feat: add support for optional env var replacements in .npmrc (#8359)

<!-- What / Why -->
This solves problem described in #8335 in a backwards-compatible way.

<!-- Describe the request in detail. What it does and why it's being
changed. -->
This PR adds possibility to have env var replacements in .npmrc configs
written as `${VAR?}` which will cause them to get replaced with an empty
string if the variable is not defined. Old behavior where undefined
variables are left unreplaced is not changed.

## References
<!-- Examples:
  Related to #0
  Depends on #0
  Blocked by #0
  Fixes #0
  Closes #0
-->
Fixes #8335

---------

Co-authored-by: Michael Smith <[email protected]>

Author: aczekajski

docs/lib/content/configuring-npm/npmrc.md

@@ -25,11 +25,14 @@ The four relevant files are:
 * npm builtin config file (`/path/to/npm/npmrc`)
 
 All npm config files are an ini-formatted list of `key = value` parameters.
-Environment variables can be replaced using `${VARIABLE_NAME}`. For
+Environment variables can be replaced using `${VARIABLE_NAME}`. By default
+if the variable is not defined, it is left unreplaced. By adding `?` after
+variable name they can be forced to evaluate to an empty string instead. For
 example:
 
 ```bash
 cache = ${HOME}/.npm-packages
+node-options = "${NODE_OPTIONS?} --use-system-ca"
 ```
 
 Each of these files is loaded, and config options are resolved in priority

workspaces/config/lib/env-replace.js

@@ -1,9 +1,11 @@
 // replace any ${ENV} values with the appropriate environ.
+// optional "?" modifier can be used like this: ${ENV?} so in case of the variable being not defined, it evaluates into empty string.
 
-const envExpr = /(?<!\\)(\\*)\$\{([^${}]+)\}/g
+const envExpr = /(?<!\\)(\\*)\$\{([^${}?]+)(\?)?\}/g
 
-module.exports = (f, env) => f.replace(envExpr, (orig, esc, name) => {
-  const val = env[name] !== undefined ? env[name] : `$\{${name}}`
+module.exports = (f, env) => f.replace(envExpr, (orig, esc, name, modifier) => {
+  const fallback = modifier === '?' ? '' : `$\{${name}}`
+  const val = env[name] !== undefined ? env[name] : fallback
 
   // consume the escape chars that are relevant.
   if (esc.length % 2) {

workspaces/config/test/env-replace.js

@@ -6,8 +6,18 @@ const env = {
   bar: 'baz',
 }
 
-t.equal(envReplace('\\${foo}', env), '${foo}')
-t.equal(envReplace('\\\\${foo}', env), '\\bar')
-t.equal(envReplace('${baz}', env), '${baz}')
-t.equal(envReplace('\\${baz}', env), '${baz}')
-t.equal(envReplace('\\\\${baz}', env), '\\${baz}')
+t.equal(envReplace('${foo}', env), 'bar', 'replaces defined variable')
+t.equal(envReplace('${foo?}', env), 'bar', 'replaces defined variable with ? modifier')
+t.equal(envReplace('${foo}${bar}', env), 'barbaz', 'replaces multiple defined variables')
+t.equal(envReplace('${foo?}${baz?}', env), 'bar', 'replaces mixed defined/undefined variables with ? modifier')
+t.equal(envReplace('\\${foo}', env), '${foo}', 'escapes normal variable')
+t.equal(envReplace('\\\\${foo}', env), '\\bar', 'double escape allows replacement')
+t.equal(envReplace('\\\\\\${foo}', env), '\\${foo}', 'triple escape prevents replacement')
+t.equal(envReplace('${baz}', env), '${baz}', 'leaves undefined variable unreplaced')
+t.equal(envReplace('\\${baz}', env), '${baz}', 'escapes undefined variable')
+t.equal(envReplace('\\\\${baz}', env), '\\${baz}', 'double escape with undefined variable')
+t.equal(envReplace('\\${foo?}', env), '${foo?}', 'escapes optional variable')
+t.equal(envReplace('\\\\${foo?}', env), '\\bar', 'double escape allows optional replacement')
+t.equal(envReplace('${baz?}', env), '', 'replaces undefined variable with empty string when using ? modifier')
+t.equal(envReplace('\\${baz?}', env), '${baz?}', 'escapes undefined optional variable')
+t.equal(envReplace('\\\\${baz?}', env), '\\', 'double escape with undefined optional variable results in empty replacement')