[Java] Add purgeOldBackups API

Summary:
1. Check status of CreateNewBackup. If status is not OK, then throw.
2. Add purgeOldBackups API

Test Plan:
make test
make sample

Reviewers: haobo, sdong, zzbennett, swapnilghike, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21753

java/org/rocksdb/BackupableDB.java

@@ -47,6 +47,15 @@ public static BackupableDB open(
   public void createNewBackup(boolean flushBeforeBackup) {
     createNewBackup(nativeHandle_, flushBeforeBackup);
   }
+
+  /**
+   * Deletes old backups, keeping latest numBackupsToKeep alive.
+   * 
+   * @param numBackupsToKeep Number of latest backups to keep.
+   */
+  public void purgeOldBackups(int numBackupsToKeep) {
+    purgeOldBackups(nativeHandle_, numBackupsToKeep);
+  }
 
 
   /**
@@ -77,4 +86,5 @@ protected BackupableDB() {
 
   protected native void open(long rocksDBHandle, long backupDBOptionsHandle);
   protected native void createNewBackup(long handle, boolean flag);
+  protected native void purgeOldBackups(long handle, int numBackupsToKeep);
 }

java/rocksjni/backupablejni.cc

@@ -40,7 +40,26 @@ void Java_org_rocksdb_BackupableDB_open(
  */
 void Java_org_rocksdb_BackupableDB_createNewBackup(
     JNIEnv* env, jobject jbdb, jlong jhandle, jboolean jflag) {
-  reinterpret_cast<rocksdb::BackupableDB*>(jhandle)->CreateNewBackup(jflag);
+  rocksdb::Status s =
+      reinterpret_cast<rocksdb::BackupableDB*>(jhandle)->CreateNewBackup(jflag);
+  if (!s.ok()) {
+    rocksdb::RocksDBExceptionJni::ThrowNew(env, s);
+  }
+}
+
+/*
+ * Class:     org_rocksdb_BackupableDB
+ * Method:    purgeOldBackups
+ * Signature: (JI)V
+ */
+void Java_org_rocksdb_BackupableDB_purgeOldBackups(
+    JNIEnv* env, jobject jbdb, jlong jhandle, jboolean jnumBackupsToKeep) {
+  rocksdb::Status s =
+      reinterpret_cast<rocksdb::BackupableDB*>(jhandle)->
+      PurgeOldBackups(jnumBackupsToKeep);
+  if (!s.ok()) {
+    rocksdb::RocksDBExceptionJni::ThrowNew(env, s);
+  }
 }
 
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