bnd-maven-plugin/m2e: BuildContext.isIncremental() is not reliable

[erwint]

Follow-up to #1346

Unfortunately, the BuildContext.isIncremental is not reliable, as an Eclipse build can get promoted to a Full Build. In this case the JDT build cleans the output directory, but bnd-maven-plugin still sees it as an incremental build. So if there are no relevant changes, this will result in a loss of the MANIFEST.MF, as bnd-maven-plugin will not generate one.

So if there is no existing MANIFEST.MF in the output directory, bnd-maven-plugin should assume a full build is going on.

[bjhargrave]

So M2E is broken and we have to work around it?

[erwint]

The problem is IMHO in the JavaBuilder of Eclipse. It has some heuristics as to when to upgrade an AUTO_BUILD to a FULL_BUILD, but the Builder framework has no means of notifying the other builders of that fact (at least I did not see any). So there's really no way for another builder to know that someone before deleted the whole output directory.

[bjhargrave]

IncrementalProjectBuilders are notified of the build type. See https://github.com/bndtools/bndtools/blob/master/bndtools.builder/src/org/bndtools/builder/BndtoolsBuilder.java#L141. I assume M2E is also an IncrementalProjectBuilder and thus can know if the build is incremental or full and then return the proper information from BuildContext.isIncremental.

[erwint]

the build type is AUTO_BUILD, but in some cases the JavaBuilder still assumes a full build, see https://github.com/maxeler/eclipse/blob/master/eclipse.jdt.core/org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/builder/JavaBuilder.java#L177-209

[bjhargrave]

So unless it is an INCREMENTAL build, BuildContext.isIncremental should return false. AUTO or FULL should be assumed non-incremental. This is what the BndtoolsBuilder does.

[erwint]

That's not what the Apidocs say, however. Also, as you can see the JavaBuilder would behave the same on INCREMENTAL.... I thought you'd be interested where there are expected problems with bnd-maven-plugin. I can't do more than to report it. IMHO if there is no MANIFEST.MF at the destination location, there is a problem in any case, so it might be a good check anyways.

[bjhargrave]

Well the manifest is just one file the plugin writes. DS xml files could also be missing if the java builder deletes files in target/classes. If we cannot trust BuildContext.isIncremental, then there is no point in even calling it.

Is the java builder deleting the complete contents of target/classes? Or just the manifest? If, the complete contents, then the missing manifest can be a proxy for the deletion event.

[erwint]

it's deleting all the content in target/classes