Add a section on infinite loops

content/blog/2017-12-15-improving-the-repository-infrastructure.md

@@ -255,6 +255,14 @@ In some cases, a change proved to be so complex that it necessitated a standalon
 
 Going through the fixtures is still a lot of work, and we are considering automating some of it. Still, the fixture app is invaluable even as documentation for the existing behavior and all the edge cases and browser bugs that React currently handles. Having it gives us confidence in making significant changes to the logic without breaking important use cases. Another improvement we're considering is to have a GitHub bot build and deploy the fixtures automatically for every pull request that touches the relevant files so anyone can help with browser testing.
 
+### Preventing Infinite Loops
+
+The React 16 codebase contains many `while` loops. They let us avoid the dreaded deep stack traces that occurred with earlier versions of React, but can make development of React really difficult. Every time there is a mistake in an exit condition our tests would just hang, and it took a while to figure out which of the loops is causing the issue.
+
+Inspired by the [strategy adopted by Repl.it](https://repl.it/site/blog/infinite-loops), we have added a [Babel plugin that prevents infinite loops](https://github.com/facebook/react/blob/d906de7f602df810c38aa622c83023228b047db6/scripts/babel/transform-prevent-infinite-loops.js) in the test environment. If some loop continues for more than the maximum allowed number of iterations, we throw an error and immediately fail it so that Jest can display where exactly this happened.
+
+This approach has a pitfall. If an error thrown from the Babel plugin gets caught and ignored up the call stack, the test will pass even though it has an infinite loop. This is really, really bad. To solve this problem, we [set a global field](https://github.com/facebook/react/blob/d906de7f602df810c38aa622c83023228b047db6/scripts/babel/transform-prevent-infinite-loops.js#L26-L30) before throwing the error. Then, after every test run, we [rethrow that error if the global field has been set](https://github.com/facebook/react/blob/d906de7f602df810c38aa622c83023228b047db6/scripts/jest/setupTests.js#L42-L56). This way any infinite loop will cause a test failure, no matter whether the error from the Babel plugin was caught or not.
+
 ## Customizing the Build
 
 There were a few things that we had to fine-tune after introducing our new build process. It took us a while to figure them out, but we're moderately happy with the solutions that we arrived at.