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Fix if/while thinko in read_stream.c edge case.

When we determine that a wanted block can't be combined with the currentpending read, it's time to start that read to get it out of the way. An"if" in that code path should have been a "while", because it might takemore than one go in case of partial reads. This was only broken forsmaller ranges, as the more common case of io_combine_limit-sized rangesis handled earlier in the code and knows how to loop, hiding the bug fora while.Discovered while testing large parallel sequential scans of partiallycached tables. The ramp-up-and-down block allocator for parallel scanscould hit the problem case and skip some blocks near the end that shouldhave been streamed.Defect in commitb5a9b18.Discussion:https://postgr.es/m/CA%2BhUKG%2Bh8Whpv0YsJqjMVkjYX%2B80fTVc6oi-V%2BzxJvykLpLHYQ%40mail.gmail.com

1 parentbeb012b commit158f581Copy full SHA for 158f581

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src/backend/storage/aio
- read_stream.c

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`‎src/backend/storage/aio/read_stream.c`

Lines changed: 1 addition & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -363,7 +363,7 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice)`
`363`	`363`	`}`
`364`	`364`
`365`	`365`	`/* We have to start the pending read before we can build another. */`
`366`		`-if (stream->pending_read_nblocks>0)`
	`366`	`+while (stream->pending_read_nblocks>0)`
`367`	`367`	`{`
`368`	`368`	`read_stream_start_pending_read(stream,suppress_advice);`
`369`	`369`	`suppress_advice= false;`

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Commit158f581

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`‎src/backend/storage/aio/read_stream.c`

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