feat: stream the metadata scan instead of Collect

CopyFolder now streams envelope metadata via Next() in a first pass (dedup +
queue new messages), then streams bodies for new ones in a second pass —
no more blocking Collect of the whole folder with zero feedback, and memory
stays flat (only new-message meta is held).

- imapx: two-pass streaming CopyFolder + CopyDeps.OnScan(scanned,total)
- orchestrator: throttled 'scan' events during the metadata pass
- web: per-account 'scanning folder: X/N' line under the progress bar;
  scan events kept out of the log to avoid flooding

Verified on greenmail: idempotency and internal-date preservation still hold.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/imapx/copy.go

@@ -23,6 +23,10 @@ type CopyDeps struct {
 	// (potentially long) envelope fetch, with the message count in the source
 	// folder — for progress visibility.
 	OnFolder func(srcFolder, dstFolder string, total int64)
+	// OnScan is called during the streaming metadata pass with how many of the
+	// folder's messages have been examined so far — so the UI shows movement
+	// while dedup decisions are made, before bodies start copying.
+	OnScan func(scanned, total int64)
 }
 
 // CopyResult summarizes the outcome of one CopyFolder run.
@@ -46,48 +50,77 @@ func CopyFolder(ctx context.Context, src, dst *imapclient.Client, srcFolder, dst
 	if err != nil {
 		return res, fmt.Errorf("examine src %q: %w", srcFolder, err)
 	}
+	total := int64(sel.NumMessages)
 	if deps.OnFolder != nil {
-		deps.OnFolder(srcFolder, dstFolder, int64(sel.NumMessages))
+		deps.OnFolder(srcFolder, dstFolder, total)
 	}
-	if sel.NumMessages == 0 {
+	if total == 0 {
 		return res, nil
 	}
 
-	// 1) Collect envelope+uid+size for every message (cheap pass, no bodies).
-	metaSet := imap.SeqSet{imap.SeqRange{Start: 1, Stop: sel.NumMessages}}
-	metas, err := src.Fetch(metaSet, &imap.FetchOptions{
-		UID: true, Envelope: true, RFC822Size: true, Flags: true, InternalDate: true,
-	}).Collect()
-	if err != nil {
-		return res, fmt.Errorf("fetch meta: %w", err)
-	}
-
 	// dst folder must exist (idempotent create; ignore "already exists").
 	_ = dst.Create(dstFolder, nil).Wait()
 
-	for _, m := range metas {
+	// Pass 1: STREAM metadata (no bodies) via Next(), dedup as we go, and queue
+	// only the new messages. Streaming (not Collect) means progress shows during
+	// the scan and memory stays flat — we hold small meta for new messages only.
+	type queued struct {
+		uid          imap.UID
+		key          string
+		flags        []imap.Flag
+		internalDate time.Time
+	}
+	var todo []queued
+	metaSet := imap.SeqSet{imap.SeqRange{Start: 1, Stop: sel.NumMessages}}
+	fc := src.Fetch(metaSet, &imap.FetchOptions{
+		UID: true, Envelope: true, RFC822Size: true, Flags: true, InternalDate: true,
+	})
+	var scanned int64
+	for {
 		if err := ctx.Err(); err != nil {
+			_ = fc.Close()
 			return res, err
 		}
-
+		msg := fc.Next()
-		key := MessageKey(m.Envelope, m.RFC822Size)
+		if msg == nil {
-		already, err := deps.IsMigrated(key)
+			break
+		}
+		buf, err := msg.Collect()
 		if err != nil {
 			res.Errors++
 			continue
 		}
-		if already {
+		scanned++
+		key := MessageKey(buf.Envelope, buf.RFC822Size)
+		already, err := deps.IsMigrated(key)
+		if err != nil {
+			res.Errors++
+		} else if already {
 			res.Skipped++
 			if deps.OnProgress != nil {
 				deps.OnProgress(res.Copied, res.Skipped)
 			}
-			continue
+		} else {
+			todo = append(todo, queued{uid: buf.UID, key: key, flags: buf.Flags, internalDate: buf.InternalDate})
 		}
-		if err := streamOne(src, dst, dstFolder, m.UID, m.Flags, m.InternalDate); err != nil {
+		if deps.OnScan != nil {
+			deps.OnScan(scanned, total)
+		}
+	}
+	if err := fc.Close(); err != nil {
+		return res, fmt.Errorf("fetch meta %q: %w", srcFolder, err)
+	}
+
+	// Pass 2: fetch bodies for the queued (new) messages, one at a time.
+	for _, q := range todo {
+		if err := ctx.Err(); err != nil {
+			return res, err
+		}
+		if err := streamOne(src, dst, dstFolder, q.uid, q.flags, q.internalDate); err != nil {
 			res.Errors++
 			continue
 		}
-		if err := deps.MarkMigrated(dstFolder, key); err != nil {
+		if err := deps.MarkMigrated(dstFolder, q.key); err != nil {
 			res.Errors++
 			continue
 		}

internal/orchestrator/orchestrator.go

@@ -301,7 +301,7 @@ func (o *Orchestrator) runAccount(ctx context.Context, task store.Task, runID in
 	var baseCopied, baseSkipped int64
 	var curFolder string
 	var curTotal int64
-	var lastEmit time.Time
+	var lastEmit, lastScanEmit time.Time
 	deps := imapx.CopyDeps{
 		IsMigrated:   func(k string) (bool, error) { return o.store.IsMigrated(ctx, a.ID, k) },
 		MarkMigrated: func(folder, k string) error { return o.store.MarkMigrated(ctx, a.ID, folder, k) },
@@ -331,6 +331,18 @@ func (o *Orchestrator) runAccount(ctx context.Context, task store.Task, runID in
 				"folder": srcFolder, "dst_folder": dstFolder, "messages": total,
 			}})
 		},
+		// Fires while streaming metadata (dedup scan) so the UI shows movement
+		// before bodies start copying. Throttled to ~4/sec, always emit the last.
+		OnScan: func(scanned, total int64) {
+			now := time.Now()
+			if now.Sub(lastScanEmit) < 250*time.Millisecond && scanned < total {
+				return
+			}
+			lastScanEmit = now
+			o.hub.Publish(wshub.Event{Type: "scan", TaskID: task.ID, Data: map[string]any{
+				"account_id": a.ID, "folder": curFolder, "scanned": scanned, "folder_total": total,
+			}})
+		},
 	}
 	for _, fp := range plan {
 		if actx.Err() != nil {

web/src/app.css

@@ -282,6 +282,10 @@
   letter-spacing: 0.04em;
 }
 
+.pscan {
+  color: var(--info);
+}
+
 /* clear-log button: mirrors the .panel-label tab on the right edge */
 .log-clear {
   position: absolute;

web/src/pages/TaskDetail.tsx

@@ -15,6 +15,9 @@ type LiveProgress = {
   startTs: number
   startCount: number
   speed: number // messages/sec, averaged since the account's run started
+  scanFolder?: string
+  scanned?: number
+  scanTotal?: number
 }
 
 function fmtDuration(sec: number): string {
@@ -89,11 +92,20 @@ export function TaskDetail({ id }: { id: number }) {
   useEffect(
     () =>
       connectTaskWS(id, (ev: TaskEvent) => {
+        // `scan` is high-frequency and shown in the progress cell, not the log.
+        if (ev.type !== 'scan') {
           setLog((l) => [{ type: ev.type, text: describeEvent(ev) }, ...l].slice(0, 300))
+        }
         const d = (ev.data ?? {}) as Record<string, number | string | undefined>
         const accId = typeof d.account_id === 'number' ? d.account_id : undefined
 
-        if (ev.type === 'plan' && accId != null) {
+        if (ev.type === 'scan' && accId != null) {
+          setLive((prev) => {
+            const cur = prev[accId]
+            const base: LiveProgress = cur ?? { copied: 0, skipped: 0, total: 0, startTs: Date.now(), startCount: 0, speed: 0 }
+            return { ...prev, [accId]: { ...base, scanFolder: d.folder as string | undefined, scanned: Number(d.scanned ?? 0), scanTotal: Number(d.folder_total ?? 0) } }
+          })
+        } else if (ev.type === 'plan' && accId != null) {
           const total = Number(d.total ?? 0)
           const now = Date.now()
           setLive((prev) => ({
@@ -453,6 +465,7 @@ export function TaskDetail({ id }: { id: number }) {
                         const done = lv.copied + lv.skipped
                         const pct = Math.min(100, Math.floor((done / lv.total) * 100))
                         const eta = lv.speed > 0 ? (lv.total - done) / lv.speed : Infinity
+                        const scanning = lv.scanned != null && lv.scanTotal != null && lv.scanned < lv.scanTotal
                         return (
                           <div className="acct-progress">
                             <div className="pbar">
@@ -462,6 +475,11 @@ export function TaskDetail({ id }: { id: number }) {
                               {done}/{lv.total} ({pct}%) · {lv.speed >= 1 ? Math.round(lv.speed) : lv.speed.toFixed(1)}/s · ETA {fmtDuration(eta)}
                               {lv.folder ? ` · ${lv.folder}` : ''}
                             </span>
+                            {scanning && (
+                              <span className="pmeta pscan mono-num">
+                                scanning {lv.scanFolder}: {lv.scanned}/{lv.scanTotal}
+                              </span>
+                            )}
                           </div>
                         )
                       })()}