[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/transport/dereference.go

@@ -20,6 +20,7 @@ package transport
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -30,6 +31,12 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/uris"
 )
 
+// ErrGone is returned from Dereference when the remote resource returns 410 GONE.
+// This is useful in cases where we're processing a delete of a resource that's already
+// been removed from the remote server, so we know we don't need to keep trying to
+// dereference it.
+var ErrGone = errors.New("remote resource returned HTTP code 410 GONE")
+
 func (t *transport) Dereference(ctx context.Context, iri *url.URL) ([]byte, error) {
 	// if the request is to us, we can shortcut for certain URIs rather than going through
 	// the normal request flow, thereby saving time and energy
@@ -66,10 +73,12 @@ func (t *transport) Dereference(ctx context.Context, iri *url.URL) ([]byte, erro
 	}
 	defer rsp.Body.Close()
 
-	// Check for an expected status code
-	if rsp.StatusCode != http.StatusOK {
+	switch rsp.StatusCode {
+	case http.StatusOK:
+		return io.ReadAll(rsp.Body)
+	case http.StatusGone:
+		return nil, ErrGone
+	default:
 		return nil, fmt.Errorf("GET request to %s failed (%d): %s", iriStr, rsp.StatusCode, rsp.Status)
 	}
-
-	return io.ReadAll(rsp.Body)
 }