gh-157678: Allow csv.Sniffer to inspect large fields - #157689
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sylvesterkaczmarek
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This temporarily changes the interpreter-wide CSV field limit, so concurrent readers can observe the raised limit, and a concurrent field_size_limit() update can also get overwritten by the finally restore. Could Sniffer avoid mutating the global limit for its trial parses, or otherwise isolate this so sniffing doesn't change another thread's parser behavior?
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gh-157678
csv.Sniffer.sniff() uses the CSV reader for trial parsing, so a sample containing a field larger than csv.field_size_limit() currently makes every candidate parse fail and raises csv.Error. Temporarily raising the parser limit to the sample length lets Sniffer determine the dialect, then restores the caller's configured limit in a finally block.
Tests: