apache/hadoop · error · IOException

Got unexpected tag end event for {} while looking for sectio

Error message

Got unexpected tag end event for {} while looking for section header tag.

What it means

While looking for the next section header inside <fsimage>, the ReverseXML StAX reader received an END_ELEMENT whose name is not fsimage. In valid OIV XML the only end tag that can legally appear at this position is </fsimage> itself, so an unexpected closing tag means the top-level element nesting is broken — typically a stray or duplicated closing tag left behind by editing or splicing.

Source

Thrown at hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/tools/offlineImageViewer/OfflineImageReconstructor.java:1775

    // Write the HDFSIMG1 magic number which begins the fsimage file.
    out.write(FSImageUtil.MAGIC_HEADER);
    // Write a series of fsimage sections.
    sectionStartOffset = FSImageUtil.MAGIC_HEADER.length;
    final HashSet<String> unprocessedSections =
        new HashSet<>(sections.keySet());
    while (!unprocessedSections.isEmpty()) {
      XMLEvent ev = expectTag("[section header]", true);
      if (ev.getEventType() == XMLStreamConstants.END_ELEMENT) {
        if (ev.asEndElement().getName().getLocalPart().equals("fsimage")) {
          if(unprocessedSections.size() == 1 && unprocessedSections.contains
                  (SnapshotDiffSectionProcessor.NAME)){
            break;
          }
          throw new IOException("FSImage XML ended prematurely, without " +
              "including section(s) " + StringUtils.join(", ",
              unprocessedSections));
        }
        throw new IOException("Got unexpected tag end event for " +
            ev.asEndElement().getName().getLocalPart() + " while looking " +
            "for section header tag.");
      } else if (ev.getEventType() != XMLStreamConstants.START_ELEMENT) {
        throw new IOException("Expected section header START_ELEMENT; " +
            "got event of type " + ev.getEventType());
      }
      String sectionName = ev.asStartElement().getName().getLocalPart();
      if (!unprocessedSections.contains(sectionName)) {
        throw new IOException("Unknown or duplicate section found for " +
            sectionName);
      }
      SectionProcessor sectionProcessor = sections.get(sectionName);
      if (sectionProcessor == null) {
        throw new IOException("Unknown FSImage section " + sectionName +
            ".  Valid section names are [" +
            StringUtils.join(", ", sections.keySet()) + "]");
      }
      unprocessedSections.remove(sectionName);

View on GitHub (pinned to 2add963021)

Solutions

Regenerate the XML from the original fsimage via `hdfs oiv -p XML`
Run `xmllint --noout image.xml` — an unmatched closing tag is a well-formedness error and xmllint names the tag and line number
Diff the edited file against pristine OIV output to find where the nesting broke

Example fix

# before: reconstruct edited XML
hdfs oiv -p ReverseXML -i edited.xml -o out
# IOException: Got unexpected tag end event for INODE while looking for section header tag.

# after: locate the mismatch before reconstructing
xmllint --noout edited.xml
# edited.xml:12: parser error : Opening and ending tag mismatch: INODE line 5 and INODE

Defensive patterns

Strategy: validation

Validate before calling

# an unmatched closing tag is a well-formedness error; xmllint names tag and line
xmllint --noout fsimage.xml
# or without xmllint:
python3 -c "import xml.etree.ElementTree as ET; ET.parse('fsimage.xml')"

Prevention

Always validate OIV XML with xmllint --noout before feeding it to ReverseXML
Prefer regenerating the XML from the fsimage over merging or hand-editing section blocks
When scripting transformations, parse and re-serialize with a real XML library instead of sed/awk

When it happens

Trigger: A top-level stray closing tag (e.g. </INODE> without its opening section), duplicated close tags from copy-paste, a section element opened inside another section and closed outside it, or a SectionProcessor stopping before consuming its own end tag on malformed inner content.

Common situations: Hand-merging two OIV XML files; scripts that cut and paste section blocks; XML from a homegrown generator that emits unbalanced tags.

Related errors

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/2449599a1d2cfd01. Report an issue: GitHub.