Bound local snapshot names for long registry refs

provider/cocoon/create.go

@@ -2,6 +2,7 @@ package cocoon
 
 import (
 	"context"
+	"crypto/sha256"
 	"fmt"
 	"path/filepath"
 	"strconv"
@@ -540,10 +541,19 @@ func isHTTPURL(ref string) bool {
 
 // localSnapshotName omits the default tag for backward compatibility.
 func localSnapshotName(repo, tag string) string {
+	name := repo
 	if tag == "" || tag == meta.DefaultSnapshotTag {
-		return repo
+		name = repo
+	} else {
+		name = repo + ":" + tag
 	}
-	return repo + ":" + tag
+	const maxSnapshotNameLength = 63
+	if len(name) <= maxSnapshotNameLength {
+		return name
+	}
+	sum := sha256.Sum256([]byte(name))
+	suffix := fmt.Sprintf("-%x", sum[:6])
+	return name[:maxSnapshotNameLength-len(suffix)] + suffix
 }
 
 func forkSnapshotName(sourceVMName string) string {

provider/cocoon/create_test.go

@@ -526,6 +526,25 @@ func TestLocalSnapshotName(t *testing.T) {
 	}
 }
 
+func TestLocalSnapshotNameBoundsLongRegistryRefs(t *testing.T) {
+	repo := "simular/ubuntu2404-base-snapshot"
+	tag := "ubuntu2404-20260716-29479663055-1"
+
+	got := localSnapshotName(repo, tag)
+	if len(got) > 63 {
+		t.Fatalf("local snapshot name length = %d, want <= 63: %q", len(got), got)
+	}
+	if got != localSnapshotName(repo, tag) {
+		t.Fatal("local snapshot name is not deterministic")
+	}
+	if got == localSnapshotName(repo, tag+"-different") {
+		t.Fatal("different registry refs produced the same local snapshot name")
+	}
+	if !strings.HasPrefix(got, "simular/ubuntu2404-base-snapshot:") {
+		t.Fatalf("local snapshot name lost its recognizable prefix: %q", got)
+	}
+}
+
 func TestAssertSnapshotBackend(t *testing.T) {
 	t.Parallel()