Avoid re-using same job

[mickaelistria]

Modifying a running or just cancelled job is not always a safe operation. This can (does when integrated in Eclipse IDE) lead to IllegalStateException when calling getRule() on some already scheduled job.
To prevent that, just create a new job when rules change.

[snjeza]

test this please

[rgrunber]

@snjeza , I see a lot of places in the tests where we use the DocumentAdapter instead of returning the compilation unit's buffer. Also at

https://github.com/eclipse/eclipse.jdt.ls/blob/e7e1418bca69e6d5862ea4e90c1c80ce55c711b1/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/commands/DiagnosticsCommand.java#L109

Is this change safe to do ?

[mickaelistria]

The part in createBuffer is not related to the intent of this change. I'll remove it.

[mickaelistria]

Done removing unrelated part of this PR.

[rgrunber]

re-test this please.

[rgrunber]

@mickaelistria , just wondering, we re-use validationTimer, in the same way. Did you not encounter any issues with that ?

[rgrunber]

re-test this please.

[rgrunber]

re-test this please.

[snjeza]

I see a lot of places in the tests where we use the DocumentAdapter instead of returning the compilation unit's buffer. Also at
...
Is this change safe to do ?

@rgrunber It is copied from https://github.com/eclipse-jdt/eclipse.jdt.ui/blob/567c1546ee76fbbd4a50ee6b0de8e046a19356fe/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/DocumentAdapter.java
Eclipse JDT Team uses it in their tests too.

[mickaelistria]

Did you not encounter any issues with that ?

Not with this one.
My impression is that the cancel didn't take immediate effect as expected for the publishDiagnosticsJob. Either it was not performed at all, or maybe it was rescheduled automatically for some reason (some job listener?) before the setRule in invoked.