[SPARK-21696][SS] Fix a potential issue that may generate partial snapshot files

## What changes were proposed in this pull request?

Directly writing a snapshot file may generate a partial file. This PR changes it to write to a temp file then rename to the target file.

## How was this patch tested?

Jenkins.

Author: Shixiong Zhu <[email protected]>

Closes apache#18928 from zsxwing/SPARK-21696.

Author: zsxwing

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/HDFSBackedStateStoreProvider.scala

@@ -386,9 +386,11 @@ private[state] class HDFSBackedStateStoreProvider extends StateStoreProvider wit
 
   private def writeSnapshotFile(version: Long, map: MapType): Unit = {
     val fileToWrite = snapshotFile(version)
+    val tempFile =
+      new Path(fileToWrite.getParent, s"${fileToWrite.getName}.temp-${Random.nextLong}")
     var output: DataOutputStream = null
     Utils.tryWithSafeFinally {
-      output = compressStream(fs.create(fileToWrite, false))
+      output = compressStream(fs.create(tempFile, false))
       val iter = map.entrySet().iterator()
       while(iter.hasNext) {
         val entry = iter.next()
@@ -403,6 +405,12 @@ private[state] class HDFSBackedStateStoreProvider extends StateStoreProvider wit
     } {
       if (output != null) output.close()
     }
+    if (fs.exists(fileToWrite)) {
+      // Skip rename if the file is alreayd created.
+      fs.delete(tempFile, true)
+    } else if (!fs.rename(tempFile, fileToWrite)) {
+      throw new IOException(s"Failed to rename $tempFile to $fileToWrite")
+    }
     logInfo(s"Written snapshot file for version $version of $this at $fileToWrite")
   }