HDDS-15962. Remove leader readiness check on the bootstrap flow.

[sadanand48]

What changes were proposed in this pull request?

Current logic rejects checkpoint requests with 503 unless isLeaderReady() is true.
In a degraded cluster (e.g., one OM down, one follower far behind), this creates a loop:

Lagging follower needs checkpoint to catch up.
Follower requests /v2/dbCheckpoint from leader.
Leader is LEADER_AND_NOT_READY and returns 503 due to isLeaderReady() check.
Follower cannot catch up, so leader readiness does not progress.
System remains stuck until another OM is brought back.
Checkpoint creation already has consistency barriers:

awaitDoubleBufferFlush()
bootstrap write lock (BOOTSTRAP_LOCK)
So requiring LEADER_AND_READY here is stricter than necessary and can block recovery. instead only reject if NOT_LEADER

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-15962

How was this patch tested?

Unit tests

[smengcl]

Thanks for the patch @sadanand48

[Copilot]

Pull request overview

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This PR relaxes OM DB checkpoint bootstrap gatekeeping by rejecting checkpoint requests only when the OM is not the leader (instead of requiring the leader to also be “ready”), preventing deadlock in degraded clusters where a lagging follower cannot catch up.

Changes:

• Add OzoneManager#isLeader() to reflect Raft leader status independent of readiness.
• Update checkpoint servlets to gate /dbCheckpoint requests on isLeader() rather than isLeaderReady().
• Extend unit tests to validate leader vs non-leader behavior and isLeader() semantics.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/OzoneManager.java	Introduces isLeader() based on Raft server status to decouple leadership from readiness.
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/OMDBCheckpointServletInodeBasedXfer.java	Switches checkpoint request rejection condition from isLeaderReady() to isLeader().
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/OMDBCheckpointServlet.java	Same leadership-only gate update for the non-inode-based servlet.
hadoop-ozone/ozone-manager/src/test/java/org/apache/hadoop/ozone/om/TestOMDBCheckpointServletInodeBasedXferNonLeader.java	Updates existing non-leader tests and adds new coverage for leader behavior + isLeader() mapping.

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[smengcl]

lgtm. just one more comment. also pls check the remaining copilot comment

[smengcl]

Thanks @sadanand48 for the patch. Thanks @jojochuang for reviewing this.