[FLINK-17012][streaming] Implemented the separated 'restore' method for StreamTask

[akalash]

This is the second part of the changes which based on #15221

What is the purpose of the change

These changes implement the restore method which was introduced in #15221. This allows restoring data independently on a invoke method and it will be possible to call the restore and the invoke in different states.

Brief change log

• The contract of MailboxProcessor#runMailboxLoop is changed. It can be suspended by calling 'suspend'.
• StreamTask#beforeInvoke renamed to the restore with small changes and it can be invoked independently of 'invoke' now.

Verifying this change

This change is mostly covered by existing tests, such as StreamTask, UnalignedCheckpoint*.

This change added tests and can be verified as follows:

• Added tests for verifying that runMailboxLoop can be suspendable correctly
• Extended StreamTask tests for checking that 'restore' method doesn't call more than 1 times

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

Thanks a lot for your contribution to the Apache Flink project. I'm the @flinkbot. I help the community
to review your pull request. We will use this comment to track the progress of the review.

Automated Checks

Last check on commit 4cc069a (Sat Aug 28 11:12:03 UTC 2021)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

_{Mention the bot in a comment to re-run the automated checks.}

Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

Please see the Pull Request Review Guide for a full explanation of the review process.

The Bot is tracking the review progress through labels. Labels are applied according to the order of the review items. For consensus, approval by a Flink committer of PMC member is required

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot approve description to approve one or more aspects (aspects: description, consensus, architecture and quality)
• @flinkbot approve all to approve all aspects
• @flinkbot approve-until architecture to approve everything until architecture
• @flinkbot attention @username1 [@username2 ..] to require somebody's attention
• @flinkbot disapprove architecture to remove an approval you gave earlier

[flinkbot]

CI report:

• 6366c66 Azure: CANCELED
• d928ce9 Azure: PENDING
• 59cca4c UNKNOWN
• 4cc069a Azure: PENDING

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run travis re-run the last Travis build
• @flinkbot run azure re-run the last Azure build

[akalash]

@flinkbot run azure

[pnowojski]

Thanks for the contribution. The general approach looks fine for me, but I've left a couple of questions/comments. The biggest question for me the is the suspendable loop addition.

[pnowojski]

I'm a bit confused. Do we have both restore() and beforeInvoke() now? Why do we need both?

[akalash]

beforeInvoke is still needed for test purposes. It helps to switch the internal isRunning flag to true without actual call the invoke. But if you think that it is problem, I can try to rewrite the test.

[pnowojski]

LGTM % two minor things:

• I've suggested some java doc changes in the last commit
• I would extract the fixup of previous PRs as a separate fixup commit

[pnowojski]

The final build is almost done, except of e2e tests. Compared to previous builds there were no significant changes (java docs vs the previous build or renames vs the build before), and those previous builds have succeeded or failed for unrelated issues, so I'm going to merge this.