Naming Maven repositories

Finding the root cause of a Maven build issue made me question my sanity, as seemingly everything I knew about Maven seemed to be wrong. Luckily, in the end logic was victorious, and quite a nasty bug was found.

Motto:

Caches are bugs waiting to happen. Rob Pike

The last two days I was hunting an issue in the Maven/Tycho based build of VIATRA: some artifacts from Maven Central were not found anymore, seemingly without any related change. A more detailed analysis has shown that the issue is specific to the build server at the Eclipse Foundation: in all other cases the builds run successfully.

When looking at the debug output of the build (the -X switch for mvn is truly a killer feature…) it was interesting that the build did not try to download the dependency from Maven Central nor from the mirror set up for builds at the Hudson instance at eclipse. Given that the first twenty-two modules compiled (with dependencies to Maven Central) this made no sense.

After managing the sanity loss and calling for help (thanks Ábel and Balázs) we have managed to identify that the settings.xml used on Hudson caused the problem. After calling for more help (thanks for the quick help from webmaster) I have finally managed to find out the root cause of the issue: repository identifier clash caused by the maven central mirror and a dependency issue.

For performance and network utilization reasons a mirror for maven central was set up about a month ago using the following fragment:


<mirrors>
    <mirror>
      <id>repo.eclipse.org</id>
      <name>Eclipse Central Proxy</name>
      <url>https://repo.eclipse.org/content/repositories/maven_central/</url>
      <mirrorOf>central</mirrorOf>
    </mirror>
</mirrors>

Our problematic bundle had a repository declaration as follows:


<repositories>
    <repository>
        <id>repo.eclipse.org</id>
        <url>https://repo.eclipse.org/content/groups/releases/</url>
    </repository>
</repositories>

What happened here is that we declared some repositories with the same identifier as the proxy repository declared by webmaster, and Maven got confused, and thought that everything from Central is reachable from our declared dependency repository. Changing the identifier to avoid the name clash solved this issue nicely.

Now only one question remains: why was this issue not found earlier. The answer is trivial: the plugins were available in the local repository, effectively hiding the unresolvable dependencies. A completely unrelated cleanup of the local repository however has brought forth this issue, causing much headache to find.


The main lesson we learned here: DO NOT reuse the same repository identifier for multiple repositories. It can cause very subtle, hard to debug issues in the long run. However, there are a few other aspects you have to keep in mind:

  • All repositories and plugin repositories defined in your parent project are also inherited, so their name should also be unique.
  • Identifier clash might not be a problem in case of deployment repositories, but have not tested this aspect yet.
  • A more specific lesson for Eclipse projects building on Foundation Servers: DO NOT use the identifier “repo.eclipse.org” for your repositories. Maybe it is worth checking whether the identifier is used (e.g. use following the github search link to check projects developed or mirrored to the Eclipse Github organization), and update accordingly. Update: the mirror repository id was updated to “eclipse.maven.central.mirror” that should not clash with manual repository names.

Parsing textual input with IncQuery

Parsing textual notation has a long history in computer science. This long time has produced many great and efficient parsing algorithms, which were optimized to the extreme by compilers for different programming languages. However in the recent years the development of integrated development environments are accelerated, adding more and more services to previously simple features like code editors and automatic background compilation. These with the rise of domain specific language frameworks which allows the user to define an arbitrary language declaratively causes that present-day computers still struggle under the resource requirements of these technologies.

While waiting in front of a textual editor, I’ve started thinking about why does the editor need to re-run the parsing algorithm every time I hit button (of course I’m aware of the fact that the main cause of the slow responsibility is probably not the parsing algorithm itself). A fully incremental approach could make these tools more responsive which would remove a lot of pain from its users. As I’m already familiar with an incremental query engine (IncQuery) the thought about using it to parse text couldn’t leave my mind.

The following article presents the experiment I’ve done as a proof-of-concept. It’s far from being useful and reveals more problems than it solves it does not produce an AST just accepts or rejects the input string, however it may be interesting as an approach and maybe in the future it will be more than just a stupid idea. Continue reading “Parsing textual input with IncQuery”

Yet another textual modeling framework

Most of the time I try to avoid reinventing the wheel. And most of the time I fail and forced to do so. That’s what happened to me when I decided to write my own textual modeling framework. I’ve used (and currently using) Xtext in several projects and tried EMFText once. Both of them are great projects with great possibilities, but I’ve come across a problem which required a different approach. I needed a more dynamic toolkit, where the language is extensible and easier to maintain. Continue reading “Yet another textual modeling framework”

Xtext Reflective Editor updated for Kepler

Today a new Eclipse version codenamed Kepler was released, with a lot of nice new features, including conflict handling during update, an update user interface for the Marketplace client or various EGit updates (my favorite is the Commit and Push button in the Commit dialog).

About the less visible stuff, in the modeling projects we use, e.g. EMF, Graphiti or Xtext also got some internal updates, resulting in the need to update our projects. As of now, I updated the Xtext Reflective Editor, as a change in EMF 2.9 made it unusable in Kepler.

I often use this tool to debug the Xtext-based parsers that use Xbase and model inferrers, as it displays the generated model using the EMF reflective editors. Version 0.5.5 is a recommended minor update – it works on both newer EMF/Xtext versions, but maintains compatibility with older EMF versions. It is still downloadable from our update site http://eclipse.cubussapiens.hu.

Thanks for all contributors the new, nice features of Kepler (and of course fixes as well). This can become a new platform for new projects for me – the reflective editor update is only the first of them.

Language Features Enhancing Trust

Alex just facepalmed.

if ( Boolean.TRUE.equals(employee.isHappy) ) {

“Wow, the human invention never ceases to come up with new ways to adorn boolean expressions! Too bad Eclipse doesn’t have a Quick Assist for simplifying them…”
Ah, one of those WWTC moments. That’s when the Show Annotation feature of Subversive comes in handy… which revealed that Bob is the author (unless he only adjusted the whitespaces in that line). “But he already left the office for today. Now, I’ll just simplify it and similar occurrences manually and tomorrow, I’m going to considerately mention to him that he could have written this condition umm… more concisely.” Alex pondered a bit over the commit message, but practiced self-restraint and wrote simply

Simplify boolean expressions...

(though he couldn’t help omitting those ellipses).
Next day, some of the tests of Bob’s module were failing.
– Hi Bob, could we have a look at your code? First, there are some red tests, and…
– Hmm, but I haven’t modified that module since yesterday. Let’s see the exception!
– Okay…

Exception in thread "main" java.lang.NullPointerException
at EmployeeLoader.loadEmployee(EmployeeLoader.java:41)

“Hmm, exactly the line I modified. But…” – thought Alex.
– …how could a simple condition without a single dereferencing cause a NPE?! – asked Bob, confused just like Alex.
– Uh-oh, I think I have a suspicion… Please show the declaration of isHappy

private Boolean isHappy;

Alex facepalmed once again, but this time the cause was himself.
– NOOO! The dreaded autoboxing! But again, why isn’t it a primitive boolean?
– Hey, now I remember! Employees are parsed from XML using an autogenerated schema. The attribute isHappy is optional, so it might very well be null.
– Sorry, Bob. I feel silly for acting without asking you in advance or running the tests before committing. See, I couldn’t have imagined how this kind of change could break.
– Take it easy, Alex. 🙂 Now you can. In fact, I thought Boolean.TRUE would refer to the attribute’s type being non-primitive unambigously, and the Yoda condition would evoke the possibility of the null value immediately.
– I understand you, but to avoid such misunderstandings in the future, would you mind writing

if ( (employee.isHappy == null) && employee.isHappy ) {

to make it totally explicit that isHappy can be null? Or rather set an optional false value for this attribute in the schema?
– OK, I’ll consider.
Fortunately, Bob took the incident very lightly and his commit message was just:

Revert Alex's "simplifications" :)

The first thing Alex did was to set a warning for boxing and unboxing among the Java compiler settings. As he was accepting the changes, he thought: “Null and primitive types as well are billion dollar mistakes coming from arbitrary language design decisions which reflect implementation details. But at least they tought me to trust my fellow’s code – or at least to inspect the types before refactoring.”