[CI] NodeEnvironmentTests testIndexCompatibilityChecks failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic #6852 / openjdk24_entitlements_true_checkpart1_java-matrix
• elasticsearch-pull-request #61424 / checkpart1_release-tests
• elasticsearch-pull-request #61213 / part-1
• elasticsearch-pull-request #61096 / part-1

Reproduction Line:

./gradlew ":server:test" --tests "org.elasticsearch.env.NodeEnvironmentTests.testIndexCompatibilityChecks" -Dtests.seed=14F69652DC3A5F7C -Dtests.jvm.argline="-Des.entitlements.enabled=true" -Dtests.locale=eu -Dtests.timezone=Pacific/Johnston -Druntime.java=24

Applicable branches:
main

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: 
Expected: (a string containing "Cannot start this node" and a string containing "it holds metadata for indices with version [4.29.21]" and a string containing "Revert this node to version [8.19.0]")
     but: a string containing "Revert this node to version [8.19.0]" was "Cannot start this node because it holds metadata for indices with version [4.29.21] with which this node of version [9.1.0] is incompatible. Revert this node to version [9.0.51] and delete any indices with versions earlier than [8.0.0] before upgrading to version [9.1.0]. If all such indices have already been deleted, revert this node to version [9.0.51] and wait for it to join the cluster to clean up any older indices from its metadata."

Issue Reasons:

• [main] 4 failures in test testIndexCompatibilityChecks (1.6% fail rate in 244 executions)
• [main] 2 failures in step part-1 (1.7% fail rate in 118 executions)
• [main] 3 failures in pipeline elasticsearch-pull-request (2.6% fail rate in 116 executions)

Note:
This issue was created using new test triage automation. Please report issues or feedback to es-delivery.

[elasticsearchmachine]

This has been muted on branch main

Mute Reasons:

• [main] 4 failures in test testIndexCompatibilityChecks (1.6% fail rate in 244 executions)
• [main] 2 failures in step part-1 (1.7% fail rate in 118 executions)
• [main] 3 failures in pipeline elasticsearch-pull-request (2.6% fail rate in 116 executions)

Build Scans:

• elasticsearch-periodic #6852 / openjdk24_entitlements_true_checkpart1_java-matrix
• elasticsearch-pull-request #61424 / checkpart1_release-tests
• elasticsearch-pull-request #61213 / part-1
• elasticsearch-pull-request #61096 / part-1

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[mosche]

@thecoop Could you have a look, if I revert #124197 this passes again.

./gradlew ":server:test" --tests "org.elasticsearch.env.NodeEnvironmentTests.testIndexCompatibilityChecks" -Dtests.seed=14F69652DC3A5F7C -Dtests.jvm.argline="-Des.entitlements.enabled=true" -Dtests.locale=eu -Dtests.timezone=Pacific/Johnston -Druntime.java=24

[prdoyle]

@mosche - is the idea that NodeEnvironment.bestDowngradeVersion is returning the wrong version? Is it really supposed to suggest 8.19.0 rather than 9.0.51?

[mosche]

I'm not sure, haven't had a chance to look into this yet @prdoyle. On my list for next week when being on duty

[prdoyle]

I'm looking at it now. Here's my take so far:

The getBestDowngradeVersion does not (and currently cannot?) implement what its javadocs say it does. The javadocs:

We return (N-1).last, unless the user is trying to upgrade from a (N-1).last.x release, in which case we return the last installed version.

This requires us to identify the difference between major versions N and N-1, which is not exposed by the BuildVersion class. Therefore, we are returning the last installed version for anything on or after (N-1).last, which includes earlier N.x versions.

The main question I have is: is is possible to introduce an index version incompatibility in a minor ES version bump?

• If it's impossible, then the difference is academic, because the failure reported here happens only when the current version is, say, N.1 and the previousNodeVersion is N.0. In this case, an index version incompatibility should be impossible so the fix would be to change the randomized test so it doesn't pick earlier versions with the same major as the current version. The javadocs are not wrong, but could be a bit clearer.
• If it's possible, then bestDowngradeVersion's javadocs need tweaking to account for older versions with the current major. It should be returning the newer of previousNodeVersion and minimumCompatibilityVersion (which is what its code already does) and the unit tests should be changed accordingly.

In either case, I believe the code inside bestDowngradeVersion is correct, so this could be downgraded to low-risk because it's the tests that are wrong; however, the fix is relatively simple, so we probably should just get it done.

[prdoyle]

I'm proposing this for the javadocs of getBestDowngradeVersion:

     * Get a useful version string to direct a user's downgrade operation
     * <p>
     * Assuming that the index was compatible with {@code previousNodeVersion},
     * the user should downgrade to that {@code previousNodeVersion},
     * unless it's prior to the minimum compatible version,
     * in which case the user should downgrade to that instead.
     * (If the index version is so old that the minimum compatible version is incompatible with the index,
     * then the cluster was already borked before the node upgrade began,
     * and we can't probably help them without more info than we have here.)

[prdoyle]

Alright, based on the above, I'm proposing a fix in #127646.

[prdoyle]

Moved to low-risk because I think the functionality is correct and the tests are wrong.