Add helpers for content-addressed dependency lock digests

Provide shared utilities to format, compare, record, and detect changes
in per-dependency tarball digests during dependency update and build.

Signed-off-by: Simone Tollardo <simone.tollardo@redhat.com>

pkg/downloader/digest.go

@@ -0,0 +1,95 @@
+/*
+Copyright The Helm Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package downloader
+
+import (
+	"fmt"
+	"strings"
+
+	chart "helm.sh/helm/v4/pkg/chart/v2"
+	"helm.sh/helm/v4/pkg/provenance"
+)
+
+// formatDigest returns a digest string in the canonical sha256: form.
+func formatDigest(hex string) string {
+	if hex == "" {
+		return ""
+	}
+	if strings.HasPrefix(hex, "sha256:") {
+		return hex
+	}
+	return "sha256:" + hex
+}
+
+// digestEqual compares two digests regardless of sha256: prefix.
+func digestEqual(a, b string) bool {
+	if a == "" || b == "" {
+		return a == b
+	}
+	return stripDigestAlgorithm(a) == stripDigestAlgorithm(b)
+}
+
+// digestFile computes the SHA256 digest of a chart archive file.
+func digestFile(path string) (string, error) {
+	hex, err := provenance.DigestFile(path)
+	if err != nil {
+		return "", err
+	}
+	return formatDigest(hex), nil
+}
+
+// recordDependencyDigest computes and stores the tarball digest on a lock dependency entry.
+func recordDependencyDigest(dep *chart.Dependency, tarballPath string) error {
+	digest, err := digestFile(tarballPath)
+	if err != nil {
+		return err
+	}
+	if dep.Digest != "" && !digestEqual(dep.Digest, digest) {
+		return fmt.Errorf("chart.lock digest mismatch for %s-%s: expected %s, got %s", dep.Name, dep.Version, formatDigest(dep.Digest), digest)
+	}
+	dep.Digest = digest
+	return nil
+}
+
+// lockNeedsWrite reports whether the new lock differs from the old lock in metadata
+// or per-dependency content digests.
+func lockNeedsWrite(old, updated *chart.Lock) bool {
+	if old == nil {
+		return true
+	}
+	if old.Digest != updated.Digest {
+		return true
+	}
+	return !dependencyDigestsEqual(old.Dependencies, updated.Dependencies)
+}
+
+func dependencyDigestsEqual(a, b []*chart.Dependency) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] == nil || b[i] == nil {
+			if a[i] != b[i] {
+				return false
+			}
+			continue
+		}
+		if a[i].Name != b[i].Name || !digestEqual(a[i].Digest, b[i].Digest) {
+			return false
+		}
+	}
+	return true
+}