Make result iteration recoverable from errors#615
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azawlocki-sbdt
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LGTM, code comments make it a good read. My only suggestion would be to cover more scenarios in tests, but it's non-blocking.
TrinoResult implemented iteration as a generator. A generator that raises an unhandled exception is finalized so every subsequent next() raises StopIteration which dbapi's fetchone() maps to None. Since spooled segments are downloaded lazily during iteration, a transient error (e.g. an S3 timeout) mid-iteration permanently killed the iterator and silently dropped the remaining rows as if the query had completed normally. Rewrite TrinoResult as a class-based iterator that keeps its state on the instance. An error propagates to the caller while the iterator stays usable, so a persistent error keeps surfacing instead of turning into StopIteration and a retried next() resumes where the failure happened. Also make SegmentIterator retry-safe. Acknowledge and advance past a segment only after it was decoded successfully so retrying after a failed segment download re-decodes the same segment instead of acknowledging and skipping it.
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Description
TrinoResult implemented iteration as a generator. A generator that raises an unhandled exception is finalized so every subsequent next() raises StopIteration which dbapi's fetchone() maps to None. Since spooled segments are downloaded lazily during iteration, a transient error (e.g. an S3 timeout) mid-iteration permanently killed the iterator and silently dropped the remaining rows as if the query had completed normally.
Rewrite TrinoResult as a class-based iterator that keeps its state on the instance. An error propagates to the caller while the iterator stays usable, so a persistent error keeps surfacing instead of turning into StopIteration and a retried next() resumes where the failure happened.
Also make SegmentIterator retry-safe. Acknowledge and advance past a segment only after it was decoded successfully so retrying after a failed segment download re-decodes the same segment instead of acknowledging and skipping it.
Non-technical explanation
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Fixes #598.