Send fetch progress at most once per second (closes #206)

progressWriter sent an update for every chunk io.Copy wrote, so fetch
logged a line per 32 KiB. It now sends at most one per second, waiting
a second before the first as gen and check do, and one more when the
last byte is written.

TestFetchProgress now checks that a download of several chunks sends no
more than one update per second plus the last.

Model: opus-5-5

internal/cli/fetch.go

@@ -679,21 +679,26 @@ func resolveManifestURL(inputURL string) (string, error) {
 	return parsed.JoinPath(defaultManifestName).String(), nil
 }
 
-// progressWriter wraps an io.Writer and reports progress to a channel.
+// progressWriter wraps an io.Writer and reports progress to a channel,
+// at most once per second and once more when the last byte is written.
 type progressWriter struct {
-	w         io.Writer
-	path      string
-	total     int64
-	written   int64
-	startTime time.Time
-	progress  chan<- DownloadProgress
+	w            io.Writer
+	path         string
+	total        int64
+	written      int64
+	startTime    time.Time
+	lastProgress time.Time
+	progress     chan<- DownloadProgress
 }
 
 func (pw *progressWriter) Write(p []byte) (int, error) {
 	n, err := pw.w.Write(p)
 
 	pw.written += int64(n)
-	if pw.progress != nil {
+
+	isLast := pw.written == pw.total
+	if pw.progress != nil &&
+		(isLast || time.Since(pw.lastProgress) >= time.Second) {
 		var (
 			bytesPerSec float64
 			eta         time.Duration
@@ -715,6 +720,8 @@ func (pw *progressWriter) Write(p []byte) (int, error) {
 			BytesPerSec: bytesPerSec,
 			ETA:         eta,
 		})
+
+		pw.lastProgress = time.Now()
 	}
 
 	return n, err
@@ -899,13 +906,16 @@ func saveResponse(
 	// Set up hash computation
 	h := sha256.New()
 
-	// Create progress-reporting writer that also computes hash
+	// Create progress-reporting writer that also computes hash. The first
+	// update waits a second, as gen's and check's do.
+	start := time.Now()
 	pw := &progressWriter{
-		w:         io.MultiWriter(out, h),
-		path:      localPath,
-		total:     totalBytes,
-		startTime: time.Now(),
-		progress:  progress,
+		w:            io.MultiWriter(out, h),
+		path:         localPath,
+		total:        totalBytes,
+		startTime:    start,
+		lastProgress: start,
+		progress:     progress,
 	}
 
 	// Copy content while hashing and reporting progress. One byte past