Reproducible pnpm issue with NPM Enterprise

When tying to install pnpm with an .npmrc file pointing to an NPM Enterprise instance, we're running an HTTP 406 error. Below are steps to reproduce this issue after checking out this repo while pointing to our internal NPM Enterprise instance (set in the .npmrc file):

1. If you have a node_modules directory, delete it

rm -rf node_modules/

2. If you have a ~/.pnpm-store directory, delete it

rm -rf ~/.pnpm-store

3. Install pnpm using npm version 3.10.10 or lower

npm install --save-dev pnpm

4. Run the local pnpm install so that pnpm can install itself

./node_modules/.bin/pnpm install

Expected result: pnpm installs successfully

Actual result:

└── pnpm@1.9.0

WARN Moving pnpm that was installed by a different package manager to "node_modules/.ignored
ERROR fetch failed with status code 406

After lots of investigating, we've determined that the following details matter a lot to get this bug to be reproducible:

1. There must be an .npmrc file that points to an NPM Enterprise instance.
   Removing it so that we point to the public repo seems to not surface the issue

2. You must be using a 3.x.x version of npm
   Installing pnpm using npm version 5.x.x does not surface the issue.

3. There has to already be a shrinkwrap.yaml file when you run pnpm install.
   If there isn't a shrinkwrap file to begin with, this issue does not surface. Confusingly, when the issue does surface, deleting the shrinkwrap.yaml file does not surface the issue, and when you compare the new shrinkwrap.yaml to the one that was deleted, there are no changes.

We did some debugging on the URL that pnpm is trying to install from.

When it installs successfully, it's pulling from:

https://npm-registry.compass.com/p/pnpm/_attachments/pnpm-1.9.0.tgz

When it fails, it's pulling from:

https://npm-registry.compass.com/pnpm/-/pnpm-1.9.0.tgz

Let us know if you need any further help reproducing this issue.

Thanks!
The Compass Frontend Foundations Team