So I found the answer, and it's Server-sent events. It basically enables one-way http-streams that the browser can handle a chunk at a time. It can be a little tricky because some existing stream libs are broken (they don't assume you have \n in your stream, and hence you get partial data), or have little documentation. But it's not hard to roll your own (once you figure it out).
You can define your sse_transform like this:
// file sse_stream.coffee
var Transform = require('stream').Transform;
var util = require('util');
util.inherits(SSEStream, Transform);
function SSEStream(option) {
Transform.call(this, option);
this.id = 0;
this.retry = (option && option.retry) || 0;
}
SSEStream.prototype._transform = function(chunk, encoding, cb) {
var data = chunk.toString();
if (data) {
this.push("id:" + this.id + "\n" +
data.split("\n").map(function (e) {
return "data:" + e
}).join("\n") + "\n\n");
//"retry: " + this.retry);
}
this.id++;
cb();
};
SSEStream.prototype._flush = function(next) {
this.push("event: end\n" + "data: end" + "\n\n");
next();
}
module.exports = SSEStream;
Then on the server side (I was using express), you can do something like this:
sse_stream = require('sse_stream')
app.get '/blob', (req, res, next) ->
sse = new sse_stream()
# It may differ here for you, but this is just a stream source.
blobStream = repo.git.streamcmd("cat-file", { p: true }, [blob.id])
if (req.headers["accept"] is "text/event-stream")
res.type('text/event-stream')
blobStream.on("end", () -> res.removeAllListeners()).stdout
.pipe(
sse.on("end", () -> res.end())
).pipe(res)
else
blobStream.stdout.pipe(res)
Then on the browser side, you can do:
source = new EventSource("/blob")
source.addEventListener('open', (event) ->
console.log "On open..."
, false)
source.addEventListener('message', (event) ->
processData(event.data)
, false)
source.addEventListener('end', (event) ->
console.log "On end"
source.close()
, false)
source.addEventListener('error', (event) ->
console.log "On Error"
if event.currentTarget.readyState == EventSource.CLOSED
console.log "Connection was closed"
source.close()
, false)
Notice that you need to listen for the event 'end', that is sent from the server in the transform stream's _flush() method. Otherwise, EventSource in the browser is just going to request the same file over and over again.
Note that you can use libraries on the server side to generate SSE. On the browser side, you can use portal.js to handle SSE. I just spelt things out, so you can see how things would work.