log store chooses where checkpoitns go (delta-io#6)

src/main/scala/org/apache/spark/sql/delta/Checkpoints.scala

@@ -229,7 +229,8 @@ object Checkpoints extends DeltaLogging {
     val checkpointSize = spark.sparkContext.longAccumulator("checkpointSize")
     val numOfFiles = spark.sparkContext.longAccumulator("numOfFiles")
     // Use the string in the closure as Path is not Serializable.
-    val path = checkpointFileSingular(snapshot.path, snapshot.version).toString
+    val resolvedPath = deltaLog.store.resolveCheckpointPath(snapshot.path)
+    val path = checkpointFileSingular(resolvedPath, snapshot.version).toString
     val base = snapshot.state
       .repartition(1)
       .map { action =>

src/main/scala/org/apache/spark/sql/delta/storage/LogStore.scala

@@ -83,6 +83,18 @@ trait LogStore {
     throw new UnsupportedOperationException()
   }
 
+  /**
+   * Let LogStore decide where checkpoints should be stored.
+   *
+   * Typically the checkpoint storage path would be the same as for Delta storage.
+   * This woudl be inside the _delta_log directory. The LogStore impl may wish to control this path
+   * and should inform the Checkpoints methods accordingly. This is only required for checkpoints
+   * as they are the only metadata file not directly written by LogStore.
+   */
+  def resolveCheckpointPath(path: Path): Path = {
+    path
+  }
+
   /**
    * Whether a partial write is visible when writing to `path`.
    *