leveldb: correct trailer boundary check in Reader::SkipToInitialBlock #33955

1. 
   sashass1315 commented at 7:13 pm on November 26, 2025: none

   replaces the magic number 6 in the trailer boundary condition with kHeaderSize and corrects an off-by-one by expressing the rule as offset_in_block > kBlockSize - kHeaderSize, which is equivalent to offset_in_block >= kBlockSize - 6. According to src/leveldb/doc/log_format.md: “A record never starts within the last six bytes of a block” and when exactly seven bytes remain a record may start (e.g., a zero-length FIRST), so the equal case must be treated as trailer. The writer already uses kHeaderSize to manage trailers, and aligning the reader with kHeaderSize removes a format-coupled magic number and ensures future-proof consistency. Functionally this avoids scanning a block when initial_offset_ is in the last six bytes, matching the documented format; it was previously a minor inefficiency and comment mismatch rather than a correctness issue.
2. leveldb: correct trailer boundary check in Reader::SkipToInitialBlock 91faa93c17
3. 
   DrahtBot commented at 7:13 pm on November 26, 2025: contributor

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4. 
   maflcko commented at 8:03 am on November 27, 2025: member

   thx, but this will need to go upstream
5. maflcko closed this on Nov 27, 2025

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sashass1315 DrahtBot maflcko