: entity xhtml-qname-extra.mod not terminated
   %xhtml-qname-extra.decl;
                           ^
Entity: line 2: 
           "http://www.w3.org/Math/DTD/mathml2/mathml2-qname-1.mod"
                                                                   ^
The problem appears to be that the latest release of libxml2 is more strict
when parsing DTD files, xhtml-math-svg.dtd in this particular case.

See also [3], which involves a similar error related to the file
xhtml1-strict.dtd.

[1] https://launchpadlibrarian.net/556859860/buildlog_ubuntu-impish-amd64.macaulay2_1.18.0.1+git202109031258-0ppa202109031444~ubuntu21.10.1_BUILDING.txt.gz
[2] https://github.com/Macaulay2/M2/issues/2225
[3] https://bugs.debian.org/993638
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