src/db.zig

@@ -1890,6 +1890,20 @@ pub const Engine = struct {
         if (self.compacting.swap(true, .acq_rel)) return;
         defer self.compacting.store(false, .release);
 
+        // Reclamation first, because it is the cheap half of the same job: a
+        // checkpoint hands back whole windows for the cost of one publish,
+        // where a rebuild copies every live byte in the database. Whatever it
+        // takes, the per-collection gate below no longer sees, so a collection
+        // whose garbage was all in empty windows is not rewritten at all.
+        //
+        // This is not a refinement, it is what makes reclamation reachable
+        // under a delete-heavy workload. A checkpoint is otherwise armed by log
+        // volume, and a delete logs only an `_id` -- so deleting half a 190 MB
+        // collection moves the log by a couple of megabytes and no checkpoint
+        // runs, while the garbage sails past the rebuild threshold. Measured
+        // with the churn harness: six rounds, six rebuilds, 1 MB reclaimed.
+        try self.checkpoint();
+
         try self.catalog_lock.lockShared(self.io);
         var rebuild_err: ?anyerror = null;
         var db_it = self.dbs.iterator();