Double check if stream must be flush to allow tests to make better as…

…sumptions of what is visible and what isn't after tragic events

core/src/main/java/org/elasticsearch/index/translog/TranslogWriter.java

@@ -266,7 +266,13 @@ public boolean syncUpTo(long offset) throws IOException {
     protected void readBytes(ByteBuffer targetBuffer, long position) throws IOException {
         if (position+targetBuffer.remaining() > getWrittenOffset()) {
             synchronized (this) {
-                outputStream.flush();
+                // we only flush here if it's really really needed - try to minimize the impact of the read operation
+                // in some cases ie. a tragic event we might still be able to read the relevant value
+                // which is not really important in production but some test can make most strict assumptions
+                // if we don't fail in this call unless absolutely necessary.
+                if (position+targetBuffer.remaining() > getWrittenOffset()) {
+                    outputStream.flush();
+                }
             }
         }
         // we don't have to have a lock here because we only write ahead to the file, so all writes has been complete