Integrate validated catalog loading (T-201)

core/catalog/loader.go

@@ -26,6 +26,19 @@ type Cache interface {
 	Store([]byte) error
 }
 
+// DocumentValidator rejects signed documents that this client cannot consume.
+// Validators run before a remote document can replace the last valid cache.
+type DocumentValidator interface {
+	Validate(VerifiedDocument) error
+}
+
+// DocumentValidatorFunc adapts a function to DocumentValidator.
+type DocumentValidatorFunc func(VerifiedDocument) error
+
+func (function DocumentValidatorFunc) Validate(document VerifiedDocument) error {
+	return function(document)
+}
+
 // LoadSource describes where a verified result came from.
 type LoadSource string
 
@@ -57,16 +70,23 @@ func (err *LoadError) Unwrap() error {
 
 // Loader verifies remote data before storing it and re-verifies cache fallback.
 type Loader struct {
-	verifier Verifier
-	fetcher  Fetcher
-	cache    Cache
+	verifier   Verifier
+	fetcher    Fetcher
+	cache      Cache
+	validators []DocumentValidator
 }
 
-func NewLoader(verifier Verifier, fetcher Fetcher, cache Cache) *Loader {
+func NewLoader(
+	verifier Verifier,
+	fetcher Fetcher,
+	cache Cache,
+	validators ...DocumentValidator,
+) *Loader {
 	return &Loader{
-		verifier: verifier,
-		fetcher:  fetcher,
-		cache:    cache,
+		verifier:   verifier,
+		fetcher:    fetcher,
+		cache:      cache,
+		validators: append([]DocumentValidator(nil), validators...),
 	}
 }
 
@@ -76,12 +96,15 @@ func (loader *Loader) Load(ctx context.Context) (LoadResult, error) {
 	if refreshErr == nil {
 		verified, verifyErr := loader.verifier.Verify(remoteBytes)
 		if verifyErr == nil {
-			storeErr := loader.cache.Store(verified.Bytes)
-			return LoadResult{
-				Document: verified,
-				Source:   SourceRemote,
-				Warning:  storeErr,
-			}, nil
+			verifyErr = loader.validate(verified)
+			if verifyErr == nil {
+				storeErr := loader.cache.Store(verified.Bytes)
+				return LoadResult{
+					Document: verified,
+					Source:   SourceRemote,
+					Warning:  storeErr,
+				}, nil
+			}
 		}
 		refreshErr = verifyErr
 	}
@@ -91,11 +114,14 @@ func (loader *Loader) Load(ctx context.Context) (LoadResult, error) {
 		var verified VerifiedDocument
 		verified, cacheErr = loader.verifier.Verify(cachedBytes)
 		if cacheErr == nil {
-			return LoadResult{
-				Document: verified,
-				Source:   SourceCache,
-				Warning:  refreshErr,
-			}, nil
+			cacheErr = loader.validate(verified)
+			if cacheErr == nil {
+				return LoadResult{
+					Document: verified,
+					Source:   SourceCache,
+					Warning:  refreshErr,
+				}, nil
+			}
 		}
 	}
 
@@ -104,3 +130,15 @@ func (loader *Loader) Load(ctx context.Context) (LoadResult, error) {
 		Cache:   cacheErr,
 	}
 }
+
+func (loader *Loader) validate(document VerifiedDocument) error {
+	for _, validator := range loader.validators {
+		if validator == nil {
+			continue
+		}
+		if err := validator.Validate(document); err != nil {
+			return err
+		}
+	}
+	return nil
+}