[feature] Read + Write tombstones for deleted Actors (#1005)

* [feature] Read + Write tombstones for deleted Actors

* copyTombstone

* update to use resultcache instead of old ttl cache

Signed-off-by: kim <grufwub@gmail.com>

* update go-cache library to fix result cache capacity / ordering bugs

Signed-off-by: kim <grufwub@gmail.com>

* bump go-cache/v3 to v3.1.6 to fix bugs

Signed-off-by: kim <grufwub@gmail.com>

* switch on status code

* better explain ErrGone reasoning

Signed-off-by: kim <grufwub@gmail.com>
Co-authored-by: kim <grufwub@gmail.com>

internal/federation/gone.go

@@ -0,0 +1,34 @@
+package federation
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/id"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+)
+
+// CheckGone checks if a tombstone exists in the database for AP Actor or Object with the given uri.
+func (f *federator) CheckGone(ctx context.Context, uri *url.URL) (bool, error) {
+	return f.db.TombstoneExistsWithURI(ctx, uri.String())
+}
+
+// HandleGone puts a tombstone in the database, which marks an AP Actor or Object with the given uri as gone.
+func (f *federator) HandleGone(ctx context.Context, uri *url.URL) error {
+	tombstoneID, err := id.NewULID()
+	if err != nil {
+		err = fmt.Errorf("HandleGone: error generating id for new tombstone %s: %s", uri, err)
+		log.Error(err)
+		return err
+	}
+
+	tombstone := &gtsmodel.Tombstone{
+		ID:     tombstoneID,
+		Domain: uri.Host,
+		URI:    uri.String(),
+	}
+
+	return f.db.PutTombstone(ctx, tombstone)
+}