ipn/localapi: add support for multipart POST to file-put

This allows sending multiple files via Taildrop in one request.
Progress is tracked via ipn.Notify.

Updates tailscale/corp#18202

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ipn/ipnlocal/local.go

@@ -319,6 +319,9 @@ type LocalBackend struct {
 	// lastNotifiedTailFSShares keeps track of the last set of shares that we
 	// notified about.
 	lastNotifiedTailFSShares atomic.Pointer[views.SliceView[*tailfs.Share, tailfs.ShareView]]
+
+	// outgoingFiles keeps track of Taildrop outgoing files
+	outgoingFiles map[string]*ipn.OutgoingFile
 }
 
 type updateStatus struct {

ipn/ipnlocal/taildrop.go

@@ -0,0 +1,34 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ipnlocal
+
+import (
+	"slices"
+	"strings"
+
+	"tailscale.com/ipn"
+)
+
+func (b *LocalBackend) UpdateOutgoingFiles(updates map[string]ipn.OutgoingFile) {
+	b.mu.Lock()
+	if b.outgoingFiles == nil {
+		b.outgoingFiles = make(map[string]*ipn.OutgoingFile, len(updates))
+	}
+	for id, file := range updates {
+		b.outgoingFiles[id] = &file
+	}
+	outgoingFiles := make([]*ipn.OutgoingFile, 0, len(b.outgoingFiles))
+	for _, file := range b.outgoingFiles {
+		outgoingFiles = append(outgoingFiles, file)
+	}
+	b.mu.Unlock()
+	slices.SortFunc(outgoingFiles, func(a, b *ipn.OutgoingFile) int {
+		t := a.Started.Compare(b.Started)
+		if t != 0 {
+			return t
+		}
+		return strings.Compare(a.Name, b.Name)
+	})
+	b.send(ipn.Notify{OutgoingFiles: outgoingFiles})
+}