Improve C/R exception printout

[rvansa]

Some users might get confused by the inner exceptions reported during C/R as suppressed exceptions. This PR changes the printout to make it look as if the exception had multiple causes. For example the DryRunTest will report this:

jdk.crac.CheckpointException: Failed with 2 inner exceptions
Cause 1/2: java.lang.RuntimeException: should not pass
	at DryRunTest$CRResource.beforeCheckpoint(DryRunTest.java:47)
	at java.base/jdk.crac.impl.AbstractContext.invokeBeforeCheckpoint(AbstractContext.java:44)
	... (redacted)
Cause 2/2: jdk.crac.impl.CheckpointOpenFileException: /tmp/jtreg-DryRunTest6956725915963168340.tmp
	at java.base/jdk.internal.crac.JDKFileResource.lambda$beforeCheckpoint$1(JDKFileResource.java:89)
	at java.base/jdk.crac.Core.checkpointRestore1(Core.java:174)
        ... (redacted)

---

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[openjdk]

@rvansa This change now passes all automated pre-integration checks.

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Improve C/R exception printout

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At the time when this comment was updated there had been 2 new commits pushed to the crac branch:

• 93000a3: Adjusting CRaC tests
• a3d56c2: 8321660: [CRaC] Trim native heap before checkpoint to decrease image size

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[mlbridge]

Webrevs

• 01: Full - Incremental (5f011c94)
• 00: Full (474b1638)

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[rvansa]

@AntonKozlov Could you review please?

[AntonKozlov]

Thanks. I agree that reporting exceptions as suppressed may be confusing. The new report looks good. But it introduces a new term "inner" which is not specified anywhere. Another side of the same problem is that programmatic access to the "inner" exception still remains through getSuppressed(), per javadoc and the fact there is no other mean to get an inner exception. I would suggest to go away completely from suppression (which was introduced for try-with-resources) to a completely distinct mechanism to record inner/nested/causing exceptions for Checkpoint... or RestoreException. How does it sound? My point is to have a mechanism that is consistent in 1) user-visible report; 2) in programmatic API; 3) the textual part of javadoc specification, e.g. package-level javadoc [1].

[1]

crac/src/java.base/share/classes/jdk/crac/package-info.java

Line 63 in 474b163

* Exceptions from restore notification are provided as suppressed ones by a {@code RestoreException} (in a sense of {@link Throwable#addSuppressed}).

[rvansa]

"Nested" is probably a better term than "inner". Having method getNestedExceptions() that actually calls getSuppressed() would be trivial, and would provide space for javadocs. A completely distinct field is probably cleaner, though; however it feels that we are getting more code duplicated for Checkpoint and RestoreException; do you think it would be useful to have them share a common parent? (ofc. we will still have some duplication with the javax.crac package...).

[AntonKozlov]

I also would prefer "nested" exceptions not to overlap with suppressed in implementation, to make sure the correct interface is always called. Indeed, a common parent class makes sense, and sometime in handling of the exception I had to write

catch (CheckpointException | RestoreException e) { ... }

Which is probably correct way to indicate both checkpoint and restore failures are handled there. But if you want to just to print the "nested" exception, apprantly you'll need a common base exception. I.e. a common base looks good.

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[rvansa]

@AntonKozlov Updated, dropping all use of suppressed exceptions.

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[rvansa]

Still relevant.

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[rvansa]

Still relevant, waiting for @AntonKozlov review.

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[rvansa]

/open

[openjdk]

@rvansa This pull request is now open

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