Fixes re-downloading data corrupt for the same url

[zhongwuzw]

Fixes #489 , we use cacheKey to get the task, actually it would mix up data if previous task cancelled but download delegate still not finished , and next task with the same url started at the same time. This is the race condition for the same url downloading, we can keep the weak reference to task and get it in session delegate.

[bolsinga]

Please add a test.

[garrettmoon]

This is a really great fix! Would you mind adding a test as @bolsinga suggested?

[zhongwuzw]

@bolsinga @garrettmoon Please review again. :)

[bolsinga]

Thanks for the test!

[garrettmoon]

Can you add an entry to CHANGELOG.md for this?

[ghost]

🚫 CI failed with log

[garrettmoon]

Mind rebasing against master? I think I fixed a flakey test.

[ghost]

🚫 CI failed with log

[zhongwuzw]

@garrettmoon Tests failed like below frequently, I think we may need to add a command like rm -rf ~/Library/Developer/Xcode/DerivedData/ before build project in CI?

[jparise]

Sorry for the long delay @zhongwuzw. The CI tests should now be fixed. If you could rebase again, we can do a final review of your change.

[zhongwuzw]

@jparise Done.

[jparise]

This is looking really good, @zhongwuzw! Just a few more bits of feedback.

[zhongwuzw]

This is looking really good, @zhongwuzw! Just a few more bits of feedback.

Done.

[jparise]

Looks good to me! We can merge this once the other folks who have left comments (@garrettmoon, @bolsinga) give it a 👍, too.

[garrettmoon]

This looks great, thank you so much for the fix!

[bolsinga]

accepted; good fix; opens up some good questions on how to test the framework better going forward!