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>

util/progresstracking/progresstracking.go

@@ -0,0 +1,39 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package progresstracking provides wrappers around io.Reader and io.Writer
+// that track progress.
+package progresstracking
+
+import (
+	"io"
+	"time"
+)
+
+// NewReader wraps the given Reader with a progress tracking Reader that
+// reports progress at the following points:
+//
+// - First read
+// - Every read spaced at least interval since the prior read
+// - Last read
+func NewReader(r io.Reader, interval time.Duration, onProgress func(totalRead int, err error)) io.Reader {
+	return &reader{Reader: r, interval: interval, onProgress: onProgress}
+}
+
+type reader struct {
+	io.Reader
+	interval    time.Duration
+	onProgress  func(int, error)
+	lastTracked time.Time
+	totalRead   int
+}
+
+func (r *reader) Read(p []byte) (int, error) {
+	n, err := r.Reader.Read(p)
+	r.totalRead += n
+	if time.Since(r.lastTracked) > r.interval || err != nil {
+		r.onProgress(r.totalRead, err)
+		r.lastTracked = time.Now()
+	}
+	return n, err
+}