Utility function to download a URL to a file path with progress reporting

tools/utils/download_file.go

@@ -0,0 +1,72 @@
+// License: GPLv3 Copyright: 2022, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strconv"
+)
+
+var _ = fmt.Print
+
+type ReportFunc = func(done, total uint64) error
+
+type write_counter struct {
+	done, total uint64
+	report      ReportFunc
+}
+
+func (self *write_counter) Write(p []byte) (int, error) {
+	n := len(p)
+	self.done += uint64(n)
+	if self.report != nil {
+		err := self.report(self.done, self.total)
+		if err != nil {
+			return 0, err
+		}
+	}
+	return n, nil
+}
+
+func DownloadFile(destpath, url string, progress_callback ReportFunc) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	destpath, err = filepath.EvalSymlinks(destpath)
+	if err != nil {
+		return err
+	}
+	dest, err := os.CreateTemp(filepath.Dir(destpath), filepath.Base(destpath)+".partial-download.")
+	if err != nil {
+		return err
+	}
+	dest_removed := false
+	defer func() {
+		dest.Close()
+		if !dest_removed {
+			os.Remove(dest.Name())
+		}
+	}()
+	wc := write_counter{report: progress_callback}
+	cl, err := strconv.Atoi(resp.Header.Get("Content-Length"))
+	if err == nil {
+		wc.total = uint64(cl)
+	}
+	_, err = io.Copy(dest, io.TeeReader(resp.Body, &wc))
+	if err != nil {
+		return err
+	}
+	dest.Close()
+	err = os.Rename(dest.Name(), destpath)
+	if err != nil {
+		return err
+	}
+	dest_removed = true
+	return nil
+}