Saxon

NPE occurs in the following code (dita2wml_global_drawing.xsl):

<xsl:for-each select="$map/descendant::*[contains(@class,' bookmap/toc ')]">
    <xsl:variable name="toc" as="element()" select="."/>
    <xsl:variable name="tocId" as="xs:string" select="ahf:generateId($toc)"/>
    <xsl:sequence select="$tocId"/>
    <xsl:for-each select="$map/descendant::*[ahf:seqContains(string(@class),(' bookmap/chapter ',' bookmap/appendix ',' bookmap/indexlist'))]">
        <xsl:variable name="topicRef" as="element()" select="."/>
        <xsl:variable name="chapterNo" as="xs:string" select="ahf:getChapterNo($topicRef)"/>
        <xsl:sequence select="concat($tocId,'-',$chapterNo)"/>
    </xsl:for-each>
</xsl:for-each></code>

If I modified such as following, the NPE has been vanished.

<xsl:variable name="toc" as="element()?" select="$map/descendant::*[contains(@class,' bookmap/toc ')][1]"/>
<xsl:if test="exists($toc)">
    <xsl:variable name="tocId" as="xs:string" select="ahf:generateId($toc)"/>
    <xsl:sequence select="$tocId"/>
    <xsl:for-each select="$map/descendant::*[ahf:seqContains(string(@class),(' bookmap/chapter ',' bookmap/appendix ',' bookmap/indexlist'))]">
        <xsl:variable name="topicRef" as="element()" select="."/>
        <xsl:variable name="chapterNo" as="xs:string" select="ahf:getChapterNo($topicRef)"/>
        <xsl:sequence select="concat($tocId,'-',$chapterNo)"/>
    </xsl:for-each>
</xsl:if></code>

I have reproduced the problem, but so far I'm no nearer to working out why the relevant expression has no retainedStaticContext. It had one when it was first created, but it's been through a number of complex rewrites (function inlining, loop lifting etc) before the failure occurs.

The two methods Expression.setRetainedStaticContext() and Expression.setRetainedStaticContextThoroughly() don't check that the supplied static context is non-null. Since an expression can have a null retained static context transiently while it is being created, this creates the risk that the null value for retainedStaticContext will be propagated and overwrite the retained static context of expressions that already have an RSC.

Changing this methods to do nothing if the supplied RSC is null solves the problem for this test case. It's not a 100% solution that ensures the retained static context will never be null, but it's a useful defensive measure.