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Fix a bug in the _upload_file_part_concurrent method #910

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merged 2 commits into from
Nov 6, 2024

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nils-braun
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The _upload_file_part_concurrent method is used as part of the put_file function to upload the file in multiple parts (when the file is larger than a certain limit).
The function basically reads from the original file in chunks (by default 50MB) and then schedules 10 upload calls in one block. It has two different "branches": if there is more than one chunk left, it schedules them in parallel - if not, it just runs it directly.

This last branch has a bug: it uses a variable chunk which is actually defined in another scope (in the for-loop before it). This leads to wrong data on the remote location: if you upload a file which has e.g. between 20 * 50MB and 21 * 50MB size, it will always be truncated to to exactly 20 * 50MB on s3. This bug is fixed in this PR.

@martindurant
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Thanks for the fix. It should be easy to test this, right?

@martindurant
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martindurant commented Nov 5, 2024

Maye a simpler fix would be to run "in parallel" even for just one remaining chunk

@nils-braun
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@martindurant - I added a test and simplified the code to only use a single branch ("in parallel" for both cases)

@martindurant
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Perfect, thank you

@martindurant martindurant merged commit ff8e4fe into fsspec:main Nov 6, 2024
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@schlagercollin
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Thanks for the fix here!

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3 participants