[crac] RFR: Draft: Move more FD tracking to java layer

Anton Kozlov akozlov at openjdk.org
Thu Jun 8 17:30:23 UTC 2023

On Wed, 7 Jun 2023 10:51:41 GMT, Anton Kozlov <akozlov at openjdk.org> wrote:

> The PR develops the idea of file descriptors tracking in Java started in #43. In general, that PR had two purposes. First, it provides CheckpointExceptions in terms that are clear for Java developers, improving the experience of developing for CRac. So if a FileDescriptor causes an exception, it's possible to look at the heap dump and find references to the offending FD, or to look at the stack trace when FD was created. And second, Java FD tracking is independent of the platform, so that was the first step to bring CRaC to non-Linux platforms, but that is a bit longer road.
> We can eliminate manual heap inspection, and this is proposed in this PR. A FileDescriptor does not exist on its own but it is owned by some higher-level Java object implementation. So an object can "claim" a FileDescriptor and define how and if to report the FD to the user. E.g. Socket can describe the its port and address without deep inspection of the process internals. Turns out, Socket.toString() provides enough information (but the reporting can be extended later if required).
> 	Suppressed: jdk.crac.impl.CheckpointOpenSocketException: Socket[addr=localhost/,port=39957,localport=41464]
> 		at java.base/java.net.SocketImpl$SocketResource.lambda$beforeCheckpoint$0(SocketImpl.java:123)
> 		at java.base/jdk.crac.Core.lambda$checkpointRestore1$0(Core.java:128)
> 		... 7 more
> A FileDescriptor is claiming itself in case there is a bug in JDK that no higher-level object is claiming the FD. FD provides just a very short description just for debugging. With stack trace to FD (which is a very nice debugging aid!), that should be enough to find the containing object and implement claiming.
> I believe this overlaps with #69, which at first glance would benefit a lot from being able to define policies in the domain objects. I'll comment on this after a closer look at the other PR.

Motiviation for the PR is described in the description. You're right that the change mostly a refactoring. To repeat, this continues non-Linux CRaC implementations with the semantic shared between them, and this is also a foundation for optional checkpoint-restore resource policies implemented in the Socket, File,.. classes.

A higher level object like Socket or File usually has more information regarding FD and the uses, thus it can provide more information or implement wider set of policies. The chain of ownership will become a real problem once on every other layer we have more information that we want to report. In your example, FileWriter will have to claim FD if it has more information about that. But immediatly this is not clear. 

Stack traces are a great addition to the debug workflow, they describe the state on the moment of creation. But heap dump describes the state at the checkpoint exception. They provide different aspects, one or another may suit better different conditions.


PR Comment: https://git.openjdk.org/crac/pull/79#issuecomment-1583064896

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