util/progresstracking: add Ticker, NewWriter, and CountingWriter

Add three new helpers to the existing progresstracking package:

  - Ticker: spawns a 1 Hz goroutine that calls a report function with
    the current value of an atomic counter and a total. Returns a stop
    function (safe to call multiple times via sync.OnceFunc) that fires
    one final report and blocks until the goroutine exits.

  - NewWriter: wraps an io.Writer and calls onProgress at most once per
    interval with the cumulative byte count.

  - CountingWriter: an io.Writer that atomically counts bytes written,
    for use with Ticker.

These will be used by the appliance flash and OTA update code in
subsequent commits.

Updates #1866

Change-Id: If353cea6506f5351b6fb19bfdb7bc9b78fe7855e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

clientupdate/distsign/distsign.go

@@ -60,6 +60,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/util/httpm"
 	"tailscale.com/util/must"
+	"tailscale.com/util/progresstracking"
 )
 
 const (
@@ -373,7 +374,10 @@ func (c *Client) download(ctx context.Context, url, dst string, limit int64) ([]
 		return nil, 0, err
 	}
 	defer of.Close()
-	pw := &progressWriter{total: res.ContentLength, logf: c.logf}
+	total := res.ContentLength
+	pw := progresstracking.NewWriter(io.Discard, total, 2*time.Second, func(done int64) {
+		c.logf("Downloaded %v/%v (%.1f%%)", done, total, float64(done)/float64(total)*100)
+	})
 	h := NewPackageHash()
 	n, err := io.Copy(io.MultiWriter(of, h, pw), io.LimitReader(dlRes.Body, limit))
 	if err != nil {
@@ -388,31 +392,10 @@ func (c *Client) download(ctx context.Context, url, dst string, limit int64) ([]
 	if err := of.Close(); err != nil {
 		return nil, n, err
 	}
-	pw.print()
 
 	return h.Sum(nil), h.Len(), nil
 }
 
-type progressWriter struct {
-	done      int64
-	total     int64
-	lastPrint time.Time
-	logf      logger.Logf
-}
-
-func (pw *progressWriter) Write(p []byte) (n int, err error) {
-	pw.done += int64(len(p))
-	if time.Since(pw.lastPrint) > 2*time.Second {
-		pw.print()
-	}
-	return len(p), nil
-}
-
-func (pw *progressWriter) print() {
-	pw.lastPrint = time.Now()
-	pw.logf("Downloaded %v/%v (%.1f%%)", pw.done, pw.total, float64(pw.done)/float64(pw.total)*100)
-}
-
 func parsePrivateKey(data []byte, typeTag string) (ed25519.PrivateKey, error) {
 	b, rest := pem.Decode(data)
 	if b == nil {