Write safely to the shared plugin cache

[finnag]

Change terraform to write safely to the shared plugin cache. Also, if a write fails, but the existing contents are correct, we now consider that a success.

Terraform currently fails terraform init if an executable it is trying to write is running
("text file busy"), or if it cannot write to the file for some other reason. There is also a race condition while it is writing the files that other readers can get partial content, or that partial content is left in the cache.

Fixes #25849, #31964, #32901, #32915

Target Release

1.4.x

Draft CHANGELOG entry

ENHANCEMENTS

• The plugin cache now supports multiple simultaneous users

[crw]

Thanks for this submission! I'll raise it in triage.

[crw]

Hi @finnag, thanks for this submission. At triage it was decided that this fix would be better applied directly to go-getter rather than have the code ported here. Although in general it has been more difficult to build and include new releases of go-getter, if you would like to pursue this in that repo I will do my best to move it forward. Thanks for your contribution!

[jgournet]

Thank you for this fix !
Just mentioning: it does help a lot with the original issue, but we still get "text file is busy" errors, as seen in this ticket:
hashicorp/terraform-cdk#2741

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