Add no-overwrite option to `s3 cp` and `s3 mv` commands

[vz10]

Issue #, if available: #2874

Description of changes:

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[codecov-io]

Codecov Report

Merging #6095 (f82c4a3) into v2 (4172ada) will increase coverage by 0.00%. The diff coverage is 97.36%.

:exclamation: Current head f82c4a3 differs from pull request most recent head 2bda3e5. Consider uploading reports for the commit 2bda3e5 to get more accurate results

@@           Coverage Diff           @@
##               v2    #6095   +/-   ##
=======================================
  Coverage   94.14%   94.15%           
=======================================
  Files         271      271           
  Lines       21550    21608   +58     
=======================================
+ Hits        20288    20344   +56     
- Misses       1262     1264    +2     

Impacted Files	Coverage Δ
awscli/customizations/s3/filters.py	98.43% <97.22%> (-1.57%)	:arrow_down:
awscli/customizations/s3/subcommands.py	97.24% <100.00%> (+0.01%)	:arrow_up:

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[four43]

This seems like it would be prone to race condition issues since it's just a HEAD then a write.

[jfstephe]

@vz10 - hi! Any update on this PR?

[jjchoi08]

Any update to this PR? I in need for no-overwrite option.. thanks!

[orf]

you can't implement a no-overwrite option in S3. it's not possible to do without race conditions.

[jfstephe]

This doesn't fix the problem but may help some people out there in specific scenarios! https://github.com/jfstephe/aws-s3-lock

[grosser]

you can't implement a no-overwrite option in S3. it's not possible to do without race conditions.

"get item, do not upload if it exists" is what is requested here, and that is trivial to implement, nobody said it needs to be atomic

[RyanFitzSimmonsAK]

Hey everyone, thanks for this pull request and the discussion around it. We've made the decision to close this pull request, and leave the feature request open for tracking purposes. There is certainly value in the feature being requested here, but this isn't something we can really accept a pull request for, as there are implementation details that need to be handled internally. Please continue to 👍 the tracking issue (https://github.com/aws/aws-cli/issues/2874), and describe your use cases for such a feature in the replies.