fix: half-written asset zips can be uploaded if process is interrupted

[rix0rrr]

Here's a possible scenario that can lead to "Uploaded file must be a non-empty zip":

• Bundling starts
• A partial zip file is written
• The process is interrupted (^C or similar) while writing.
• On next run, the destination file already exists, is taken to be complete, and uploaded.

In this fix, we'll write the zip to a temporary file and atomically rename it into place once finished, thereby avoiding this possible scenario.

Attempts to fix #18459.

I don't have tests for this, I don't know how to write them properly. To my mind, if the code looks good and we pass integration tests, that should be sufficient. Add a global asset salting mechanism so that we know all asset hashes are always unique.

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[gitpod-io]

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[MrArnoldPalmer]

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[jogold]

• On next run, the destination file already exists, is taken to be complete, and uploaded.

How about checking the file size in the cache against EMPTY_ZIP_FILE_SIZE here? (and rezip even if it exists in this case)

aws-cdk/packages/cdk-assets/lib/private/handlers/files.ts

Lines 140 to 143 in e0c0b56

	if (await pathExists(packagedPath)) {
	this.host.emitMessage(EventType.CACHED, `From cache ${packagedPath}`);
	return { packagedPath, contentType };
	}

[rix0rrr]

How about checking the file size in the cache against EMPTY_ZIP_FILE_SIZE here? (and rezip even if it exists in this case)

That will only catch the case if it's actually empty, not if we succesfully wrote 10MB and yet still failed because were supposed to write 100MB.

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Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).