Add structured logging and better error handling

internal/fetch/worker.go

@@ -68,11 +68,26 @@ func (worker *segmentWorker) record(count int64) error {
 
 func (worker *segmentWorker) reconnect(ctx context.Context, cause error) (*http.Response, error) {
 	progress := worker.session.request.Progress
+	logger := worker.session.request.logger().With("part_start", worker.segment.start)
 	for {
 		worker.failedAttempts++
-		if ctx.Err() != nil || !isRetryable(cause) || worker.failedAttempts > maxRetriesWithoutProgress {
+		if ctx.Err() != nil {
 			return nil, cause
 		}
+		if !isRetryable(cause) {
+			logger.Debug("error cannot be retried", "error", cause)
+			return nil, cause
+		}
+		if worker.failedAttempts > maxRetriesWithoutProgress {
+			logger.Error("giving up after repeated connection failures", "attempts", maxRetriesWithoutProgress, "error", cause)
+			return nil, fmt.Errorf("gave up after %d reconnect attempts: %w", maxRetriesWithoutProgress, cause)
+		}
+
+		logger.Warn("connection lost, reconnecting",
+			"attempt", worker.failedAttempts,
+			"position", worker.segment.position(),
+			"error", cause,
+		)
 		progress.SetNotice(fmt.Sprintf("Connection lost, reconnecting (attempt %d of %d)", worker.failedAttempts, maxRetriesWithoutProgress))
 		if err := waitBeforeRetry(ctx, worker.failedAttempts); err != nil {
 			return nil, err
@@ -80,6 +95,7 @@ func (worker *segmentWorker) reconnect(ctx context.Context, cause error) (*http.
 
 		response, err := worker.open(ctx)
 		if err == nil {
+			logger.Info("reconnected", "position", worker.segment.position())
 			progress.SetNotice("")
 			return response, nil
 		}