Friday, August 16, 2019

Debugging the Karmabug

Debugging the Karmabug

For reasons I will not go through right now, I needed a new library for making synchronous HTTP requests in Node.js. I know what you are saying, “But that’s one of the seven deadly sins of JavaScript!” 1 Well, just know I had my reasons, and I wanted to replace sync-request.

Since I was already using the Fetch API with node-fetch in the async part of my library, I thought: why not build a sync-fetch, using node-fetch under the hood like sync-request uses (then-)request. Two days later, it was actually working, as far as I could tell. However, if I wanted to publish this horror I need some actual testing.

Luckily node-fetch has a nice test suite, I only needed to convert 194 test cases to use the synchronous API. Not fun work, but worth its while, maybe. Anyway, the first test cases worked, but then it got stuck on the first actual request.

This is were I have to introduce you to the Karmabug. You see, after some testing I figured out that only the combination of my sync-fetch and the test server just… stopped. The arguments were correct, my fetch works with and the test server works with node-fetch, but this combination simply did not. Investigating either pointed to the other, and I had no idea what to do next.

That would make a good tweet, I thought. “Karma for making sync HTTP requests I guess.” Literally two minutes later it hit me: that was exactly what was going on. The test server could not respond to the requests because the request itself was blocking the event loop.

1 Incidentally, the seven deadly sins of JavaScript all happen to be Sloth.

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