Introduce a Semaphore to fully honor RUSTUP_CONCURRENT_DOWNLOADS
#4455
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rami3l
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Aug 25, 2025
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That's quite straightforward, thanks!
It might be interesting to introduce this to check_updates() as well.
…g "n" concurrent downloads Using this environment variable with a value between 2 and 5 would mean that concurrency was not being totally maximized as there could be less than n futures running at a some point in time. See: https://docs.rs/futures-util/0.3.31/futures_util/stream/trait.StreamExt.html#method.buffered Adding a semaphore allows US to control how many futures are running at a time, fixing this problem and ensuring that the env var is always honored and the downloads are maximizing concurrency.
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rami3l
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Aug 26, 2025
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I think this was just a spurious failure :/ |
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Follow-up on #4450.
Although introducing the
RUSTUP_CONCURRENT_DOWNLOADSenvironment variable allows the user to opt in to concurrency, there may be moments where this variable is not fully honored.An example would be when installing a toolchain (6 components) and the user had
RUSTUP_CONCURRENT_DOWNLOADS=2.This would create some situations where there would only be one download being done, where two could have been downloaded instead; this situation has been reported here.
This is not a surprise as per the nature of the
.buffered()method which, unfortunately, cannot ensure thatnfutures will always be buffered at a same point in time.To overcome this, and ensure that the environment variable is honored at all times, I decided to introduce a
Semaphorethat guarantees that there are alwaysndownloads happening concurrently.For reference, I leave below a small animation of the old (faulty) behavior:
... and the new (corrected) behavior: