[SPARK-19707][CORE] Improve the invalid path check for sc.addJar

## What changes were proposed in this pull request?

Currently in Spark there're two issues when we add jars with invalid path:

* If the jar path is a empty string {--jar ",dummy.jar"}, then Spark will resolve it to the current directory path and add to classpath / file server, which is unwanted. This is happened in our programatic way to submit Spark application. From my understanding Spark should defensively filter out such empty path.
* If the jar path is a invalid path (file doesn't exist), `addJar` doesn't check it and will still add to file server, the exception will be delayed until job running. Actually this local path could be checked beforehand, no need to wait until task running. We have similar check in `addFile`, but lacks similar similar mechanism in `addJar`.

## How was this patch tested?

Add unit test and local manual verification.

Author: jerryshao <sshao@hortonworks.com>

Closes #17038 from jerryshao/SPARK-19707.

(cherry picked from commit b0a8c16f)
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1727,10 +1727,18 @@ class SparkContext(config: SparkConf) extends Logging {
           // A JAR file which exists only on the driver node
           case null | "file" =>
             try {
+              val file = new File(uri.getPath)
+              if (!file.exists()) {
+                throw new FileNotFoundException(s"Jar ${file.getAbsolutePath} not found")
+              }
+              if (file.isDirectory) {
+                throw new IllegalArgumentException(
+                  s"Directory ${file.getAbsoluteFile} is not allowed for addJar")
+              }
               env.rpcEnv.fileServer.addJar(new File(uri.getPath))
             } catch {
-              case exc: FileNotFoundException =>
-                logError(s"Jar not found at $path")
+              case NonFatal(e) =>
+                logError(s"Failed to add $path to Spark environment", e)
                 null
             }
           // A JAR file which exists locally on every worker node

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -2016,7 +2016,7 @@ private[spark] object Utils extends Logging {
     if (paths == null || paths.trim.isEmpty) {
       ""
     } else {
-      paths.split(",").map { p => Utils.resolveURI(p) }.mkString(",")
+      paths.split(",").filter(_.trim.nonEmpty).map { p => Utils.resolveURI(p) }.mkString(",")
     }
   }
 

core/src/test/scala/org/apache/spark/SparkContextSuite.scala

@@ -289,6 +289,22 @@ class SparkContextSuite extends SparkFunSuite with LocalSparkContext {
     }
   }
 
+  test("add jar with invalid path") {
+    val tmpDir = Utils.createTempDir()
+    val tmpJar = File.createTempFile("test", ".jar", tmpDir)
+
+    sc = new SparkContext(new SparkConf().setAppName("test").setMaster("local"))
+    sc.addJar(tmpJar.getAbsolutePath)
+
+    // Invaid jar path will only print the error log, will not add to file server.
+    sc.addJar("dummy.jar")
+    sc.addJar("")
+    sc.addJar(tmpDir.getAbsolutePath)
+
+    sc.listJars().size should be (1)
+    sc.listJars().head should include (tmpJar.getName)
+  }
+
   test("Cancelling job group should not cause SparkContext to shutdown (SPARK-6414)") {
     try {
       sc = new SparkContext(new SparkConf().setAppName("test").setMaster("local"))

core/src/test/scala/org/apache/spark/util/UtilsSuite.scala

@@ -484,6 +484,7 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
       assertResolves("""hdfs:/jar1,file:/jar2,jar3,C:\pi.py#py.pi,C:\path to\jar4""",
         s"hdfs:/jar1,file:/jar2,file:$cwd/jar3,file:/C:/pi.py#py.pi,file:/C:/path%20to/jar4")
     }
+    assertResolves(",jar1,jar2", s"file:$cwd/jar1,file:$cwd/jar2")
   }
 
   test("nonLocalPaths") {