em-http-request middlewares get double-called

[ajvondrak]

Specifically, the #request methods are invoked in two places.

1. em-http-request calls them from EventMachine::HttpClient#connection_completed: https://github.com/igrigorik/em-http-request/blob/157d5ff281c503656192825c388b28e7f35e04ce/lib/em-http/client.rb#L57-L59
2. webmock calls them from EventMachine::WebMockHttpClient#build_request_signature:

   webmock/lib/webmock/http_lib_adapters/em_http_request_adapter.rb

   Lines 169 to 171 in 382d84c

   	@conn.middleware.select {|m| m.respond_to?(:request) }.each do |m|
   	headers, body = m.request(self, headers, body)
   	end

The request signature is built at the start of EventMachine::WebMockHttpConnection#activate_connection here:

webmock/lib/webmock/http_lib_adapters/em_http_request_adapter.rb

Line 56 in 382d84c

request_signature = client.request_signature

This happens prior to duplicating the logic of EventMachine::HttpConnection#activate_connection (ref) to call #finalize_request here:

webmock/lib/webmock/http_lib_adapters/em_http_request_adapter.rb

Lines 60 to 69 in 382d84c

	post_init
	@deferred = false
	@conn = conn
	conn.parent = self
	conn.pending_connect_timeout = @connopts.connect_timeout
	conn.comm_inactivity_timeout = @connopts.inactivity_timeout
	finalize_request(client)

Then the #finalize_request call hits the superclass EventMachine::HttpConnection#finalize_request, which adds a callback to EventMachine::HttpClient#connection_completed: https://github.com/igrigorik/em-http-request/blob/157d5ff281c503656192825c388b28e7f35e04ce/lib/em-http/http_connection.rb#L159

Which, as noted at the start, runs the #request middleware again.

The middleware handling was added waaay back in #111. Since the #request hook can amend the request headers & body, it makes sense that you'd want it to run before building & checking the webmock signature. But the circuitous control flow means that if the middleware has a side-effect (e.g., adding another callback), those side-effects will get triggered twice, which can lead to some really bizarre (and incorrect) behavior in the test suite.

[bblimke]

@ajvondrak thank you for investigating this and presenting the detailed explanation of the problem. Since you know the problem, do you perhaps also know how this can be fixed? If yes, then would you fancy submitting a PR with the fix?