2016. július 2., szombat 8:15:19 UTC+2 időpontban Sri G a következőt írta:
>
> I'm working on receiving uploads through a form.
>
> The tricky part is validation.
>
> I attempt to read the first 1024 bytes to check the mime of the file and
> then if valid read the rest and hash it and also save it to disk. Reading
> the mime type is successful and I've gotten it to work by chaining
> TeeReader but it seems very hackish. Whats the idiomatic way to do this?
>
> I'm trying something like this:
>
>
> // Parse my multi part form
> ...
> // Get file handle
> file, err := fh.Open()
>
> var a bytes.Buffer
>
> io.CopyN(&a, file, 1024)
>
> mime := mimemagic.Match("", a.Bytes())
> // Check mime type (this works fine)
>
> I'm trying to seek a stream so this should be no-op
> file.Seek(0, 0)
>
> The file stored on disk is 1KB larger than the original so it appears to
> be re-copying the entire file and appending it to bytes.Buffer
> io.Copy(&a, file)
>
> checksum := md5.New()
> b := io.TeeReader(&a, checksum)
>
> md5hex := hex.EncodeToString(checksum.Sum(nil))
> fmt.Println("md5=", md5hex)
>
> //Open file f for writing to disk
> ...
> //Save file
> io.Copy(f, b)
>
>
> Checked the md5 of (1KB of orig + orig), and (orginal - first 1 KB),
> neither match the md5 of the file being hashed.
>
> Why can't I append the rest of the stream to the byte buffer to get the
> complete file in memory and why is the byte buffer being "consumed"?
>
> I simply need to read the same array of byte multiple times, I don't need
> to "copy" them. I'm coming from a C background so I'm wondering what is
> going on behind the scenes as well.
>
If you know you'll have to read the whole file into memory, then do that,
and use bytes.NewReader to create a reader for that byte slice.
If you read partly, to decide whether to go on, then use fh.Read or
io.ReadAtLeast with a byte slice.
If you read sth, then want to read the whole from the beginning, construct
a Reader with io.MultiReader(bytes.NewReader(b), fh).
You can combine these approaches, but if the while file size is less than a
few KiB, I think it is easier, simpler and more performant (!) to read the
whole file up into memory,
into a bytes.Buffer, and construct the needed readers with
bytes.NewReader(buf.Bytes()).
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