Corrupted contents when downloading on top of wrong file #21
Description
Grab looked like a good fit for a project I'm working on so I gave it a spin. I found that it downloaded a file perfectly and when asked to download the same file again managed to avoid downloading all the bytes again, which was just what I was looking for.
I then overwrote the downloaded file with completely different contents and then downloaded again using grab. The message:
206 Partial Content
was emitted and the download was apparently successful. The downloaded file even had the same number of bytes as the original, but unfortunately the contents were corrupted.
Fortunately, this problem is easily reproduced using the example program in the README:
$ go run main.go
Downloading http://www.golang-book.com/public/pdf/gobook.pdf...
200 OK
Download saved to ./gobook.pdf
$ mv gobook.pdf gobook.pdf.good
$ cp main.go gobook.pdf
$ go run main.go
Downloading http://www.golang-book.com/public/pdf/gobook.pdf...
206 Partial Content
Download saved to ./gobook.pdf
$ diff gobook.pdf gobook.pdf.good
Binary files gobook.pdf and gobook.pdf.good differ
$ ls -l
total 11320
-rw-r--r-- 1 gnormington staff 2893557 1 Jan 1970 gobook.pdf
-rw-r--r-- 1 gnormington staff 2893557 1 Jan 1970 gobook.pdf.good
-rw-r--r-- 1 gnormington staff 1139 3 Nov 11:10 main.go
The environment is go version go1.9.2 darwin/amd64 on macOS 10.13.1.
In case the README changes, the contents of main.go above is:
package main
import (
"fmt"
"os"
"time"
"github.com/cavaliercoder/grab"
)
func main() {
// create client
client := grab.NewClient()
req, _ := grab.NewRequest(".", "http://www.golang-book.com/public/pdf/gobook.pdf")
// start download
fmt.Printf("Downloading %v...\n", req.URL())
resp := client.Do(req)
fmt.Printf(" %v\n", resp.HTTPResponse.Status)
// start UI loop
t := time.NewTicker(500 * time.Millisecond)
defer t.Stop()
Loop:
for {
select {
case <-t.C:
fmt.Printf(" transferred %v / %v bytes (%.2f%%)\n",
resp.BytesComplete(),
resp.Size,
100*resp.Progress())
case <-resp.Done:
// download is complete
break Loop
}
}
// check for errors
if err := resp.Err(); err != nil {
fmt.Fprintf(os.Stderr, "Download failed: %v\n", err)
os.Exit(1)
}
fmt.Printf("Download saved to ./%v \n", resp.Filename)
// Output:
// Downloading http://www.golang-book.com/public/pdf/gobook.pdf...
// 200 OK
// transferred 42970 / 2893557 bytes (1.49%)
// transferred 1207474 / 2893557 bytes (41.73%)
// transferred 2758210 / 2893557 bytes (95.32%)
// Download saved to ./gobook.pdf
}