mirror of https://github.com/rwf2/Rocket.git
c24a96308b
Problem: To support Server-Side Events (SSE, aka JS EventSource) it is necessary for the server to keep open an HTTP request and dribble out data (the event stream) as it is generated. Currently, Rocket handles this poorly. It likes to fill in complete chunks. Also there is no way to force a flush of the underlying stream: in particular, there is a BufWriter underneath hyper. hyper would honour a flush request, but there is no way to send one. Options: Ideally the code which is producing the data would be able to explicitly designate when a flush should occur. Certainly it would not be acceptable to flush all the time for all readers. 1. Invent a new kind of Body (UnbufferedChunked) which translates the data from each Read::read() call into a single call to the stream write, and which always flushes. This would be a seriously invasive change. And it would mean that SSE systems with fast event streams might work poorly. 2. Invent a new kind of Body which doesn't use Read at all, and instead has a more sophisticated API. This would be super-invasive and heavyweight. 3. Find a way to encode the necessary information in the Read trait protocol. Chosen solution: It turns out that option 3 is quite easy. The read() call can return an io::Error. There are at least some errors that clearly ought not to occur here. An obvious one is ErrorKind::WouldBlock. Rocket expects the reader to block. WouldBlock is only applicable to nonblocking objects. And indeed the reader will generally want to return it (once) when it is about to block. We have the Stream treat io::Error with ErrorKind::WouldBlock, from its reader, as a request to flush. There are two effects: we stop trying to accumulate a full chunk, and we issue a flush call to the underlying writer (which, eventually, makes it all the way down into hyper and BufWriter). Implementation: We provide a method ReadExt::read_max_wfs which is like read_max but which handles the WouldBlock case specially. It tells its caller whether a flush was wanted. This is implemented by adding a new code to read_max_internal. with a boolean to control it. This seemed better than inventing a trait or something. (The other read_max call site is reading http headers in data.rs, and I think it wants to tread WouldBlock as an error.) Risks and downsides: Obviously this ad-hoc extension to the Read protocol is not particularly pretty. At least, people who aren't doing SSE (or similar) won't need it and can ignore it. If for some reason the data source is actually nonblocking, this new arrangement would spin, rather than calling the situation a fatal error. This possibility seems fairly remote, in production settings at least. To migitate this it might be possible for the loop in Rocket::issue_response to bomb out if it notices it is sending lots of consecutive empty chunks. It is possible that async Rocket will want to take a different approach entirely. But it will definitely need to solve this problem somehow, and naively it seems like the obvious transformation to eg the Tokio read trait would have the same API limitation and admit the same solution. (Having a flush occur every time the body stream future returns Pending would not be good for performance, I think.) |
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