Provide a method for disabling Adapters

[techfg]

Background

Adapters are a great path forward for Gatsby, however the current functionality compared to previous methods (e.g., gatsby-plugin-gatsby-cloud, gatsby-plugin-netlify) is not yet at the same level. For example, these two plugins offer allPageHeaders and transformHeaders properties simplifying the ability to create a header for pages only and then transforming special cases for any header on any path. With the migration to Netlify from Gatsby Cloud, and for those already on Netlify (since Netlify is the only adapter currently), losing the functionality can have significant effects.

Additionally, there are situations where you might "build" on a platform that has an adapter but be deploying to another platform entirely - or simply want full control of your build/deploy without side-effects. For example, on Netlify, the adapter will still execute even when Build Settings->Runtime is not Gatsby. Ironically, Netlify does provide a way to still execute gatsby-plugin-netlify via its netlify-plugin-gatsby (NETLIFY_SKIP_GATSBY_BUILD_PLUGIN), however the gatsby-adapter-netlify doesn't respect this environment variable and removes gatsby-plugin-netlify regardless.

Currently, there is no way to disable Adapters - if the manifest finds a matching adapter on build, it will use it.

Given above (and likely others will have other reasons), there should be a method for disabling adapters.

Proposal

Allow gatsby config adapter property to be set to false which would disable adapters (or provide an environment variable option).

Workaround

Fortunately, Gatsby has an extensible adapter model so create a noop adapter essentially yields the same outcome as disabling (+/- a little overhead because noOpAdapterManager is not used).

Warning

This code has not been fully tested for functionality, performance, etc., it is just a proof-of-concept - use at your own risk.

gatsby-config.ts

....

const noOpAdapter: AdapterInit<AdapterOptions> = () => ({
  name: `gatsby-adapter-noop`,
  adapt() {
    // noop
  },
})

....

const config: GatsbyConfig = {
  adapter: noOpAdapter(),

Thoughts/input welcome!

[techfg]

Tagging #38542 for cross-reference