PY-23996 In PYTHONPATH keep the order in which source roots were added

It follows the behavior of the platform and IntelliJ IDEA in this regard.

python/src/com/jetbrains/python/run/PythonCommandLineState.java

@@ -15,6 +15,7 @@
  */
 package com.jetbrains.python.run;
 
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 import com.google.common.collect.Sets;
@@ -433,10 +434,11 @@ public abstract class PythonCommandLineState extends CommandLineState {
     }
   }
 
-  protected static Collection<String> collectPythonPath(Project project, PythonRunParams config, boolean isDebug) {
+  @VisibleForTesting
+  public static Collection<String> collectPythonPath(Project project, PythonRunParams config, boolean isDebug) {
     final Module module = getModule(project, config);
     final HashSet<String> pythonPath =
-      Sets.newHashSet(collectPythonPath(module, config.shouldAddContentRoots(), config.shouldAddSourceRoots()));
+      Sets.newLinkedHashSet(collectPythonPath(module, config.shouldAddContentRoots(), config.shouldAddSourceRoots()));
 
     if (isDebug && PythonSdkFlavor.getFlavor(config.getSdkHome()) instanceof JythonSdkFlavor) {
       //that fixes Jython problem changing sys.argv on execfile, see PY-8164

python/testSrc/com/jetbrains/python/toolbox/PyRunConfigurationTest.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2000-2017 JetBrains s.r.o.
+ *
+ * Licensed 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.
+ */
+package com.jetbrains.python.toolbox;
+
+import com.intellij.execution.RunManager;
+import com.intellij.execution.RunnerAndConfigurationSettings;
+import com.intellij.execution.configurations.ConfigurationFactory;
+import com.intellij.openapi.module.Module;
+import com.intellij.openapi.vfs.VirtualFile;
+import com.intellij.testFramework.PsiTestUtil;
+import com.jetbrains.python.fixtures.PyTestCase;
+import com.jetbrains.python.run.PythonCommandLineState;
+import com.jetbrains.python.run.PythonConfigurationType;
+import com.jetbrains.python.run.PythonRunConfiguration;
+
+import java.util.Collection;
+
+/**
+ * @author Mikhail Golubev
+ */
+public class PyRunConfigurationTest extends PyTestCase {
+  
+  public void testPythonPathPreservesAdditionOrderOfSourceRoots() {
+    myFixture.copyDirectoryToProject(getTestName(false), "");
+    final VirtualFile sourceRoot1 = myFixture.findFileInTempDir("src1");
+    final VirtualFile sourceRoot2 = myFixture.findFileInTempDir("src2");
+    
+    final Module module = myFixture.getModule();
+    PsiTestUtil.addSourceRoot(module, sourceRoot1);
+    PsiTestUtil.addSourceRoot(module, sourceRoot2);
+    
+    final ConfigurationFactory factory = PythonConfigurationType.getInstance().getConfigurationFactories()[0];
+    final RunnerAndConfigurationSettings settings = RunManager.getInstance(myFixture.getProject()).createRunConfiguration("test", factory);
+    final PythonRunConfiguration configuration = (PythonRunConfiguration)settings.getConfiguration();
+    configuration.setAddSourceRoots(true);
+
+    Collection<String> path = PythonCommandLineState.collectPythonPath(myFixture.getProject(), configuration, false);
+    assertContainsOrdered(path, "/src", "/src/src1", "/src/src2");
+
+    PsiTestUtil.removeSourceRoot(module, sourceRoot1);
+    PsiTestUtil.addSourceRoot(module, sourceRoot1);
+
+    path = PythonCommandLineState.collectPythonPath(myFixture.getProject(), configuration, false);
+    assertContainsOrdered(path, "/src", "/src/src2", "/src/src1");
+  }
+
+  @Override
+  protected String getTestDataPath() {
+    return super.getTestDataPath() + "/runConfig/";
+  }
+}