Address issue with DateFieldMapper#isFieldWithinQuery(...) (elastic#138032)

If a date field has doc values skippers enabled, then it isn't guaranteed that every segment will actually have skippers. The logic was currently assuming that it did.

Closes elastic#138024

Author: martijnvg

server/src/main/java/org/elasticsearch/index/mapper/DateFieldMapper.java

@@ -841,9 +841,10 @@ public Relation isFieldWithinQuery(
                 }
                 for (LeafReaderContext ctx : leaves) {
                     DocValuesSkipper skipper = ctx.reader().getDocValuesSkipper(name());
-                    assert skipper != null : "no skipper for field:" + name() + " and reader:" + reader;
-                    minValue = Long.min(minValue, skipper.minValue());
-                    maxValue = Long.max(maxValue, skipper.maxValue());
+                    if (skipper != null) {
+                        minValue = Long.min(minValue, skipper.minValue());
+                        maxValue = Long.max(maxValue, skipper.maxValue());
+                    }
                 }
                 return isFieldWithinQuery(minValue, maxValue, from, to, includeLower, includeUpper, timeZone, dateParser, context, name());
             }

server/src/test/java/org/elasticsearch/index/mapper/DateFieldTypeTests.java

@@ -113,8 +113,44 @@ public void testIsFieldWithinQueryDateNanosDocValueSkipper() throws IOException
         isFieldWithinRangeTestCase(ft);
     }
 
-    public void isFieldWithinRangeTestCase(DateFieldType ft) throws IOException {
+    public void testIsFieldWithinQueryDocValueSkipperNotInAllSegments() throws IOException {
+        var ft = new DateFieldType(
+            "my_date",
+            IndexType.skippers(),
+            false,
+            DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER,
+            Resolution.NANOSECONDS,
+            null,
+            null,
+            Collections.emptyMap()
+        );
 
+        try (Directory dir = newDirectory()) {
+            try (IndexWriter w = new IndexWriter(dir, new IndexWriterConfig(null))) {
+                // Simulates one segment have no my_date field
+                LuceneDocument doc = new LuceneDocument();
+                doc.add(SortedNumericDocValuesField.indexedField("my_other_date", 123456789000L));
+                w.addDocument(doc);
+                w.flush();
+
+                doc = new LuceneDocument();
+                Field field = SortedNumericDocValuesField.indexedField("my_date", ft.parse("2015-10-12"));
+                doc.add(field);
+                w.addDocument(doc);
+                field.setLongValue(ft.parse("2016-04-03"));
+                w.addDocument(doc);
+                try (DirectoryReader reader = DirectoryReader.open(w)) {
+                    DateMathParser alternateFormat = DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER.toDateMathParser();
+                    doTestIsFieldWithinQuery(ft, reader, null, null);
+                    doTestIsFieldWithinQuery(ft, reader, null, alternateFormat);
+                    doTestIsFieldWithinQuery(ft, reader, ZoneOffset.UTC, null);
+                    doTestIsFieldWithinQuery(ft, reader, ZoneOffset.UTC, alternateFormat);
+                }
+            }
+        }
+    }
+
+    public void isFieldWithinRangeTestCase(DateFieldType ft) throws IOException {
         Directory dir = newDirectory();
         IndexWriter w = new IndexWriter(dir, new IndexWriterConfig(null));
         LuceneDocument doc = new LuceneDocument();