feat(gatsby): DSG support for FS routes

[vladar]

Description

This PR adds support for DSG config for FS routes.

Caveat: it only runs in gatsby build right now. So there is no way to debug it in develop. Wiring it up to develop will be a follow-up. The reason why this is not a priority is that DSG in develop has no effect anyways.

Usage example

Say you have a template src/pages/posts/{MarkdownRemark.fields__slug}.js (using FS routes), then you can optionally mark some of the pages created with this template as defer using the following code:

import React from "react"
import { graphql } from "gatsby"

export default function Component(props) {
  return <pre>{JSON.stringify(props, null, 2)}</pre>
}

export async function config() {
  const { data } = graphql`
    {
      oldPosts: allMarkdownRemark(filter: { frontmatter: { date: { lt: "2015-05-27" } } }) {
        nodes { fields { slug } }
      }
    }
  `
  // Trim slashes because FS routes params have no slashes
  const oldPosts = new Set(data.oldPosts.nodes.map((n) => n.fields.slug.replace(/^\/|\/$/g, ``)))

  // Return a function that when called will return a config for FS route pages
  // (right now only `defer` is supported)
  return ({ params }) => {
    return {
      defer: oldPosts.has(params.fields__slug),
    }
  }
}

export const pageQuery = graphql`
  query BlogPost($id: String!) {
    markdownRemark(id: { eq: $id }) {
      id
      excerpt(pruneLength: 160)
      html
      frontmatter {
        title
        date(formatString: "MMMM DD, YYYY")
        description
      }
    }
  }
`

[ch38465]

[pieh]

🚢 it ;)