fix dtt

internal/imapx/keepalive.go

@@ -0,0 +1,47 @@
+package imapx
+
+import (
+	"context"
+	"time"
+
+	"github.com/emersion/go-imap/v2/imapclient"
+)
+
+// KeepaliveInterval is how often an otherwise-idle IMAP connection is pinged
+// with NOOP so the server does not drop it.
+//
+// The destination connection sits completely idle for the entire duration of
+// the source-side metadata scan (Pass 1 of CopyFolder), which on a large
+// mailbox runs for many minutes across all folders. With no traffic, the
+// server closes the idle connection; go-imap's reader then tears the client
+// down, and every subsequent APPEND fails with "use of closed network
+// connection" — aborting each folder and copying nothing. A periodic NOOP
+// keeps the connection warm. 60s is well under the idle timeout of any common
+// IMAP server.
+const KeepaliveInterval = 60 * time.Second
+
+// Keepalive pings c with a NOOP every interval until ctx is cancelled, keeping
+// an idle connection from being dropped by the server. It is meant to run in
+// its own goroutine.
+//
+// It is safe to run concurrently with other commands on c: go-imap serializes
+// command submission and supports multiple in-flight commands over a single
+// connection (RFC 9051 §5.5 pipelining). A NOOP issued while another command is
+// in flight simply queues behind it and completes when the server responds.
+//
+// Keepalive returns when ctx is done or when a NOOP fails — a failed NOOP means
+// the connection is already gone, so there is nothing left to keep alive.
+func Keepalive(ctx context.Context, c *imapclient.Client, interval time.Duration) {
+	t := time.NewTicker(interval)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			if err := c.Noop().Wait(); err != nil {
+				return
+			}
+		}
+	}
+}