[sha512] image/copy: allow users to force digest algorithm in CLI (e.g. skopeo copy --force-digest)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,146 @@ | ||
| // Package digests provides an internal representation of users’ digest use preferences. | ||
| // | ||
| // Something like this _might_ be eventually made available as a public API: | ||
| // before doing so, carefully think whether the API should be modified before we commit to it. | ||
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| package digests | ||
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| import ( | ||
| "errors" | ||
| "fmt" | ||
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| "github.com/opencontainers/go-digest" | ||
| ) | ||
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| // Options records users’ preferences for used digest algorithm usage. | ||
| // It is a value type and can be copied using ordinary assignment. | ||
| // | ||
| // It can only be created using one of the provided constructors. | ||
| type Options struct { | ||
| initialized bool // To prevent uses that don’t call a public constructor; this is necessary to enforce the .Available() promise. | ||
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| // If any of the fields below is set, it is guaranteed to be .Available(). | ||
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| mustUse digest.Algorithm // If not "", written digests must use this algorithm. | ||
| prefer digest.Algorithm // If not "", use this algorithm whenever possible. | ||
| defaultAlgo digest.Algorithm // If not "", use this algorithm if there is no reason to use anything else. | ||
| } | ||
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| // CanonicalDefault is Options which default to using digest.Canonical if there is no reason to use a different algorithm | ||
| // (e.g. when there is no pre-existing digest). | ||
| // | ||
| // The configuration can be customized using .WithPreferred() or .WithDefault(). | ||
| func CanonicalDefault() Options { | ||
| // This does not set .defaultAlgo so that .WithDefault() can be called (once). | ||
| return Options{ | ||
| initialized: true, | ||
| } | ||
| } | ||
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| // MustUse constructs Options which always use algo. | ||
| func MustUse(algo digest.Algorithm) (Options, error) { | ||
| // We don’t provide Options.WithMustUse because there is no other option that makes a difference | ||
| // once .mustUse is set. | ||
| if !algo.Available() { | ||
| return Options{}, fmt.Errorf("attempt to use an unavailable digest algorithm %q", algo.String()) | ||
| } | ||
| return Options{ | ||
| initialized: true, | ||
| mustUse: algo, | ||
| }, nil | ||
| } | ||
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| // WithPreferred returns a copy of o with a “preferred” algorithm set to algo. | ||
| // The preferred algorithm is used whenever possible (but if there is a strict requirement to use something else, it will be overridden). | ||
| func (o Options) WithPreferred(algo digest.Algorithm) (Options, error) { | ||
| if err := o.ensureInitialized(); err != nil { | ||
| return Options{}, err | ||
| } | ||
| if o.prefer != "" { | ||
| return Options{}, errors.New("digests.Options already have a 'prefer' algorithm configured") | ||
| } | ||
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| if !algo.Available() { | ||
| return Options{}, fmt.Errorf("attempt to use an unavailable digest algorithm %q", algo.String()) | ||
| } | ||
| o.prefer = algo | ||
| return o, nil | ||
| } | ||
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| // WithDefault returns a copy of o with a “default” algorithm set to algo. | ||
| // The default algorithm is used if there is no reason to use anything else (e.g. when there is no pre-existing digest). | ||
| func (o Options) WithDefault(algo digest.Algorithm) (Options, error) { | ||
| if err := o.ensureInitialized(); err != nil { | ||
| return Options{}, err | ||
| } | ||
| if o.defaultAlgo != "" { | ||
| return Options{}, errors.New("digests.Options already have a 'default' algorithm configured") | ||
| } | ||
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| if !algo.Available() { | ||
| return Options{}, fmt.Errorf("attempt to use an unavailable digest algorithm %q", algo.String()) | ||
| } | ||
| o.defaultAlgo = algo | ||
| return o, nil | ||
| } | ||
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| // ensureInitialized returns an error if o is not initialized. | ||
| func (o Options) ensureInitialized() error { | ||
| if !o.initialized { | ||
| return errors.New("internal error: use of uninitialized digests.Options") | ||
| } | ||
| return nil | ||
| } | ||
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| // Situation records the context in which a digest is being chosen. | ||
| type Situation struct { | ||
| Preexisting digest.Digest // If not "", a pre-existing digest value (frequently one which is cheaper to use than others) | ||
| CannotChangeAlgorithmReason string // The reason why we must use Preexisting, or "" if we can use other algorithms. | ||
| } | ||
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| // Choose chooses a digest algorithm based on the options and the situation. | ||
| func (o Options) Choose(s Situation) (digest.Algorithm, error) { | ||
| if err := o.ensureInitialized(); err != nil { | ||
| return "", err | ||
| } | ||
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| if s.CannotChangeAlgorithmReason != "" && s.Preexisting == "" { | ||
| return "", fmt.Errorf("internal error: digests.Situation.CannotChangeAlgorithmReason is set but Preexisting is empty") | ||
| } | ||
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| var choice digest.Algorithm // = what we want to use | ||
| switch { | ||
| case o.mustUse != "": | ||
| choice = o.mustUse | ||
| case s.CannotChangeAlgorithmReason != "": | ||
| choice = s.Preexisting.Algorithm() | ||
| if !choice.Available() { | ||
| return "", fmt.Errorf("existing digest uses unimplemented algorithm %s", choice) | ||
| } | ||
| case o.prefer != "": | ||
| choice = o.prefer | ||
| case s.Preexisting != "" && s.Preexisting.Algorithm().Available(): | ||
| choice = s.Preexisting.Algorithm() | ||
| case o.defaultAlgo != "": | ||
| choice = o.defaultAlgo | ||
| default: | ||
| choice = digest.Canonical // We assume digest.Canonical is always available. | ||
| } | ||
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| if s.CannotChangeAlgorithmReason != "" && choice != s.Preexisting.Algorithm() { | ||
| return "", fmt.Errorf("requested to always use digest algorithm %s but we cannot replace existing digest algorithm %s: %s", | ||
| choice, s.Preexisting.Algorithm(), s.CannotChangeAlgorithmReason) | ||
| } | ||
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| return choice, nil | ||
| } | ||
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| // MustUseSet returns an algorithm if o is set to always use a specific algorithm, "" if it is flexible. | ||
| func (o Options) MustUseSet() digest.Algorithm { | ||
| // We don’t do .ensureInitialized() because that would require an extra error value just for that. | ||
| // This should not be a part of any public API either way. | ||
| if o.mustUse != "" { | ||
| return o.mustUse | ||
| } | ||
| return "" | ||
| } |
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This is redundant / conflicting with
PutBlob…making its own choice. If we had the generic code choosing a specific algorithm and expecting precisely that one to be used, then it should do that beforePutBloband provide that to transports as a parameter.I’m not sure where the decision should be, at this point [and it’s a private interface, so we can change our mind] — I guess the transport might have some information (e.g. about remote registry capabilities) that the generic code doesn’t — but, for now, we don’t have all transports updated yet, either way…
AFAICS this only exists as a sanity check (both config and layer copies later enforce equality if
cannotModifyManifest…), so I think an “if algorithms match the whole values must match” check here would be sufficient.