[SPARK-17613] S3A base paths with no '/' at the end return empty DataFrames

## What changes were proposed in this pull request?

Consider you have a bucket as `s3a://some-bucket`
and under it you have files:
```
s3a://some-bucket/file1.parquet
s3a://some-bucket/file2.parquet
```
Getting the parent path of `s3a://some-bucket/file1.parquet` yields
`s3a://some-bucket/` and the ListingFileCatalog uses this as the key in the hash map.

When catalog.allFiles is called, we use `s3a://some-bucket` (no slash at the end) to get the list of files, and we're left with an empty list!

This PR fixes this by adding a `/` at the end of the `URI` iff the given `Path` doesn't have a parent, i.e. is the root. This is a no-op if the path already had a `/` at the end, and is handled through the Hadoop Path, path merging semantics.

## How was this patch tested?

Unit test in `FileCatalogSuite`.

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15169 from brkyvz/SPARK-17613.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningAwareFileCatalog.scala

@@ -76,7 +76,15 @@ abstract class PartitioningAwareFileCatalog(
       paths.flatMap { path =>
         // Make the path qualified (consistent with listLeafFiles and listLeafFilesInParallel).
         val fs = path.getFileSystem(hadoopConf)
-        val qualifiedPath = fs.makeQualified(path)
+        val qualifiedPathPre = fs.makeQualified(path)
+        val qualifiedPath: Path = if (qualifiedPathPre.isRoot && !qualifiedPathPre.isAbsolute) {
+          // SPARK-17613: Always append `Path.SEPARATOR` to the end of parent directories,
+          // because the `leafFile.getParent` would have returned an absolute path with the
+          // separator at the end.
+          new Path(qualifiedPathPre, Path.SEPARATOR)
+        } else {
+          qualifiedPathPre
+        }
 
         // There are three cases possible with each path
         // 1. The path is a directory and has children files in it. Then it must be present in

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileCatalogSuite.scala

@@ -18,10 +18,12 @@
 package org.apache.spark.sql.execution.datasources
 
 import java.io.File
+import java.net.URI
 
+import scala.collection.mutable
 import scala.language.reflectiveCalls
 
-import org.apache.hadoop.fs.Path
+import org.apache.hadoop.fs.{FileStatus, Path, RawLocalFileSystem}
 
 import org.apache.spark.sql.catalyst.util._
 import org.apache.spark.sql.test.SharedSQLContext
@@ -78,4 +80,45 @@ class FileCatalogSuite extends SharedSQLContext {
       assert(catalog1.listLeafFiles(catalog1.paths).isEmpty)
     }
   }
+
+  test("SPARK-17613 - PartitioningAwareFileCatalog: base path w/o '/' at end") {
+    class MockCatalog(
+      override val paths: Seq[Path]) extends PartitioningAwareFileCatalog(spark, Map.empty, None) {
+
+      override def refresh(): Unit = {}
+
+      override def leafFiles: mutable.LinkedHashMap[Path, FileStatus] = mutable.LinkedHashMap(
+        new Path("mockFs://some-bucket/file1.json") -> new FileStatus()
+      )
+
+      override def leafDirToChildrenFiles: Map[Path, Array[FileStatus]] = Map(
+        new Path("mockFs://some-bucket/") -> Array(new FileStatus())
+      )
+
+      override def partitionSpec(): PartitionSpec = {
+        PartitionSpec.emptySpec
+      }
+    }
+
+    withSQLConf(
+        "fs.mockFs.impl" -> classOf[FakeParentPathFileSystem].getName,
+        "fs.mockFs.impl.disable.cache" -> "true") {
+      val pathWithSlash = new Path("mockFs://some-bucket/")
+      assert(pathWithSlash.getParent === null)
+      val pathWithoutSlash = new Path("mockFs://some-bucket")
+      assert(pathWithoutSlash.getParent === null)
+      val catalog1 = new MockCatalog(Seq(pathWithSlash))
+      val catalog2 = new MockCatalog(Seq(pathWithoutSlash))
+      assert(catalog1.allFiles().nonEmpty)
+      assert(catalog2.allFiles().nonEmpty)
+    }
+  }
+}
+
+class FakeParentPathFileSystem extends RawLocalFileSystem {
+  override def getScheme: String = "mockFs"
+
+  override def getUri: URI = {
+    URI.create("mockFs://some-bucket")
+  }
 }