Fix mutable path refresh: default mutable_ttl, revalidate after expiry (#151)

A remote created through the API without `mutable_ttl` was stored as 0, so its mutable paths never refreshed. With `check_mutable`, the ETag expired together with the freshness key, so an expired mutable path was always refetched in full and never revalidated.

- default an omitted `mutable_ttl` to 3600 on create; keep the stored value on update; explicit 0 still means never expire
- keep mutable ETags for `max(10 × mutable_ttl, 7d)`, re-armed on each 304, only when `check_mutable` is set and `mutable_ttl` is non-zero
- drop the stored ETag when a refetch returns none; never store one for immutable paths
- flush a remote's Redis keys on delete
- update docs/common/caching.md

Reviewed-on: #151
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

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()
 }