Stream attachments directly to storage

[aksiksi]

Reqwest implements From<Response> for Body. This means that a request body can be set for a pending response. It is not clear if the content is streamed or not.

For Attachment, we can set the content filed to point to an enum like so:

enum AttachmentBody {
    Data(Vec<u8>),
    Stream(reqwest::Response),
}

Note that you can convert a Response into a Stream, and then pass that on. Or we can use the chunk method. See: https://docs.rs/reqwest/0.10.0/reqwest/struct.Response.html#method.bytes_stream

[aksiksi]

For each attachment:

1. Pass in sender, recipients, content type, email ID, and filename as HTTP headers.
2. Set attachment body as the body.
3. On server side, validate headers and return relevant error (graceful or otherwise).
4. Stream HTTP body to Dropbox with given filename.

[aksiksi]

I did some basic profiling (release build) to compare approaches. For an ~8 MB email with 3 attachments:

• With non-streaming version, peak memory usage is ~9 MB. Basically, each attachment was allocated once, which makes sense.
• With streaming version, peak usage is ~140 KB... which is mind boggling!

See graphs (courtesy of massif-visualizer):

See https://github.com/seanmonstar/warp/issues/448 for how to use this.