SOLR-18098: Fix replication failure for files with exact MB sizes

[shubhamranjan]

https://issues.apache.org/jira/browse/SOLR-18098

Description

Replication fails with EOFException when transferring files whose size is an exact multiple of PACKET_SZ (1 MB). For example, replicating a file that is exactly 1 MB, 2 MB, etc. causes the follower to crash.

Solution

The root cause is in IndexFetcher.FileFetcher.fetchPackets(). The replication packet protocol has three packet types:

1. Data packet: int(size) + long(checksum) + byte[size]
2. Zero-length data packet: int(0) + long(checksum) — sent when the last chunk fills exactly PACKET_SZ
3. EOF marker: int(0) — no checksum follows

The old code treated any packetSize == 0 as a loop-continue, skipping the checksum at step 2. Those 8 unread checksum bytes were then interpreted as the next packet size → garbage value → EOFException.

The fix reorders fetchPackets() to:

1. Detect the EOF marker (size=0 and fis.peek() == -1)
2. Read the checksum for all data packets, including zero-length ones
3. Skip zero-length data packets only after consuming their checksum

AI Disclosure: Claude (Anthropic) was used as an aid during diagnosis and development — specifically for analyzing the packet protocol interaction between DirectoryFileStream.write() and fetchPackets(), reasoning through the checksum read misalignment, and drafting test cases. All changes were reviewed, verified, and refined by a human (me) before submission.

Tests

Added IndexFetcherPacketProtocolTest with 18 unit tests that exercise the packet protocol between DirectoryFileStream (sender) and FileFetcher.fetchPackets (receiver) in isolation:

• Exact multiples of PACKET_SZ: 1 MB, 2 MB, 3 MB, 63 MB
• Non-multiples: empty, 1 byte, 100 bytes, 100 KB, 512 KB
• Boundary cases: PACKET_SZ ± 1 byte, 1.5× PACKET_SZ, 2× PACKET_SZ ± 1
• Error handling: checksum mismatch detection
• Buffer resize: large multi-packet file (5 MB + 12345 bytes)
• Successive transfers: multiple exact-size files in sequence

Run with: ./gradlew :solr:core:test --tests "IndexFetcherPacketProtocolTest"

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.