materialize-boilerplate: fix staged files cleanup again

In f4a82c4 a partial fix was implemented to prevent attempts to cleanup the same
files again when a binding had data for one but not a subsequent transaction.

That change fixed that issue, but it also made it so that staged files just
never got cleaned up, because `f.started` is set to `false` by `f.flush()`,
which is always called before the cleanup.

A complete fix is here, which clears out the list of uploaded files after
sending them off for deletion.

materialize-boilerplate/staged_files.go

@@ -46,7 +46,6 @@ type StagedFiles[T any] struct {
 	flushOnNextBinding bool
 	stagedFiles        []stagedFile[T]
 	lastBinding        int
-	didCleanupCurrent  bool
 }
 
 // NewStagedFiles creates a StagedFiles instance, which is used for staging data
@@ -94,7 +93,6 @@ func (sf *StagedFiles[T]) EncodeRow(ctx context.Context, binding int, row []any)
 		}
 	}
 	sf.lastBinding = binding
-	sf.didCleanupCurrent = false
 
 	return sf.stagedFiles[binding].encodeRow(ctx, row)
 }
@@ -110,24 +108,24 @@ func (sf *StagedFiles[T]) Flush(binding int) ([]string, error) {
 // to use both as part of a deferred call, and an inline call to check for
 // deletion errors.
 func (sf *StagedFiles[T]) CleanupCurrentTransaction(ctx context.Context) error {
-	if !sf.didCleanupCurrent {
-		sf.didCleanupCurrent = true
-
-		var uris []string
-		for _, f := range sf.stagedFiles {
-			if !f.started {
-				continue
-			}
-
-			for _, u := range f.uploaded {
-				uris = append(uris, sf.client.URI(u))
-			}
+	var uris []string
+	for i := range sf.stagedFiles {
+		f := &sf.stagedFiles[i]
+		if len(f.uploaded) == 0 {
+			continue
 		}
 
-		return sf.client.Delete(ctx, uris)
+		for _, u := range f.uploaded {
+			uris = append(uris, sf.client.URI(u))
+		}
+		f.uploaded = nil
 	}
 
-	return nil
+	if len(uris) == 0 {
+		return nil
+	}
+
+	return sf.client.Delete(ctx, uris)
 }
 
 // CleanupCheckpoint deletes files specified int he list of URIs. This can be