[SPARK-17513][SQL] Make StreamExecution garbage-collect its metadata

## What changes were proposed in this pull request?
This PR modifies StreamExecution such that it discards metadata for batches that have already been fully processed. I used the purge method that was added as part of SPARK-17235.

This is based on work by frreiss in #15067, but fixed the test case along with some typos.

## How was this patch tested?
A new test case in StreamingQuerySuite. The test case would fail without the changes in this pull request.

Author: petermaxlee <petermaxlee@gmail.com>
Author: frreiss <frreiss@us.ibm.com>

Closes #15126 from petermaxlee/SPARK-17513.

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

@@ -24,6 +24,7 @@ package org.apache.spark.sql.execution.streaming
  *  - Allow the user to query the latest batch id.
  *  - Allow the user to query the metadata object of a specified batch id.
  *  - Allow the user to query metadata objects in a range of batch ids.
+ *  - Allow the user to remove obsolete metadata
  */
 trait MetadataLog[T] {
 

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

@@ -290,6 +290,13 @@ class StreamExecution(
       assert(offsetLog.add(currentBatchId, availableOffsets.toCompositeOffset(sources)),
         s"Concurrent update to the log. Multiple streaming jobs detected for $currentBatchId")
       logInfo(s"Committed offsets for batch $currentBatchId.")
+
+      // Now that we have logged the new batch, no further processing will happen for
+      // the previous batch, and it is safe to discard the old metadata.
+      // Note that purge is exclusive, i.e. it purges everything before currentBatchId.
+      // NOTE: If StreamExecution implements pipeline parallelism (multiple batches in
+      // flight at the same time), this cleanup logic will need to change.
+      offsetLog.purge(currentBatchId)
     } else {
       awaitBatchLock.lock()
       try {

sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingQuerySuite.scala

@@ -125,6 +125,30 @@ class StreamingQuerySuite extends StreamTest with BeforeAndAfter {
     )
   }
 
+  testQuietly("StreamExecution metadata garbage collection") {
+    val inputData = MemoryStream[Int]
+    val mapped = inputData.toDS().map(6 / _)
+
+    // Run 3 batches, and then assert that only 1 metadata file is left at the end
+    // since the first 2 should have been purged.
+    testStream(mapped)(
+      AddData(inputData, 1, 2),
+      CheckAnswer(6, 3),
+      AddData(inputData, 1, 2),
+      CheckAnswer(6, 3, 6, 3),
+      AddData(inputData, 4, 6),
+      CheckAnswer(6, 3, 6, 3, 1, 1),
+
+      AssertOnQuery("metadata log should contain only one file") { q =>
+        val metadataLogDir = new java.io.File(q.offsetLog.metadataPath.toString)
+        val logFileNames = metadataLogDir.listFiles().toSeq.map(_.getName())
+        val toTest = logFileNames // Workaround for SPARK-17475
+        assert(toTest.size == 1 && toTest.head == "2")
+        true
+      }
+    )
+  }
+
   /**
    * A [[StreamAction]] to test the behavior of `StreamingQuery.awaitTermination()`.
    *