fix(fs): prevent TOCTOU race in InMemoryFs::append_file()#454
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append_file() previously read file state under a read lock, dropped it, checked size limits with stale data, then acquired a write lock. Another thread could modify the file between locks, bypassing per-file size limits. Fix: use a single write lock for the entire read-check-write operation. File existence check, size limit validation, and the actual append all happen under the same write lock, eliminating the race window. Also inline the file-creation path to avoid deadlock from calling write_file() while already holding the entries lock. Closes #415
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Summary
append_file()to prevent TOCTOU race where file size changes between lock acquisitionsTest plan
test_append_file_no_toctou_race- concurrent appends respect file size limitstest_append_creates_new_file_atomic- append to nonexistent file creates correctlytest_append_to_directory_fails- append to directory returns errorCloses #415