Fixes DocStats to not report index size < -1 (#27863)

Previously to this change when DocStats are added together (for example when adding the index size of all primary shards for an index)  we naively added the `totalSizeInBytes` together. This worked most of the time but not when the index size on one or multiple shards was reported to be `-1` (no value).

This change improves the logic by considering if the current value or the value to be added is `-1`:
* If the current and new value are both `-1` the value remains at `-1`
* If the current value is `-1` and the new value is not `-1`, current value is changed to be equal to the new value
* If the current value is not `-1` and the new value is `-1` the new value is ignored and the current value is not changed
* If both the current and new values are not `-1` the current value is changed to be equal to the sum of the current and new values.

The change also re-enables the failing rollover YAML test that was failing due to this bug.

Author: colings86

core/src/main/java/org/elasticsearch/index/shard/DocsStats.java

@@ -49,7 +49,11 @@ public void add(DocsStats other) {
         if (other == null) {
             return;
         }
-        this.totalSizeInBytes += other.totalSizeInBytes;
+        if (this.totalSizeInBytes == -1) {
+            this.totalSizeInBytes = other.totalSizeInBytes;
+        } else if (other.totalSizeInBytes != -1) {
+            this.totalSizeInBytes += other.totalSizeInBytes;
+        }
         this.count += other.count;
         this.deleted += other.deleted;
     }

core/src/test/java/org/elasticsearch/index/shard/DocsStatsTests.java

@@ -41,6 +41,24 @@ public void testCalculateAverageDocSize() throws Exception {
         assertThat(stats.getTotalSizeInBytes(), equalTo(600L));
         assertThat(stats.getAverageSizeInBytes(), equalTo(12L));
     }
+    
+    public void testUninitialisedShards() {
+        DocsStats stats = new DocsStats(0, 0, -1);
+        assertThat(stats.getTotalSizeInBytes(), equalTo(-1L));
+        assertThat(stats.getAverageSizeInBytes(), equalTo(0L));
+        stats.add(new DocsStats(0, 0, -1));
+        assertThat(stats.getTotalSizeInBytes(), equalTo(-1L));
+        assertThat(stats.getAverageSizeInBytes(), equalTo(0L));
+        stats.add(new DocsStats(1, 0, 10));
+        assertThat(stats.getTotalSizeInBytes(), equalTo(10L));
+        assertThat(stats.getAverageSizeInBytes(), equalTo(10L));
+        stats.add(new DocsStats(0, 0, -1));
+        assertThat(stats.getTotalSizeInBytes(), equalTo(10L));
+        assertThat(stats.getAverageSizeInBytes(), equalTo(10L));
+        stats.add(new DocsStats(1, 0, 20));
+        assertThat(stats.getTotalSizeInBytes(), equalTo(30L));
+        assertThat(stats.getAverageSizeInBytes(), equalTo(15L));
+    }
 
     public void testSerialize() throws Exception {
         DocsStats originalStats = new DocsStats(randomNonNegativeLong(), randomNonNegativeLong(), randomNonNegativeLong());

rest-api-spec/src/main/resources/rest-api-spec/test/indices.rollover/10_basic.yml

@@ -1,5 +1,7 @@
 ---
 "Rollover index via API":
+
+
   # create index with alias
   - do:
       indices.create: