Add structured logging and better error handling

internal/fetch/downloader.go

@@ -3,6 +3,7 @@ package fetch
 import (
 	"context"
 	"errors"
+	"log/slog"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -34,6 +35,14 @@ type Request struct {
 	State         *State
 	Progress      Progress
 	Checkpoint    func(State)
+	Logger        *slog.Logger
+}
+
+func (request Request) logger() *slog.Logger {
+	if request.Logger != nil {
+		return request.Logger
+	}
+	return slog.Default()
 }
 
 type Downloader struct {
@@ -90,11 +99,19 @@ func (downloader *Downloader) Download(ctx context.Context, request Request) (st
 }
 
 func (downloader *Downloader) prepare(ctx context.Context, source *remoteFile, request Request, partPath string) (State, *http.Response, error) {
-	if request.State != nil && request.State.valid() && fileExists(partPath) {
-		state := *request.State
-		source.validator = state.Validator
-		source.totalBytes = state.TotalBytes
-		return state, nil, nil
+	if request.State != nil {
+		if request.State.valid() && fileExists(partPath) {
+			state := *request.State
+			source.validator = state.Validator
+			source.totalBytes = state.TotalBytes
+			request.logger().Info("resuming from saved progress",
+				"bytes_done", state.DoneBytes(),
+				"bytes_total", state.TotalBytes,
+				"parts", len(state.Segments),
+			)
+			return state, nil, nil
+		}
+		request.logger().Warn("saved progress is unusable, starting over", "part_file_exists", fileExists(partPath))
 	}
 
 	probe, err := downloader.open(ctx, source, 0, 0)
@@ -113,6 +130,13 @@ func (downloader *Downloader) prepare(ctx context.Context, source *remoteFile, r
 		TotalBytes: source.totalBytes,
 		Segments:   planSegments(source.totalBytes, connections),
 	}
+	request.logger().Info("connected to server",
+		"file", state.FileName,
+		"bytes_total", state.TotalBytes,
+		"supports_ranges", probe.Header.Get("Accept-Ranges") == "bytes",
+		"has_validator", state.Validator != "",
+		"parts", len(state.Segments),
+	)
 	return state, probe, nil
 }