Force direct IO for fdt tmp file read access when flushing stored fields.

Author: martijnvg

server/src/main/java/org/elasticsearch/index/store/FsDirectoryFactory.java

@@ -171,6 +171,15 @@ protected boolean useDirectIO(String name, IOContext context, OptionalLong fileL
 
         @Override
         public IndexInput openInput(String name, IOContext context) throws IOException {
+            // Force direct I/O for stored field temp fdt files, given that these are used once and then removed:
+            // (tmp fdt files should only exist when index sorting is enabled)
+            if (directIODelegate != null && LuceneFilesExtensions.TMP.getExtension().equals(getExtension(name)) && name.contains("fdt")) {
+                // we need to do these checks on the outer directory since the inner doesn't know about pending deletes
+                ensureOpen();
+                ensureCanRead(name);
+                return directIODelegate.openInput(name, context);
+            }
+
             if (useDelegate(name, context)) {
                 // we need to do these checks on the outer directory since the inner doesn't know about pending deletes
                 ensureOpen();