core branch: EMFILE too many open files error on Mac build

[robbear]

On the core branch, if you build example/docs with sufficient content (I copied about.en.md to about1.en.md, about2.en.md, ... about39.en.md), you run into UnhandledPromiseRejectionWarning errors: EMFILE: too many open files

This appears to be related to the Promise.all approach of getPageMap/getFiles in packages/nextra/src/loader.js. Making that operation synchronous (I tweaked the code with a synchronous version of getPageMap) allows the Mac build to succeed.

This open files build error seems limited to my Mac builds. My Linux/Ubuntu builds don't run into this.

I'm running macOS Big Sur version 11.6 on a 2019 MacBook Pro Intel Core i7 with 32 GB memory.

$ next build
You have i18n enabled for Nextra.
info  - Checking validity of types  
warn  - No ESLint configuration detected. Run next lint to begin setup
(node:5683) UnhandledPromiseRejectionWarning: Error: EMFILE: too many open files, open '/Users/robbear/dev/robbear/nextra/examples/docs/pages/nextra/about2.en.md'
(Use `node --trace-warnings ...` to show where the warning was created)
(node:5683) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:5683) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:5683) UnhandledPromiseRejectionWarning: Error: EMFILE: too many open files, open '/Users/robbear/dev/robbear/nextra/examples/docs/pages/blog/hello-world.en.md'
(node:5683) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)

[dimaMachina]

Closing as completed #318