Add a default directory for Netlify Functions

[ehmicky]

Background at https://github.com/netlify/pod-the-builder/issues/91 and https://github.com/netlify/product/issues/257

When @netlify/config is run and both:

• no Functions directory was specified neither in the netlify.toml nor in the UI build settings
• a netlify-automatic-functions directory exists in the build directory

The build.functions configuration property should be set to netlify-automatic-functions.

This means, users will now be able to use Netlify Functions without specifying a Functions directory setting, by using the default directory name instead.

Based on the feedback we get in https://github.com/netlify/buildbot/issues/1033, we might also want to rename this default value to netlify-functions or functions instead.

[ehmicky]

Done at #2049

The name is currently netlify-automatic-functions. After https://github.com/netlify/buildbot/issues/1033, we might rename it to something simpler. Until then, this should be left undocumented.

[ehmicky]

The following Humio queries are showing potential conflicts if we were to rename it to: functions, netlify-functions

From early results it seems that functions is used by too many sites for other purposes than Netlify Functions. This is an example where functions is used for Firebase functions.

However, netlify-functions is a more likely candidate. Let's run this for one week and see the results.

[ehmicky]

We have added warning logs for the upcoming default directories netlify/ without netlify/functions/ and with netlify/functions/.

@verythorough

[ehmicky]

The final behavior should look like this for users:

• If a user has a netlify/functions/ directory in their build directory, this will be used as their functions directory. Previously, users had to define a Functions directory (there was no default value).
• The netlify/functions/ directory can also be created at build time. This can happen either in the build command or any build plugin.

Some additional details:

• @netlify/config assigns netlify/functions/ as a default value for the build.functions configuration property, but only if that directory exists (link to source).
• @netlify/build sets constants.FUNCTIONS_SRC using the return value from @netlify/config (link to source). This means this value is defined if either:
  • The Functions directory was configured by the user (netlify.toml or UI build settings)
  • A netlify/functions/ directory exists.
• If a build command or Build plugin creates netlify/functions/ and constants.FUNCTIONS_SRC is not set yet, it is automatically set. This allows the Functions directory to be created at build time (link to source). This is currently used by our Next.js plugin (link to source).
• If the user specifies a path to a Functions directory but that directory does not exist (or is a regular file instead of a directory), the build fails (link to source)

At the moment, netlify/functions/ is using a different undocumented name netlify-automatic-functions

• We want to remove that name with netlify/functions/ instead.
• This will be a breaking change for sites already using that directory location. We are printing warnings for a few weeks to give users a chance to migrate.
• netlify/functions/ is a sub-directory while netlify-automatic-functions. We'll need to make sure this is not a problem with the current implementation.

Those are the remaining steps to complete this feature:

• Remove the warning messages printed for sites with netlify/ or netlify/functions/ directories (link to issue).
• Remove the warning by CLI messages printed for sites with netlify/ or netlify/functions directories (link to issue)
• Rename netlify-automatic-functions to netlify/functions in @netlify/build. This should only be released in February.
• Rename netlify-automatic-functions to netlify/functions in @netlify/plugin-nextjs. This should be done in the same release to prevent any problems.
• Document this new behavior (link to issue).
• Update the Netlify community post
• Update and close the related issues: https://github.com/netlify/pod-the-builder/issues/91, https://github.com/netlify/product/issues/257, https://github.com/netlify/pod-the-builder/issues/89