Default omitted mutable_ttl and keep the ETag past TTL expiry

- default mutable_ttl to 3600 on create and keep the stored value on update
- store mutable ETags without a TTL so expiry revalidates with If-None-Match
- drop the stored ETag when a refetch returns none

internal/cache/cache_test.go

@@ -86,6 +86,12 @@ func TestETagAndToken(t *testing.T) {
 	if v, _ := testRedis.GetETag(ctx, "r", "p"); v != `"abc"` {
 		t.Errorf("etag = %q", v)
 	}
+	if err := testRedis.SetETag(ctx, "r", "p", "", 0); err != nil {
+		t.Fatal(err)
+	}
+	if v, _ := testRedis.GetETag(ctx, "r", "p"); v != "" {
+		t.Errorf("empty etag should drop the stored one, got %q", v)
+	}
 
 	if v, _ := testRedis.GetToken(ctx, "missing"); v != "" {
 		t.Error("missing token should be empty")

internal/cache/redis.go

@@ -57,8 +57,12 @@ func (r *Redis) ReleaseLock(ctx context.Context, remote, path string) error {
 	return r.client.Del(ctx, key).Err()
 }
 
+// SetETag stores etag for path, or drops the stored one when etag is empty.
 func (r *Redis) SetETag(ctx context.Context, remote, path, etag string, ttl time.Duration) error {
 	key := fmt.Sprintf("etag:%s:%s", remote, path)
+	if etag == "" {
+		return r.client.Del(ctx, key).Err()
+	}
 	return r.client.Set(ctx, key, etag, ttl).Err()
 }