[performance] reduce InboxForward->Create calls by partially implementing Exists() (#3647)

* alphabetical reordering

* keep a cache of activity IDs we have handled creates for

* reduce number of inbox forwarding create calls by partially implementing Exists()

* increase cache size, since all we're storing is string keys

1 files changed, 3 insertions, 7 deletions

diff --git a/internal/federation/federatingdb/exists.go b/internal/federation/federatingdb/exists.go
index abac9960e..ec996f72f 100644
--- a/internal/federation/federatingdb/exists.go
+++ b/internal/federation/federatingdb/exists.go
@@ -22,12 +22,8 @@ import (
 	"net/url"
 )
 
-// Exists returns true if the database has an entry for the specified
-// id. It may not be owned by this application instance.
-//
-// The library makes this call only after acquiring a lock first.
-//
-// Implementation note: this just straight up isn't implemented, and doesn't *really* need to be either.
+// Exists is an implementation of pub.Database{}.Exists(), optimized specifically for
+// the only usecase in which go-fed/activity/pub actually calls it. Do not use otherwise!
 func (f *federatingDB) Exists(ctx context.Context, id *url.URL) (exists bool, err error) {
-	return false, nil
+	return f.activityIDs.Has(id.String()), nil
 }