HeartSaVioR commented on a change in pull request #27620:
URL: https://github.com/apache/spark/pull/27620#discussion_r411779675



##########
File path: 
sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala
##########
@@ -1935,6 +1928,120 @@ class FileStreamSourceSuite extends 
FileStreamSourceTest {
     assert(expectedDir.exists())
     assert(expectedDir.list().exists(_.startsWith(filePrefix)))
   }
+
+  private def withCountListingLocalFileSystemAsLocalFileSystem(body: => Unit): 
Unit = {
+    val optionKey = s"fs.${CountListingLocalFileSystem.scheme}.impl"
+    val originClassForLocalFileSystem = spark.conf.getOption(optionKey)
+    try {
+      spark.conf.set(optionKey, classOf[CountListingLocalFileSystem].getName)
+      body
+    } finally {
+      originClassForLocalFileSystem match {
+        case Some(fsClazz) => spark.conf.set(optionKey, fsClazz)
+        case _ => spark.conf.unset(optionKey)
+      }
+    }
+  }
+
+  test("Caches and leverages unread files") {
+    withCountListingLocalFileSystemAsLocalFileSystem {
+      withThreeTempDirs { case (src, meta, tmp) =>
+        val options = Map("latestFirst" -> "false", "maxFilesPerTrigger" -> 
"5")
+        val scheme = CountListingLocalFileSystem.scheme
+        val source = new FileStreamSource(spark, 
s"$scheme:///${src.getCanonicalPath}/*/*", "text",
+          StructType(Nil), Seq.empty, meta.getCanonicalPath, options)
+        val _metadataLog = 
PrivateMethod[FileStreamSourceLog](Symbol("metadataLog"))
+        val metadataLog = source invokePrivate _metadataLog()
+
+        def verifyBatch(
+            offset: FileStreamSourceOffset,
+            expectedBatchId: Long,
+            inputFiles: Seq[File],
+            expectedListingCount: Int): Unit = {
+          val batchId = offset.logOffset
+          assert(batchId === expectedBatchId)
+
+          val files = 
metadataLog.get(batchId).getOrElse(Array.empty[FileEntry])
+          assert(files.forall(_.batchId == batchId))
+
+          val actualInputFiles = files.map { p => new 
Path(p.path).toUri.getPath }
+          val expectedInputFiles = inputFiles.slice(batchId.toInt * 5, 
batchId.toInt * 5 + 5)
+            .map(_.getCanonicalPath)
+          assert(actualInputFiles === expectedInputFiles)
+
+          assert(expectedListingCount === 
CountListingLocalFileSystem.pathToNumListStatusCalled
+            .get(src.getCanonicalPath).map(_.get()).getOrElse(0))
+        }
+
+        // provide 20 files in src, with sequential "last modified" to 
guarantee ordering
+        var lastModified = 0
+        val inputFiles = (0 to 19).map { idx =>
+          val f = createFile(idx.toString, new File(src, idx.toString), tmp)
+          f.setLastModified(lastModified)
+          lastModified += 10000
+          f
+        }
+
+        // 4 batches will be available for 20 input files
+        (0 to 3).foreach { batchId =>
+          val offsetBatch = source.latestOffset(FileStreamSourceOffset(-1L), 
ReadLimit.maxFiles(5))
+            .asInstanceOf[FileStreamSourceOffset]
+          verifyBatch(offsetBatch, expectedBatchId = batchId, inputFiles, 
expectedListingCount = 1)
+        }
+
+        val offsetBatch = source.latestOffset(FileStreamSourceOffset(-1L), 
ReadLimit.maxFiles(5))
+          .asInstanceOf[FileStreamSourceOffset]
+        // latestOffset returns the offset for previous batch which means no 
new batch is presented
+        assert(3 === offsetBatch.logOffset)
+        // listing should be performed after the list of unread files are 
exhausted
+        assert(2 === CountListingLocalFileSystem.pathToNumListStatusCalled
+          .get(src.getCanonicalPath).map(_.get()).getOrElse(0))
+      }
+    }
+  }
+
+  test("Don't cache unread files when latestFirst is true") {
+    withCountListingLocalFileSystemAsLocalFileSystem {
+      withThreeTempDirs { case (src, meta, tmp) =>
+        val options = Map("latestFirst" -> "true", "maxFilesPerTrigger" -> "5")
+        val scheme = CountListingLocalFileSystem.scheme
+        val source = new FileStreamSource(spark, 
s"$scheme:///${src.getCanonicalPath}/*/*", "text",
+          StructType(Nil), Seq.empty, meta.getCanonicalPath, options)
+
+        // provide 20 files in src, with sequential "last modified" to 
guarantee ordering
+        var lastModified = 0
+        (0 to 19).map { idx =>
+          val f = createFile(idx.toString, new File(src, idx.toString), tmp)
+          f.setLastModified(lastModified)
+          lastModified += 10000
+          f
+        }
+
+        source.latestOffset(FileStreamSourceOffset(-1L), ReadLimit.maxFiles(5))
+          .asInstanceOf[FileStreamSourceOffset]
+        assert(1 === CountListingLocalFileSystem.pathToNumListStatusCalled

Review comment:
       I'm not sure we want to verify whole behavior of file stream source in 
this PR. This test only makes sure the call of listing input directory (and 
input files as well) is expected, other checks are redundant and error-prone. 
E.g. Suppose file stream source employs some changes to read side due to some 
changes, then this test will fail unintentionally.




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