Fix sneaky date_histogram bug (elastic#65707)

`date_histogram` has a bug with `offset` and `extended_bounds` when it
needs to create an "empty" aggregation result: it includes the bounds
twice! Wooops!

I broke this a while back when I started trying to merge `offset` into
`Rounding`. I never finished that merge, sadly. Interestingly, we've
discovered that the merge is required to properly handle daylight
savings time (elastic#56305) but it isn't really something we're looking to
solve today. For now, this just stops counting the offset twice.

Closes elastic#65624

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/10_histogram.yml

@@ -573,3 +573,62 @@ setup:
   - match: { aggregations.histo.buckets.0.doc_count: 1 }
   - match: { aggregations.histo.buckets.20.key: 20 }
   - match: { aggregations.histo.buckets.20.doc_count: 1 }
+
+
+---
+"date_histogram with extended_bounds and offset and unmapped":
+  - skip:
+      version: " - 7.99.99"
+      reason:  fixed in 8.0 to be backported to 7.11
+
+  - do:
+      indices.create:
+        index: test_nested
+        body:
+          mappings:
+            properties:
+              foo:
+                type: nested
+
+  - do:
+      bulk:
+        index: test_nested
+        refresh: true
+        body:
+            - '{"index": {}}'
+            - '{"foo": {"date": "2016-01-02"}}'
+            - '{"index": {}}'
+            - '{"foo": {"date": "2016-01-03"}}'
+
+  - do:
+      indices.create:
+        index: test_unmapped
+
+  - do:
+      search:
+        index: test_*
+        body:
+          size: 0
+          aggs:
+            foo:
+              nested:
+                path: foo
+              aggs:
+                histo:
+                  date_histogram:
+                    field: foo.date
+                    calendar_interval: day
+                    offset: 1h
+                    extended_bounds:
+                      min: "2016-01-01"
+                      max: "2016-01-04"
+  - match: { hits.total.value: 2 }
+  - length: { aggregations.foo.histo.buckets: 4 }
+  - match: { aggregations.foo.histo.buckets.0.key_as_string: 2016-01-01T01:00:00.000Z }
+  - match: { aggregations.foo.histo.buckets.0.doc_count: 1 }
+  - match: { aggregations.foo.histo.buckets.1.key_as_string: 2016-01-02T01:00:00.000Z }
+  - match: { aggregations.foo.histo.buckets.1.doc_count: 1 }
+  - match: { aggregations.foo.histo.buckets.2.key_as_string: 2016-01-03T01:00:00.000Z }
+  - match: { aggregations.foo.histo.buckets.2.doc_count: 0 }
+  - match: { aggregations.foo.histo.buckets.3.key_as_string: 2016-01-04T01:00:00.000Z }
+  - match: { aggregations.foo.histo.buckets.3.doc_count: 0 }

server/src/main/java/org/elasticsearch/search/aggregations/bucket/histogram/DateHistogramAggregator.java

@@ -312,7 +312,7 @@ public InternalAggregation[] buildAggregations(long[] owningBucketOrds) throws I
     @Override
     public InternalAggregation buildEmptyAggregation() {
         InternalDateHistogram.EmptyBucketInfo emptyBucketInfo = minDocCount == 0
-                ? new InternalDateHistogram.EmptyBucketInfo(rounding, buildEmptySubAggregations(), extendedBounds)
+                ? new InternalDateHistogram.EmptyBucketInfo(rounding.withoutOffset(), buildEmptySubAggregations(), extendedBounds)
                 : null;
         return new InternalDateHistogram(name, Collections.emptyList(), order, minDocCount, rounding.offset(), emptyBucketInfo, formatter,
                 keyed, metadata());

server/src/test/java/org/elasticsearch/search/aggregations/bucket/histogram/DateHistogramAggregatorTests.java

@@ -1235,6 +1235,25 @@ public void testIllegalInterval() throws IOException {
         assertWarnings("[interval] on [date_histogram] is deprecated, use [fixed_interval] or [calendar_interval] in the future.");
     }
 
+    public void testBuildEmpty() throws IOException {
+        withAggregator(
+            new DateHistogramAggregationBuilder("test").field(AGGREGABLE_DATE).calendarInterval(DateHistogramInterval.YEAR).offset(10),
+            new MatchAllDocsQuery(),
+            iw -> {},
+            (searcher, aggregator) -> {
+                InternalDateHistogram histo = (InternalDateHistogram) aggregator.buildEmptyAggregation();
+                /*
+                 * There was a time where we including the offset in the
+                 * rounding in the emptyBucketInfo which would cause us to
+                 * include the offset twice. This verifies that we don't do
+                 * that.
+                 */
+                assertThat(histo.emptyBucketInfo.rounding.prepareForUnknown().round(0), equalTo(0L));
+            },
+            aggregableDateFieldType(false, true)
+        );
+    }
+
     private void testSearchCase(Query query, List<String> dataset,
                                 Consumer<DateHistogramAggregationBuilder> configure,
                                 Consumer<InternalDateHistogram> verify, boolean useNanosecondResolution) throws IOException {