StackOverFlowError with arabic characters - FOP issue or font issue?

[Tim.Weller]

Hi,

I am having a problem when using FOP with an Arabic font (Hasubi, 
https://github.com/eliheuer/hasubi-mono) and the following string: لأ

Input XML:
<?xml version="1.0" encoding="UTF-8"?>
<string>لأ</string>

Stylesheet:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
  xmlns:fo="http://www.w3.org/1999/XSL/Format"; version="1.0">
  <xsl:output method="xml" indent="yes" encoding="UTF-8"/>
  <xsl:template match="/">
    <fo:root>
      <fo:layout-master-set>
        <fo:simple-page-master master-name="A4" page-height="29.7cm" 
page-width="21cm" margin="1cm">
          <fo:region-body/>
        </fo:simple-page-master>
      </fo:layout-master-set>
      <fo:page-sequence master-reference="A4">
        <fo:flow flow-name="xsl-region-body">
          <xsl:apply-templates select="string"/>
        </fo:flow>
      </fo:page-sequence>
    </fo:root>
  </xsl:template>
  <xsl:template match="string">
    <fo:block font-family="CustomFont" font-size="10pt">
      <xsl:value-of select="."/>
    </fo:block>
  </xsl:template>
</xsl:stylesheet>

Custom font Hasubi is defined in my fop.xconf:
<?xml version="1.0"?>
<fop version="1.0">
    <strict-configuration>true</strict-configuration>
    <strict-validation>false</strict-validation>
    <base>.</base>
    <renderers>
        <renderer mime="application/pdf">
            <fonts>
                <font kerning="yes" embed-url="file:///C:/Temp/Hasubi-Mono.ttf">
                  <font-triplet name="CustomFont" style="normal" 
weight="normal"/>
                </font>
            </fonts>
        </renderer>
    </renderers>
</fop>

I have tested with OpenJDK 17 and FOP 2.11 on windows starting via commandline:
fop-2.11\fop\fop.bat -c fop.xconf -xml input.xml -xsl stylesheet.xsl -pdf 
out.pdf

Resulting in the following stacktrace:
Aug. 21, 2025 10:11:31 AM org.apache.fop.apps.FOUserAgent processEvent
INFORMATION: Rendered page #1.
Exception in thread "main" java.lang.StackOverflowError
        at java.base/java.util.HashMap.putVal(HashMap.java:627)
        at java.base/java.util.HashMap.put(HashMap.java:610)
        at java.base/java.util.HashSet.add(HashSet.java:221)
        at 
org.apache.fop.fonts.truetype.GlyfTable.retrieveComposedGlyphs(GlyfTable.java:225)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:158)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:160)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:160)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:160)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:160)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:160)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:160)
        at 
org.apache.fop.fonts.truetype.GlyfTable.scanGlyphsRecursively(GlyfTable.java:160)
(and so on)

So, the question: Is this a FOP issue (stack overflow should be 
detected/avoided) or is the font simply broken in some way? – Testing with 
other fonts (e.g. Cascadia Mono or Noto Sans Arabic) works without issues.


Thanks,
Tim Weller