pkg/chunked/GetDiffer must differentiate between failures and errors that can be ignored

[giuseppe]

we currently do not differentiate between critical errors (hard failures) and ignorable errors, so that the caller (containers/image in our case) can fallback to using the traditional pull code path.

Especially with composefs and fs-verity, if any error happens in this phase, the entire pull operation must fail and containers/image must not attempt any fallback.

Suggestion: define a new error typefor the errors that can be ignored and treat anything else as an hard failure, so that the caller can differentiate between them.

[giuseppe]

@mtrmac what do you think?

[mtrmac]

For context, what are the “ignorable” error situations? I can see:

• c/image/internal/private.BadPartialRequestError ~ pkg/chunked/ErrBadRequest: missing registry support
• enable_partial_images disabled
• layer is not chunked / missing TOC annotations.

And those are currently always ignorable, but might be fatal in an enforcing configuration, I guess.

Are there any other significant classes of ignorable errors?

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Yes, having a special error type (returned by chunked.GetDiffer?), and handling it specially in c/image so that other errors don’t trigger a fallback to the full layer copy, LGTM.

[giuseppe]

yes, the only "ignorable" errors are the ones where the image doesn't support partial pulls and there is no conversion enabled. Missing registry support should be a hard error when convert_images is configured

[giuseppe]

thanks, opened a PR to implement it: #2118