JHSUYU opened a new pull request, #4820:
URL: https://github.com/apache/solr/pull/4820
SOLR-18406: Restart replication after index generation expires
# Description
During leader/follower index replication, the follower first requests the
file list for generation G and then downloads the files using separate
`filecontent` requests.
If the reservation for G expires before a file download starts, and a
leader commit removes G `DirectoryFileStream.initWrite()` attempts to save the
stale commit point and throws an `IllegalStateException`. This was returned as
HTTP 500.
The existing `filelist` path already handles the same condition as an
invalid index generation, but the `filecontent` path did not.
# Solution
Convert the stale-generation failure in `DirectoryFileStream.initWrite()`
into an explicit HTTP 409 conflict with an `invalid index generation` message.
When `IndexFetcher` receives this response, it stops retrying the same file
from the already unavailable generation and restarts the complete replication
cycle once, beginning with a new `indexversion` request. This allows the
follower to fetch the leader's current generation instead of failing the entire
replication attempt.
The change also ensures that a commit point is only reserved and released
when it was successfully saved.
# Tests
Added
`TestReplicationHandler.testFollowerRestartsWhenCommitExpiresBeforeFileDownload
`.
The test uses real leader and follower Jetty instances and performs this
sequence:
1. The leader commits generation G.
2. The follower starts an explicit `fetchindex` request.
3. The follower pauses after receiving `filelist(G)` and before requesting
`filecontent(G)`.
4. The commit reservation expires.
5. The leader commits G+1, allowing G to be deleted.
6. The follower resumes, detects that G is unavailable, restarts
replication, and installs G+1.
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