fix(mcp): start wait_for_event at the beginning of a half-written line - #31
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wait_for_event took the event log's length as its starting point. The sampler appends a line in more than one write now and then, so a call that began between two of them had its offset in the middle of a line: the second half arrived alone, failed to parse, and the event was lost. That is the intermittent CI failure of partial_lines_wait_for_their_newline (the writer thread's first write beat the reader's metadata() on a loaded runner), and a real miss for an agent whose wait happened to start at that moment. The starting offset is now the first byte of any unterminated last line, found by scanning the tail back to the nearest newline; a log that ends cleanly still starts at its end, so nothing already written is replayed. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01MuezDnJq5Qh34SoGGAgzZU
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Why
partial_lines_wait_for_their_newlinefailed once on CI for #30 and passed on re-run. Not a flaky assertion:wait_for_eventtook the event log's length as its starting point, and when the writer's first half-line landed before the reader measured the file, the offset sat mid-line — the second half arrived alone, failed to parse, and the wait timed out. The sampler appends lines in more than one write now and then, so an agent whose wait began at that moment lost a real event the same way.What
start_of_unfinished_line, scanning the tail back to the nearest newline in 4 KiB chunks). A log that ends cleanly still starts at its end, so nothing already written is replayed.Tested
cargo test -p sint mcp::(13 passed),cargo fmt --check,cargo clippy -p sint --all-targets -- -D warnings— in an acompile allocation.🤖 Generated with Claude Code
https://claude.ai/code/session_01MuezDnJq5Qh34SoGGAgzZU