Capybara, WebMock, and too many open files

Getting set up with a new codebase is always a fun, sometimes head-scratching experience where you have to discern whether the issues you’re facing are because you missed some piece of documentation (or it was missing), or have some bug in your setup.

I had this recently, joining a team where I just couldn’t get our RSpec feature tests to run on a fresh machine and checkout. Many tests failed with “Too many open files”:

Failure/Error: initialize_tcp host, port, local_host, local_port

          Errno::EMFILE:
            Failed to open TCP connection to 127.0.0.1:9526 (Too many open files - socket(2) for 127.0.0.1 port 9526)

This led me to a gotcha in Capybara/WebMock that was causing the error. I still have no idea why this affected my Ruby build on MacOS Ventura, but none of my colleagues 🤷‍♂️.

The error occurs because Net::HTTP has separate connect and request phases, but WebMock was designed with connect being part of the request step. Because there’s no information about the request in the connect step (just the destination address), WebMock can’t decide whether to stub or pass through the connection at that point, so they made the design decision to delay connect until a request is made. This is the approach of many HTTP libraries, but not Net::HTTP.

Fixing the error appeared to be as simple as setting net_http_connect_on_start: true to undo WebMock’s workaround for Net::HTTP, however this introduced a new SSL certificate error for a mocked request. This was curious, because the requests are supposed to be mocked, so we don’t expect any external connection to be made.

It turns out that setting net_http_connect_on_start: true works around the Capybara gotcha by enabling all Net::HTTP connections to be made¹, while still intercepting and mocking the requests appropriately. We happened to see an error because of an SSL certificate problem on a host we didn’t really want to connect to.

Scoping eager connections

Thankfully, net_http_connect_on_start is smarter than just accepting true or false. There are a bunch of tests showing net_http_connect_on_start accepts:

• Regular expression matching host
• Host
• Port
• Scheme (e.g. http, https)

For our test suite, we disallow all connections except the Capybara server, so the working solution for us was to scope net_http_connect_on_start to match the allowed host:

WebMock.disallow_net_connect!(
    allow_localhost: true, 
    allow: Capybara.server_host, 
    net_http_connect_on_start: Capybara.server_host
  )

For another gotcha: WebMock supports passing a list of hosts to allow, but this isn’t supported by net_http_connect_on_start, where you’ll need a regular expression instead if you have multiple disparate hosts.

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1. Mostly thanks to this comment ↩