[SPARK-4504][Examples] fix run-example failure if multiple assembly jars exist

Fix run-example script to fail fast with useful error message if multiple
example assembly JARs are present.

Author: Venkata Ramana Gollamudi <ramana.gollamudi@huawei.com>

Closes #3377 from gvramana/run-example_fails and squashes the following commits:

fa7f481 [Venkata Ramana Gollamudi] Fixed review comments, avoiding ls output scanning.
6aa1ab7 [Venkata Ramana Gollamudi] Fix run-examples script error during multiple jars

(cherry picked from commit 74de94e)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>

Conflicts:
	bin/compute-classpath.sh
	bin/run-example

Author: gvramana

bin/compute-classpath.sh

@@ -69,22 +69,25 @@ else
   assembly_folder="$ASSEMBLY_DIR"
 fi
 
-num_jars=$(ls "$assembly_folder" | grep "spark-assembly.*hadoop.*\.jar" | wc -l)
-if [ "$num_jars" -eq "0" ]; then
-  echo "Failed to find Spark assembly in $assembly_folder"
-  echo "You need to build Spark before running this program."
-  exit 1
-fi
+num_jars=0
+
+for f in ${assembly_folder}/spark-assembly*hadoop*.jar; do
+  if [[ ! -e "$f" ]]; then
+    echo "Failed to find Spark assembly in $assembly_folder" 1>&2
+    echo "You need to build Spark before running this program." 1>&2
+    exit 1
+  fi
+  ASSEMBLY_JAR="$f"
+  num_jars=$((num_jars+1))
+done
+
 if [ "$num_jars" -gt "1" ]; then
-  jars_list=$(ls "$assembly_folder" | grep "spark-assembly.*hadoop.*.jar")
-  echo "Found multiple Spark assembly jars in $assembly_folder:"
-  echo "$jars_list"
-  echo "Please remove all but one jar."
+  echo "Found multiple Spark assembly jars in $assembly_folder:" 1>&2
+  ls ${assembly_folder}/spark-assembly*hadoop*.jar 1>&2
+  echo "Please remove all but one jar." 1>&2
   exit 1
 fi
 
-ASSEMBLY_JAR=$(ls "$assembly_folder"/spark-assembly*hadoop*.jar 2>/dev/null)
-
 # Verify that versions of java used to build the jars and run Spark are compatible
 jar_error_check=$("$JAR_CMD" -tf "$ASSEMBLY_JAR" nonexistent/class/path 2>&1)
 if [[ "$jar_error_check" =~ "invalid CEN header" ]]; then

bin/run-example

@@ -35,17 +35,32 @@ else
 fi
 
 if [ -f "$FWDIR/RELEASE" ]; then
-  export SPARK_EXAMPLES_JAR=`ls "$FWDIR"/lib/spark-examples-*hadoop*.jar`
-elif [ -e "$EXAMPLES_DIR"/target/scala-$SCALA_VERSION/spark-examples-*hadoop*.jar ]; then
-  export SPARK_EXAMPLES_JAR=`ls "$EXAMPLES_DIR"/target/scala-$SCALA_VERSION/spark-examples-*hadoop*.jar`
+  JAR_PATH="${FWDIR}/lib"
+else
+  JAR_PATH="${EXAMPLES_DIR}/target/scala-${SPARK_SCALA_VERSION}"
 fi
 
-if [[ -z $SPARK_EXAMPLES_JAR ]]; then
-  echo "Failed to find Spark examples assembly in $FWDIR/lib or $FWDIR/examples/target" 1>&2
-  echo "You need to build Spark before running this program" 1>&2
+JAR_COUNT=0
+
+for f in ${JAR_PATH}/spark-examples-*hadoop*.jar; do
+  if [[ ! -e "$f" ]]; then
+    echo "Failed to find Spark examples assembly in $FWDIR/lib or $FWDIR/examples/target" 1>&2
+    echo "You need to build Spark before running this program" 1>&2
+    exit 1
+  fi
+  SPARK_EXAMPLES_JAR="$f"
+  JAR_COUNT=$((JAR_COUNT+1))
+done
+
+if [ "$JAR_COUNT" -gt "1" ]; then
+  echo "Found multiple Spark examples assembly jars in ${JAR_PATH}" 1>&2
+  ls ${JAR_PATH}/spark-examples-*hadoop*.jar 1>&2
+  echo "Please remove all but one jar." 1>&2
   exit 1
 fi
 
+export SPARK_EXAMPLES_JAR
+
 EXAMPLE_MASTER=${MASTER:-"local[*]"}
 
 if [[ ! $EXAMPLE_CLASS == org.apache.spark.examples* ]]; then