Speed up async streaming.

[graebm]

• Fix bug where async-streaming prevented a meta-request from doing ANY parallelism 😅.
  • We accidentally limited "number of parts in flight" when we meant to limit "number of parts pending read"
• Reduce gap between sequential reads by running client->update() as soon as the read completes, rather than waiting until after signing is complete.
• Rename ~num_parts_sent~ -> num_parts_started, for clarity
• Rename (rework) ~num_parts_read~ -> num_parts_pending_read.
  • Note this slightly changes the meaning as well, this number goes up and down, and it's never more than 1 for async-streaming.
  • This fixes a bug when resuming an upload. We'd forgot to increment the old num_parts_read along with num_parts_started and num_parts_completed which threw off the math and lead to the upload stalling. It seemed simpler and more obvious to just track the number we care about, rather than have so many different numbers ticking up, from many different locations, and their meaning is weird when skipping is involved.
• If a stream-reading failure occurs during "resume from pause", reveal the actual error code. Stop covering it with AWS_ERROR_S3_RESUME_FAILED.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #313 (f4c3ba0) into main (f5837e5) will decrease coverage by 0.01%. The diff coverage is 100.00%.

Additional details and impacted files

[](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/313?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) ```diff @@ Coverage Diff @@ ## main #313 +/- ## ========================================== - Coverage 88.99% 88.99% -0.01% ========================================== Files 17 17 Lines 4936 4933 -3 ========================================== - Hits 4393 4390 -3 Misses 543 543 ``` | [Impacted Files](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/313?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) | Coverage Δ | | |---|---|---| | [source/s3\_auto\_ranged\_put.c](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/313?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3MzX2F1dG9fcmFuZ2VkX3B1dC5j) | `93.24% <100.00%> (+0.12%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/313/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs)