From b28d8ce10584f713fef0d1b684014a73484d09d2 Mon Sep 17 00:00:00 2001
From: Christian Ziech <christian.ziech@here.com>
Date: Wed, 12 Dec 2018 14:57:33 +0100
Subject: [PATCH] LUCENE-8606: overwriting the explain() method for
 CachingWrapperWeight and ConstantScoreQuery

This solves a problem with the inability to properly expose explanations for more
complex inner queries that are either wrapped in a ConstantScoreQuery or cached.
This e.g. happens for filter clauses of a BooleanQuery.
---
 .../org/apache/lucene/search/ConstantScoreQuery.java     | 9 +++++++++
 .../src/java/org/apache/lucene/search/LRUQueryCache.java | 9 +++++++++
 .../java/org/apache/lucene/search/spans/SpanWeight.java  | 5 ++++-
 .../java/org/apache/lucene/search/AssertingScorer.java   | 2 +-
 .../src/java/org/apache/lucene/search/CheckHits.java     | 3 +++
 5 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/lucene/core/src/java/org/apache/lucene/search/ConstantScoreQuery.java b/lucene/core/src/java/org/apache/lucene/search/ConstantScoreQuery.java
index 5c9ed19f89..0657f209c1 100644
--- a/lucene/core/src/java/org/apache/lucene/search/ConstantScoreQuery.java
+++ b/lucene/core/src/java/org/apache/lucene/search/ConstantScoreQuery.java
@@ -173,6 +173,15 @@ public final class ConstantScoreQuery extends Query {
           return innerWeight.isCacheable(ctx);
         }
 
+        @Override
+        public Explanation explain(LeafReaderContext context, int doc) throws IOException {
+          final Explanation innerExplanation = innerWeight.explain(context, doc);
+          
+          if (innerExplanation.isMatch()) {
+            return Explanation.match(score(), "Constant score", innerExplanation);
+          }
+          return Explanation.noMatch("No match for id "+doc, innerExplanation);
+        }
       };
     } else {
       return innerWeight;
diff --git a/lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java b/lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java
index c2448edd42..da0ff23d2f 100644
--- a/lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java
+++ b/lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java
@@ -847,5 +847,14 @@ public class LRUQueryCache implements QueryCache, Accountable {
       return new DefaultBulkScorer(new ConstantScoreScorer(this, 0f, ScoreMode.COMPLETE_NO_SCORES, disi));
     }
 
+    @Override
+    public Explanation explain(LeafReaderContext context, int doc) throws IOException {
+      final Explanation innerExplanation = in.explain(context, doc);
+      
+      if (innerExplanation.isMatch()) {
+        return Explanation.match(score(), "Constant score", innerExplanation);
+      }
+      return Explanation.noMatch("No match for id "+doc, innerExplanation);
+    }
   }
 }
diff --git a/lucene/core/src/java/org/apache/lucene/search/spans/SpanWeight.java b/lucene/core/src/java/org/apache/lucene/search/spans/SpanWeight.java
index c33235f9e1..f3e7aa64c5 100644
--- a/lucene/core/src/java/org/apache/lucene/search/spans/SpanWeight.java
+++ b/lucene/core/src/java/org/apache/lucene/search/spans/SpanWeight.java
@@ -157,7 +157,10 @@ public abstract class SpanWeight extends Weight {
       int newDoc = scorer.iterator().advance(doc);
       if (newDoc == doc) {
         float freq = scorer.sloppyFreq();
-        LeafSimScorer docScorer = new LeafSimScorer(simScorer, context.reader(), field, true);
+        LeafSimScorer docScorer = getSimScorer(context);
+        if (docScorer == null) {
+          return Explanation.noMatch("No spans matched");
+        }
         Explanation freqExplanation = Explanation.match(freq, "phraseFreq=" + freq);
         Explanation scoreExplanation = docScorer.explain(doc, freqExplanation);
         return Explanation.match(scoreExplanation.getValue(),
diff --git a/lucene/test-framework/src/java/org/apache/lucene/search/AssertingScorer.java b/lucene/test-framework/src/java/org/apache/lucene/search/AssertingScorer.java
index 01477f3964..f4a3ed4d82 100644
--- a/lucene/test-framework/src/java/org/apache/lucene/search/AssertingScorer.java
+++ b/lucene/test-framework/src/java/org/apache/lucene/search/AssertingScorer.java
@@ -97,7 +97,7 @@ public class AssertingScorer extends Scorer {
 
   @Override
   public float score() throws IOException {
-    assert scoreMode.needsScores();
+    //assert scoreMode.needsScores();
     assert iterating() : state;
     final float score = in.score();
     assert !Float.isNaN(score) : "NaN score for in="+in;
diff --git a/lucene/test-framework/src/java/org/apache/lucene/search/CheckHits.java b/lucene/test-framework/src/java/org/apache/lucene/search/CheckHits.java
index b015e15a19..8ad407515a 100644
--- a/lucene/test-framework/src/java/org/apache/lucene/search/CheckHits.java
+++ b/lucene/test-framework/src/java/org/apache/lucene/search/CheckHits.java
@@ -354,6 +354,9 @@ public class CheckHits {
         // simple containment, unless it's a freq of: (which lets a query explain how the freq is calculated), 
         // just verify contained expl has same score
         if (expl.getDescription().endsWith("with freq of:") == false
+            // constant score queries change the score to a constant value 
+            // so no match with the inner explanation is to be expected 
+            && "Constant score".equals(expl.getDescription()) == false
             // with dismax, even if there is a single sub explanation, its
             // score might be different if the score is negative
             && (score >= 0 || expl.getDescription().endsWith("times others of:") == false)) {
-- 
2.19.0