Keep the progress line of a multi-path snapshot within 100% (closes #271)

One progress reporter spans every path of a snapshot. Its counts of
files and bytes processed run across all the paths, while the totals
they were divided by were reset to each path's own, so a second path
smaller than the first showed more than 100% and an ETA of `unknown`.
The totals now add up over the paths scanned so far. The rate behind
the ETA is measured from the bytes processed since the current path's
scan phase ended, so a later path's scan phase does not lower it.
SetTotalSize is renamed AddTotalSize because it now adds.

The issue says the ETA went negative; it was computed negative and
printed as `unknown`.

Model: opus-5-5

internal/snapshot/progress.go

@@ -56,23 +56,27 @@ const (
 
 // ProgressStats holds atomic counters for progress tracking
 type ProgressStats struct {
-	FilesScanned     atomic.Int64 // Total files seen during scan (includes skipped)
-	FilesProcessed   atomic.Int64 // Files actually processed in phase 2
-	FilesSkipped     atomic.Int64 // Files skipped due to no changes
-	BytesScanned     atomic.Int64 // Bytes from new/changed files only
-	BytesSkipped     atomic.Int64 // Bytes from unchanged files
-	BytesProcessed   atomic.Int64 // Actual bytes processed (for ETA calculation)
-	ChunksCreated    atomic.Int64
-	BlobsCreated     atomic.Int64
-	BlobsUploaded    atomic.Int64
-	BytesUploaded    atomic.Int64
-	CurrentFile      atomic.Value // stores string
-	TotalSize        atomic.Int64 // Total size to process (set after scan phase)
-	TotalFiles       atomic.Int64 // Total files to process in phase 2
-	ProcessStartTime atomic.Value // stores time.Time when processing starts
-	StartTime        time.Time
-	mu               sync.RWMutex
-	lastDetailTime   time.Time
+	FilesScanned   atomic.Int64 // Total files seen during scan (includes skipped)
+	FilesProcessed atomic.Int64 // Files actually processed in phase 2
+	FilesSkipped   atomic.Int64 // Files skipped due to no changes
+	BytesScanned   atomic.Int64 // Bytes from new/changed files only
+	BytesSkipped   atomic.Int64 // Bytes from unchanged files
+	BytesProcessed atomic.Int64 // Actual bytes processed (for ETA calculation)
+	ChunksCreated  atomic.Int64
+	BlobsCreated   atomic.Int64
+	BlobsUploaded  atomic.Int64
+	BytesUploaded  atomic.Int64
+	CurrentFile    atomic.Value // stores string
+	TotalSize      atomic.Int64 // Size to process in the paths scanned so far
+	TotalFiles     atomic.Int64 // Files to process in the paths scanned so far
+	StartTime      time.Time
+	mu             sync.RWMutex
+	lastDetailTime time.Time
+
+	// Guarded by mu: when the current path's processing started, and
+	// BytesProcessed at that moment.
+	processStartTime  time.Time
+	processStartBytes int64
 
 	// Upload tracking
 	CurrentUpload    atomic.Value // stores *UploadInfo
@@ -148,10 +152,36 @@ func (pr *ProgressReporter) GetStats() *ProgressStats {
 	return pr.stats
 }
 
-// SetTotalSize sets the total size to process (after scan phase)
-func (pr *ProgressReporter) SetTotalSize(size int64) {
-	pr.stats.TotalSize.Store(size)
-	pr.stats.ProcessStartTime.Store(time.Now().UTC())
+// AddTotalSize adds the size one path of the snapshot has to process to
+// the total, once that path's scan phase is done, and starts measuring
+// the processing rate again. The processed counts run across every path,
+// so the total does too. The rate covers the current path's processing
+// only, so that the scan phase of a later path, which processes nothing,
+// does not lower it.
+func (pr *ProgressReporter) AddTotalSize(size int64) {
+	pr.stats.TotalSize.Add(size)
+
+	pr.stats.mu.Lock()
+	defer pr.stats.mu.Unlock()
+
+	pr.stats.processStartTime = time.Now().UTC()
+	pr.stats.processStartBytes = pr.stats.BytesProcessed.Load()
+}
+
+// processRate returns the bytes processed per second since the current
+// path's processing started, or 0 before the first path's scan phase is
+// done.
+func (s *ProgressStats) processRate() float64 {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	if s.processStartTime.IsZero() {
+		return 0
+	}
+
+	processed := s.BytesProcessed.Load() - s.processStartBytes
+
+	return float64(processed) / time.Since(s.processStartTime).Seconds()
 }
 
 // Helper functions
@@ -357,19 +387,12 @@ func (pr *ProgressReporter) printSummaryStatus() {
 	// Calculate ETA if we have total size and are processing
 	etaStr := ""
 
-	if totalSize > 0 && bytesProcessed > 0 {
-		processStart, ok := pr.stats.ProcessStartTime.Load().(time.Time)
-		if ok && !processStart.IsZero() {
-			processElapsed := time.Since(processStart)
-
-			rate := float64(bytesProcessed) / processElapsed.Seconds()
-			if rate > 0 {
-				remainingBytes := totalSize - bytesProcessed
-				remainingSeconds := float64(remainingBytes) / rate
-				eta := time.Duration(remainingSeconds * float64(time.Second))
-				etaStr = " | ETA: " + formatDuration(eta)
-			}
-		}
+	processRate := pr.stats.processRate()
+	if totalSize > 0 && processRate > 0 {
+		remainingBytes := totalSize - bytesProcessed
+		remainingSeconds := float64(remainingBytes) / processRate
+		eta := time.Duration(remainingSeconds * float64(time.Second))
+		etaStr = " | ETA: " + formatDuration(eta)
 	}
 
 	rate := float64(bytesScanned+bytesSkipped) / elapsed.Seconds()
@@ -421,25 +444,18 @@ func (pr *ProgressReporter) printDetailedStatus() {
 	log.Info("Elapsed time", "duration", formatDuration(elapsed))
 
 	// Calculate and show ETA if we have data
-	if totalSize > 0 && bytesProcessed > 0 {
-		processStart, ok := pr.stats.ProcessStartTime.Load().(time.Time)
-		if ok && !processStart.IsZero() {
-			processElapsed := time.Since(processStart)
-
-			processRate := float64(bytesProcessed) / processElapsed.Seconds()
-			if processRate > 0 {
-				remainingBytes := totalSize - bytesProcessed
-				remainingSeconds := float64(remainingBytes) / processRate
-				eta := time.Duration(remainingSeconds * float64(time.Second))
-				percentComplete := float64(bytesProcessed) / float64(totalSize) * percentScale
-				log.Info("Overall progress",
-					"percent", fmt.Sprintf("%.1f%%", percentComplete),
-					"processed", humanize.Bytes(safeUint64(bytesProcessed)),
-					"total", humanize.Bytes(safeUint64(totalSize)),
-					"rate", humanize.Bytes(uint64(processRate))+"/s",
-					"eta", formatDuration(eta))
-			}
-		}
+	processRate := pr.stats.processRate()
+	if totalSize > 0 && processRate > 0 {
+		remainingBytes := totalSize - bytesProcessed
+		remainingSeconds := float64(remainingBytes) / processRate
+		eta := time.Duration(remainingSeconds * float64(time.Second))
+		percentComplete := float64(bytesProcessed) / float64(totalSize) * percentScale
+		log.Info("Overall progress",
+			"percent", fmt.Sprintf("%.1f%%", percentComplete),
+			"processed", humanize.Bytes(safeUint64(bytesProcessed)),
+			"total", humanize.Bytes(safeUint64(totalSize)),
+			"rate", humanize.Bytes(uint64(processRate))+"/s",
+			"eta", formatDuration(eta))
 	}
 
 	log.Info("Files processed",