A multipart.Writer exists in one of three states currently:
1. CreatePart has not been called.
2. CreatePart has been called at least once, but Close has not been called
yet.
3. Close has been called.
This proposal would introduce a fourth state:
4. FlushLastPart has been called. CreatePart or Close have not been called
since then.
This needs to be a distinct state, since FlushLastPart (perhaps better
called ClosePart?) would write the boundary separator terminating the
previous part. If we call Close in this state, we need to write "--\r\n",
not "\r\n--BOUNDARY--\r\n".
Given how simple multipart.Writer is, I wonder if it isn't better to just
write the multipart message manually in your case.
On Friday, May 7, 2021 at 11:11:59 AM UTC-7 Ohad Lutzky wrote:
> Hi all, I'm looking at multipart/x-mixed-replace-based image streaming
> (mjpeg-style, but with PNG in my case), and would like to allow code to
> push a new image immediately. This look roughly like this (error handling
> elided for brevity):
>
> m := multipart.NewWriter()
> defer m.Close()
> w.Header().Set("Content-Type", "multipart/x-mixed-replace;
> boundary="+m.Boundary())
> h := textproto.MIMEHeader{}
> h.Set("Content-Type", "image/png")
> for {
> buf := <-c // Channel with PNG data
> h.Set("Content-Length", fmt.Sprint(len(buf)))
> mw, _ := m.CreatePart(h)
> mw.Write(buf)
> w.(http.Flusher).Flush()
> }
>
> Writing PNGs to channel c will indeed display the image to the screen, but
> one image later. Specifically, it's the m.CreatePart
> <https://golang.org/src/mime/multipart/writer.go?s=2406:2481#L87> call
> that completes the last part by emitting the final --BOUNDARY\r\n.
> Unfortunately, I can't call it immediately after mw.Write, because I don't
> necessarily know the Content-Length for the next image. A workaround that
> does work is this:
>
> mw, _ := m.CreatePart(h)
> mw.Write(buf)
> mw, _ = m.CreatePart(h) // Yes, again
> mw.Write(buf)
> w.(http.Flusher).Flush()
>
> Notably, calling m.Close() wouldn't be appropriate here. While CreatePart
> write \r\n--BOUNDARY\r\n, Close would write \r\n--BOUNDARY--\r\n.
> Apparently that extra -- is enough to make the browser disregard everything
> that follows.
>
> While this can be worked around by not using the mime/multipart library, I
> think it's a reasonable API change (but an API change nonetheless) to add a
> FlushLastPart method to Writer. WDYT?
>
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