[LUCENE-6806] FunctionQuery.AllScorer.explain overwrites FunctionWeight.queryNorm in trappy fashion

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 6.0
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Terry Smith
• Created: 15/Sep/15 14:34
• Updated: 28/Aug/22 14:43

Description

FunctionQuery.AllScorer.explain is:

    public Explanation explain(int doc, float queryNorm) throws IOException {
      float sc = qWeight * vals.floatVal(doc);

      return Explanation.match(sc, "FunctionQuery(" + func + "), product of:",
          vals.explain(doc),
          Explanation.match(queryNorm, "boost"),
          Explanation.match(weight.queryNorm = 1f, "queryNorm"));
    }

The following line has a subtle assignment that overwrites weight.queryNorm.

          Explanation.match(weight.queryNorm = 1f, "queryNorm"));

Because weights aren't reused between search and explain this doesn't break anything but it's awfully subtle.

Seeing as queryNorm is ALWAYS 1 here, could we just drop this extra line from the explain output and use the following instead?

    public Explanation explain(int doc, float queryNorm) throws IOException {
      float sc = qWeight * vals.floatVal(doc);

      return Explanation.match(sc, "FunctionQuery(" + func + "), product of:",
          vals.explain(doc),
          Explanation.match(queryNorm, "boost"));
    }

Issue Links

• is related to: LUCENE-7209 Fix FunctionQuery explanations