SPARK-1911: Emphasize that Spark jars should be built with Java 6.

This commit requires the user to manually say "yes" when buiding Spark
without Java 6. The prompt can be bypassed with a flag (e.g. if the user
is scripting around make-distribution).

Author: Patrick Wendell <pwendell@gmail.com>

Closes #859 from pwendell/java6 and squashes the following commits:

4921133 [Patrick Wendell] Adding Pyspark Notice
fee8c9e [Patrick Wendell] SPARK-1911: Emphasize that Spark jars should be built with Java 6.

(cherry picked from commit 75a03277)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>

make-distribution.sh

@@ -46,27 +46,6 @@ set -e
 FWDIR="$(cd `dirname $0`; pwd)"
 DISTDIR="$FWDIR/dist"
 
-if [ -z "$JAVA_HOME" ]; then
-  echo "Error: JAVA_HOME is not set, cannot proceed."
-  exit -1
-fi
-
-JAVA_CMD="$JAVA_HOME"/bin/java
-JAVA_VERSION=$("$JAVA_CMD" -version 2>&1)
-if ! [[ "$JAVA_VERSION" =~ "1.6" ]]; then
-  echo "***NOTE***: JAVA_HOME is not set to a JDK 6 installation. The resulting"
-  echo "            distribution will not support Java 6. See SPARK-1703."
-  echo "Output from 'java -version' was:"
-  echo "$JAVA_VERSION"
-fi
-
-VERSION=$(mvn help:evaluate -Dexpression=project.version 2>/dev/null | grep -v "INFO" | tail -n 1)
-if [ $? != 0 ]; then
-    echo -e "You need Maven installed to build Spark."
-    echo -e "Download Maven from https://maven.apache.org/"
-    exit -1;
-fi
-
 # Initialize defaults
 SPARK_HADOOP_VERSION=1.0.4
 SPARK_YARN=false
@@ -88,6 +67,9 @@ while (( "$#" )); do
     --with-hive)
       SPARK_HIVE=true
       ;;
+    --skip-java-test)
+      SKIP_JAVA_TEST=true
+      ;;
     --with-tachyon)
       SPARK_TACHYON=true
       ;;
@@ -102,6 +84,34 @@ while (( "$#" )); do
   shift
 done
 
+if [ -z "$JAVA_HOME" ]; then
+  echo "Error: JAVA_HOME is not set, cannot proceed."
+  exit -1
+fi
+
+VERSION=$(mvn help:evaluate -Dexpression=project.version 2>/dev/null | grep -v "INFO" | tail -n 1)
+if [ $? != 0 ]; then
+    echo -e "You need Maven installed to build Spark."
+    echo -e "Download Maven from https://maven.apache.org/"
+    exit -1;
+fi
+
+JAVA_CMD="$JAVA_HOME"/bin/java
+JAVA_VERSION=$("$JAVA_CMD" -version 2>&1)
+if [[ ! "$JAVA_VERSION" =~ "1.6" && -z "$SKIP_JAVA_TEST" ]]; then
+  echo "***NOTE***: JAVA_HOME is not set to a JDK 6 installation. The resulting"
+  echo "            distribution may not work well with PySpark and will not run"
+  echo "            with Java 6 (See SPARK-1703 and SPARK-1911)."
+  echo "            This test can be disabled by adding --skip-java-test."
+  echo "Output from 'java -version' was:"
+  echo "$JAVA_VERSION"
+  read -p "Would you like to continue anyways? [y,n]: " -r
+  if [[ ! $REPLY =~ ^[Yy]$ ]]; then
+    echo "Okay, exiting."
+    exit 1
+  fi 
+fi
+
 if [ "$NAME" == "none" ]; then
   NAME=$SPARK_HADOOP_VERSION
 fi