Makefile: add 'check' target, consolidating xmllint invocations

Previously the html and pdf targets would also verify the input via
xmllint. This caused some code duplication and overloaded the targets
which an arguably unrelated task (verification).

Furthermore, something changed and the

! xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" $< 2>/dev/null && true

no fails. It seems that xmllint does no longer (?) report an error
exit status if the XPath result set is empty [1]. Therfore, this rule
is currently commented out, which fixed #1316.

1: https://gitlab.gnome.org/GNOME/libxml2/-/issues/673

.github/workflows/xep-validation.yml

@@ -31,5 +31,11 @@ jobs:
             if ! tools/validate-xep0001-conformance.sh "$xep"; then
               result=1
             fi
+            if [[ ${xep} == xep-* ]]; then
+              echo Checking ${xep} by invoking \"make .${xep}.check.ok\"
+              if ! make ".${xep}.check.ok"; then
+                result=1
+              fi
+            fi
           done
           exit $result

.gitignore

@@ -75,3 +75,5 @@ Session.vim
 /tools/xeps-email.conf
 /tmp/
 /pr-worktree/
+
+.*.xml.check.ok

Makefile

@@ -36,6 +36,7 @@ xep_pdfs=$(patsubst %.xml,$(OUTDIR)/%.pdf,$(xeps))
 xep_refs=$(patsubst xep-%.xml, $(REFSDIR)/reference.XSF.XEP-%.xml, $(xeps))
 xep_examples=$(patsubst xep-%.xml, $(EXAMPLESDIR)/%.xml, $(xeps))
 
+all_xep_check_ok=$(patsubst %.xml, .%.xml.check.ok, $(xeps))
 
 .PHONY: help
 help:
@@ -46,6 +47,7 @@ help:
 	@echo '                  refs  -  build all IETF refs'
 	@echo '                  html  -  build all XEPs'
 	@echo '            inbox-html  -  build all ProtoXEPs'
+	@echo '                 check  -  check all XEPs for errors'
 	@echo '                 clean  -  recursively unlink the build tree'
 	@echo '               preview  -  builds html whenever an XEP changes (requires inotify-tools)'
 	@echo '              examples  -  extract all examples'
@@ -82,6 +84,9 @@ refs: $(xep_refs)
 .PHONY: examples
 examples: $(xep_examples)
 
+.PHONY: check
+check: $(all_xep_check_ok)
+
 .PHONY: xep-%
 xep-%: $(OUTDIR)/xep-%.html $(REFSDIR)/reference.XSF.XEP-%.xml $(OUTDIR)/xep-%.pdf $(EXAMPLESDIR)/%.xml;
 
@@ -106,30 +111,24 @@ $(EXAMPLESDIR)/%.xml: xep-%.xml $(XMLDEPS) examples.xsl | $(EXAMPLESDIR)
 $(REFSDIR)/reference.XSF.XEP-%.xml: xep-%.xml $(XMLDEPS) ref.xsl | $(REFSDIR)
 	xsltproc --path $(CURDIR) ref.xsl "$<" > "$@" && echo "Finished building $@"
 
-$(xep_htmls): $(OUTDIR)/xep-%.html: xep-%.xml $(XMLDEPS) $(HTMLDEPS) | $(OUTDIR)
+.%.xml.check.ok: %.xml
 	xmllint --nonet --noout --noent --loaddtd --valid "$<"
-	# Check for non-data URIs
-	! xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" $< 2>/dev/null && true
+	# Check for non-data URIs, the result set of the XPath expression below should be empty
+	# Disabled for now, see
+	# https://github.com/xsf/xeps/issues/1316 and https://gitlab.gnome.org/GNOME/libxml2/-/issues/673
+	# xmllint --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" $< > /dev/null
+	touch $@
 
-	# Actually build the HTML
+$(xep_htmls): $(OUTDIR)/xep-%.html: xep-%.xml $(XMLDEPS) $(HTMLDEPS) | $(OUTDIR)
 	xsltproc --path $(CURDIR) --param htmlbase "$(if $(findstring inbox,$<),'../','./')" xep.xsl "$<" > "$@" && echo "Finished building $@"
 
 $(proto_xep_htmls): $(OUTDIR)/inbox/%.html: inbox/%.xml $(XMLDEPS) $(proto_HTMLDEPS) | $(OUTDIR)
-	xmllint --nonet --noout --noent --loaddtd --valid "$<"
-	# Check for non-data URIs
-	! xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" $< 2>/dev/null && true
-
-	# Actually build the HTML
 	xsltproc --path $(CURDIR) --param htmlbase "$(if $(findstring inbox,$<),'../','./')" xep.xsl "$<" > "$@" && echo "Finished building $@"
 
 $(OUTDIR)/xmpp.pdf $(OUTDIR)/xmpp-text.pdf: | $(OUTDIR)
 	cp "resources/$(notdir $@)" "$@"
 
 $(OUTDIR)/%.pdf: %.xml $(XMLDEPS) $(TEXMLDEPS)
-	xmllint --nonet --noout --noent --loaddtd --valid "$<"
-	# Check for non-data URIs
-	! xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" $< 2>/dev/null && true
-
 	xsltproc --path $(CURDIR) xep2texml.xsl "$<" > "$(@:.pdf=.tex.xml)"
 	texml -e utf8 "$(@:.pdf=.tex.xml)" "$(@:.pdf=.tex)"
 	sed -i -e 's|\([\s"]\)\([^"]http://[^ "]*\)|\1\\path{\2}|g' \