Assert IO is still owned by the region after read and write (elastic#…

…103100)

When the code path goes to CacheFile#populateAndRead, both the read and
write are performed when holding the reference of the file region
(elastic#102843). Therefore we should be able to assert that the region
ownership throughout the processes.

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/shared/SharedBlobCacheService.java

@@ -917,6 +917,8 @@ private RangeMissingHandler writerWithOffset(RangeMissingHandler writer, CacheFi
                 return (channel, channelPos, relativePos, len, progressUpdater) -> {
                     assert assertValidRegionAndLength(fileRegion, channelPos, len);
                     adjustedWriter.fillCacheRange(channel, channelPos, relativePos, len, progressUpdater);
+                    assert regionOwners.get(fileRegion.io) == fileRegion
+                        : "File chunk [" + fileRegion.regionKey + "] no longer owns IO [" + fileRegion.io + "]";
                 };
             }
             return adjustedWriter;
@@ -932,7 +934,10 @@ private RangeAvailableHandler readerWithOffset(RangeAvailableHandler reader, Cac
             if (Assertions.ENABLED) {
                 return (channel, channelPos, relativePos, len) -> {
                     assert assertValidRegionAndLength(fileRegion, channelPos, len);
-                    return adjustedReader.onRangeAvailable(channel, channelPos, relativePos, len);
+                    final int bytesRead = adjustedReader.onRangeAvailable(channel, channelPos, relativePos, len);
+                    assert regionOwners.get(fileRegion.io) == fileRegion
+                        : "File chunk [" + fileRegion.regionKey + "] no longer owns IO [" + fileRegion.io + "]";
+                    return bytesRead;
                 };
             }
             return adjustedReader;