[SYSTEMDS-3885] Fix robustness live variable analysis

This patch adds a new function potpourri test case which produced
incorrect results in the past. Although the test itself ran fine, adding
a test condition revealed a hidden robustness issue of live variable
analysis where parameter builtin functions such as toString when used
in predicates add 'null' objects (because they are not bound to
variables) to the set of updated variables which in turn then crashes
many things in the inter-procedural analysis.

Author: mboehm7

src/main/java/org/apache/sysds/parser/VariableSet.java

@@ -38,7 +38,8 @@ public VariableSet( VariableSet vs ) {
 	}
 
 	public void addVariable(String name, DataIdentifier id) {
-		_variables.put(name,id);
+		if( name != null ) // for robustness
+			_variables.put(name,id);
 	}
 
 	public void addVariables(VariableSet vs) {

src/test/java/org/apache/sysds/test/functions/misc/FunctionPotpourriTest.java

@@ -64,6 +64,7 @@ public class FunctionPotpourriTest extends AutomatedTestBase
 		"FunPotpourriParforEvalSpark",
 		"FunPotpourriEvalNamespace3",
 		"FunPotpourriDefaultParams",
+		"FunPotpourriListHandling"
 	};
 
 	private final static String TEST_DIR = "functions/misc/";
@@ -283,6 +284,11 @@ public void testFunctionDefaultParams() {
 		runFunctionTest( TEST_NAMES[30], null, false );
 	}
 
+	@Test
+	public void testFunctionListHandling() {
+		runFunctionTest( TEST_NAMES[31], null, false );
+	}
+	
 	private void runFunctionTest(String testName, Class<?> error) {
 		runFunctionTest(testName, error, false);
 	}
@@ -306,6 +312,8 @@ private void runFunctionTest(String testName, Class<?> error, boolean evalRewrit
 				Assert.assertTrue(heavyHittersContainsString(Opcodes.PRINT.toString()));
 			if( evalRewrite && !testName.equals(TEST_NAMES[28]) )
 				Assert.assertTrue(!heavyHittersContainsString(Opcodes.EVAL.toString()));
+			if( testName.equals(TEST_NAMES[31]) ) //print used for error
+				Assert.assertFalse(heavyHittersContainsString(Opcodes.PRINT.toString()));
 		}
 		finally {
 			OptimizerUtils.ALLOW_EVAL_FCALL_REPLACEMENT = oldFlag;

src/test/scripts/functions/misc/FunPotpourriListHandling.dml

@@ -0,0 +1,53 @@
+#-------------------------------------------------------------
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
+#-------------------------------------------------------------
+
+add_and_return = function(Int i_old, List[String] l_old)
+  return(Int i, List[String] l)
+{
+  i = i_old + 1
+  if (TRUE)
+    l = append(l_old, toString(i))
+  else
+    l = append(l_old, toString(i))
+}
+
+# Works!
+init = function() return(Int i, List[String] l) {i=0; l=list();}
+[test_i, test_l] = init()
+
+[test_i, test_l] = add_and_return(test_i, test_l)
+[test_i, test_l] = add_and_return(test_i, test_l)
+[test_i, test_l] = add_and_return(test_i, test_l)
+
+#while(FALSE){} # cut
+
+# Does not work!
+test_fail_i = 0
+test_fail_l = list()
+[test_fail_i, test_fail_l] = add_and_return(test_fail_i, test_fail_l)
+[test_fail_i, test_fail_l] = add_and_return(test_fail_i, test_fail_l)
+[test_fail_i, test_fail_l] = add_and_return(test_fail_i, test_fail_l)
+
+if( test_i != test_fail_i )
+  print("test_i discrepancy: "+test_i+" vs "+test_fail_i);
+
+if( toString(test_l) != toString(test_fail_l) )
+  print("test_l discrepancy: "+toString(test_l)+" vs "+toString(test_fail_l));