[GATEWAY] copy translog file if rename fails after retries.

Today we ignore a translog file if the rename operation fails. This can be problematic
on windows if another process hodls on to the translog file. If we can't rename it we should
at least try to copy it since otherwise its content will just be lost.
This is a workaround for an already fixed issue in 2.0 since all translog files are write
once in 2.0 and renaming / copying is not needed anymore.

Conflicts:
	src/main/java/org/elasticsearch/index/gateway/local/LocalIndexShardGateway.java

src/main/java/org/elasticsearch/index/gateway/local/LocalIndexShardGateway.java

@@ -51,6 +51,7 @@
 import java.io.EOFException;
 import java.io.File;
 import java.io.IOException;
+import java.nio.file.Files;
 import java.util.Arrays;
 import java.util.Set;
 import java.util.concurrent.CountDownLatch;
@@ -204,12 +205,30 @@ public void recover(boolean indexShouldExists, RecoveryState recoveryState) thro
                 if (!tmpRecoveringFile.exists()) {
                     File tmpTranslogFile = new File(translogLocation, translogName);
                     if (tmpTranslogFile.exists()) {
+                        logger.trace("Translog file found in {} - renaming", translogLocation);
+                        boolean success = false;
                         for (int i = 0; i < RECOVERY_TRANSLOG_RENAME_RETRIES; i++) {
                             if (tmpTranslogFile.renameTo(tmpRecoveringFile)) {
+
                                 recoveringTranslogFile = tmpRecoveringFile;
+                                logger.trace("Renamed translog from {} to {}", tmpTranslogFile.getName(), recoveringTranslogFile.getName());
+                                success = true;
                                 break;
                             }
                         }
+                        if (success == false) {
+                            try {
+                                // this is a fallback logic that to ensure we can recover from the file.
+                                // on windows a virus-scanner etc can hold on to the file and after retrying
+                                // we just skip the recovery and the engine will reuse the file and truncate it.
+                                // in 2.0 this is all not needed since translog files are write once.
+                                Files.copy(tmpTranslogFile.toPath(), tmpRecoveringFile.toPath());
+                                recoveringTranslogFile = tmpRecoveringFile;
+                                logger.trace("Copied translog from {} to {}", tmpTranslogFile.getName(), recoveringTranslogFile.getName());
+                            } catch (IOException ex) {
+                                throw new ElasticsearchException("failed to copy recovery file", ex);
+                            }
+                        }
                     }
                 } else {
                     recoveringTranslogFile = tmpRecoveringFile;