May 09 2018

DSpot is a mutation testing tool that automatically generates new tests from existing test suites. It's being developed as part of the STAMP European research project (to which XWiki SAS is participating to, represented by me).

Very quickly, DSpot works as follows:

dspot.png

  • Step 1: Finds an existing test and remove some API call. Also remove assertions (but keep the calls on the code being tested). Add logs in the source to capture object states
  • Step 2: Execute the test and add assertions that validate the captured states
  • Step 3: Run a selector to decide which test to keep and which ones to discard. By default PITest/Descartes is used, meaning that only tests killing mutants than the original test didn't kill are kept. It's also possible to use other selector. For example a Clover selector exists that will keep the tests which generate more coverage than the original test.
  • Step 4: Repeat (with different API calls removed) or stop if good enough.

For full details, see this presentation by Benjamin Danglot (main contributor of DSpot).

Today I tested the latest version of DSpot (I built it from its sources to have the latest code) and tried it on several modules of xwiki-common.

FTR here's what I did to test it:

  • Cloned Dspot and built it with Maven by running mvn clean package -DskipTests. This generated a dspot/target/dspot-1.1.1-SNAPSHOT-jar-with-dependencies.jar JAR.
  • For each module on which I tested it, I created a dspot.properties file. For example for xwiki-commons-core/xwiki-commons-component/xwiki-commons-component-api, I created the following file:
    project=../../../
    targetModule=xwiki-commons-core/xwiki-commons-component/xwiki-commons-component-api
    src=src/main/java/
    srcResources=src/main/resources/
    testSrc=src/test/java/
    testResources=src/test/resources/
    javaVersion=8
    outputDirectory=output
    filter=org.xwiki.*

    Note that project is pointing to the root of the project.

  • Then I executed: java -jar /some/path/dspot/dspot/target/dspot-1.1.1-SNAPSHOT-jar-with-dependencies.jar path-to-properties dspot.properties
  • Then checked results in output/* to see if new tests have been generated

I had to test DSpot on 6 modules before getting any result, as follows:

  • xwiki-commons-core/xwiki-commons-cache/xwiki-commons-cache-infinispan/: No new test generated by DSpot. One reason was because DSpot modifies the test sources and the tests in this module were extending Abstract test classes located in other modules and this DSpot didn't touch those and was not able to modify them to generate new tests.
  • xwiki-commons-core/xwiki-commons-component/xwiki-commons-component-api/: No new test generated by DSpot.
  • xwiki-commons-core/xwiki-commons-component/xwiki-commons-component-default/: No new test generated by DSpot.
  • xwiki-commons-core/xwiki-commons-component/xwiki-commons-component-observation/: No new test generated by DSpot.
  • xwiki-commons-core/xwiki-commons-context/: No new test generated by DSpot.
  • xwiki-commons-core/xwiki-commons-crypto/xwiki-commons-crypto-cipher/: Eureka! One test was generated by Dspot emoticon_smile

Here's the original test:

@Test
public void testRSAEncryptionDecryptionProgressive() throws Exception
{
    Cipher cipher = factory.getInstance(true, publicKey);
    cipher.update(input, 0, 17);
    cipher.update(input, 17, 1);
    cipher.update(input, 18, input.length - 18);
   byte[] encrypted = cipher.doFinal();
    cipher = factory.getInstance(false, privateKey);
    cipher.update(encrypted, 0, 65);
    cipher.update(encrypted, 65, 1);
    cipher.update(encrypted, 66, encrypted.length - 66);
    assertThat(cipher.doFinal(), equalTo(input));

    cipher = factory.getInstance(true, privateKey);
    cipher.update(input, 0, 15);
    cipher.update(input, 15, 1);
    encrypted = cipher.doFinal(input, 16, input.length - 16);
    cipher = factory.getInstance(false, publicKey);
    cipher.update(encrypted);
    assertThat(cipher.doFinal(), equalTo(input));
}

And here's the new test generated by DSpot, based on this test:

@Test
public void testRSAEncryptionDecryptionProgressive_failAssert2() throws Exception {
--> try {
        Cipher cipher = factory.getInstance(true, publicKey);
        cipher.update(input, 0, 17);
        cipher.update(input, 17, 1);
        cipher.update(input, 18, ((input.length) - 18));
       byte[] encrypted = cipher.doFinal();
        cipher = factory.getInstance(false, privateKey);
        cipher.update(encrypted, 0, 65);
        cipher.update(encrypted, 65, 1);
        cipher.update(encrypted, 66, ((encrypted.length) - 66));
        cipher.doFinal();
        CoreMatchers.equalTo(input);
        cipher = factory.getInstance(true, privateKey);
        cipher.update(input, 0, 15);
        cipher.update(input, 15, 1);
        encrypted = cipher.doFinal(input, 16, ((input.length) - 16));
        cipher = factory.getInstance(false, publicKey);
        cipher.update(encrypted);
        cipher.doFinal();
        CoreMatchers.equalTo(input);
-->     cipher.doFinal();
-->     CoreMatchers.equalTo(input);
-->     org.junit.Assert.fail("testRSAEncryptionDecryptionProgressive should have thrown GeneralSecurityException");
--> } catch (GeneralSecurityException eee) {
--> }
}

I've highlighted the parts that were added with the --> prefix. In short DSpot found that by calling cipher.doFinal() twice, it generates a GeneralSecurityException and that's killing some mutants that were not killed by the original test. Note that calling doFinal() resets the cipher, which explains why the second call generates an exception.

Looking at the source code, we can see:

@Override
public byte[] doFinal(byte[] input, int inputOffset, int inputLen) throws GeneralSecurityException
{
   if (input != null) {
       this.cipher.processBytes(input, inputOffset, inputLen);
   }
   try {
       return this.cipher.doFinal();
   } catch (InvalidCipherTextException e) {
       throw new GeneralSecurityException("Cipher failed to process data.", e);
   }
}

Haha... DSpot was able to automatically generate a new test that was able to create a state that makes the code go in the catch.

Note that it would have been even nicer if DSpot had put an assert on the exception message.

Then I wanted to verify if the test coverage had increased so I ran Jacoco before and after for this module:

  • Before: 70.5%
  • After: 71.2%

Awesome!

Conclusions:

  • DSpot was able to improve the quality of our test suite automatically and as a side effect it also increased our test coverage (it's not always the case that new tests will increase the test coverage. DSpot's main intent, when executed with PIT/Descartes, is to increase the test quality - i.e. its ability to kill more mutants).
  • It takes quite a long time to execute, globally on those 6 modules it took about 15 minutes to build them with DSpot/PIT/Descartes (when it takes about 1-2 minutes normally).
  • DSpot doesn't generate a lot of tests: one test generated out of 100s of tests mutated (in this example session).
  • IMO one good strategy to use DSpot is the following:
    • Create a Jenkins pipeline job which executes DSpot on your code
    • Since it's time consuming, run it only every month or so
    • Have the pipeline automatically commit the generated tests to your SCM in a different test tree (e.g. src/test-dspot/)
    • Modify your Maven build to use the Build Helper Maven plugin to add a new test source tree so that your tests run on both your manually-written tests and the ones generated by DSpot
    • I find this an interesting strategy because it's automated and unattended. If you have to manually execute DSpot and look, find some generated tests and then manually incorporate them (with rewriting) to your existing test suite, then it's very tedious and time-consuming and IMO the ratio time spent vs added value is too low to be interesting.

WDYT?

EDIT: If you want to know more, check the presentation I gave at Devoxx France 2018 about New Generation of Tests.

Created by Vincent Massol on 2018/05/09 12:01
Created by Vincent Massol on 2018/05/09 12:01
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