The Hidden Race Condition in file-type That Corrupted Our Parallel Image Detection

A company experienced issues with their logo candidate list shrinking unpredictably, which was traced to a problem with the file-type npm package when used under high concurrency conditions. The list of logo candidates varied significantly across different runs due to MIME-type misclassification, where files like PNGs were incorrectly identified as JPEGs. Initially, the issue was hard to reproduce locally but was found to be exacerbated by concurrent processing in their production environment, where multiple file buffers were processed simultaneously. The root cause was identified as a shared tokenizer state in the file-type library, which led to incorrect file-type detection when multiple requests were processed at once. The solution involved ensuring that each file buffer was processed with a fresh FileTypeParser instance, eliminating the shared state problem and stabilizing the candidate list.

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