feat: Add support for env.NYC_CONFIG_OVERRIDE

This allows a process to spawn a child process with an environment
variable NYC_CONFIG_OVERRIDE.  This is a JSON string which overrides any
values in the NYC_CONFIG env.  It is not deleted in the child process,
so will be contagious unless unset, and it is the responsibility of the
caller to manage.

The first intended use case of this feature is to allow node-tap to map
tests to the specific portion of a system under test that they cover.
In this way, unit tests can be more focused, and only re-run when their
specific unit has changed.

There are, of course, many other uses that this could be put to, and it
did not seem appropriate to add a special hook _just_ for overriding
the include list.

bin/wrap.js

@@ -16,6 +16,12 @@ if (process.env.NYC_PROCESSINFO_EXTERNAL_ID) {
   delete process.env.NYC_PROCESSINFO_EXTERNAL_ID
 }
 
+if (process.env.NYC_CONFIG_OVERRIDE) {
+  var override = JSON.parse(process.env.NYC_CONFIG_OVERRIDE)
+  config = Object.assign(config, override)
+  process.env.NYC_CONFIG = JSON.stringify(config)
+}
+
 ;(new NYC(config)).wrap()
 
 sw.runMain()

tap-snapshots/test-config-override.js-TAP.test.js

@@ -0,0 +1,20 @@
+/* IMPORTANT
+ * This snapshot file is auto-generated, but designed for humans.
+ * It should be checked into source control and tracked carefully.
+ * Re-generate by setting TAP_SNAPSHOT=1 and running tests.
+ * Make sure to inspect the output below.  Do not ignore changes!
+ */
+'use strict'
+exports[`test/config-override.js TAP spawn that does config overriding > stdout 1`] = `
+in parent { include: 'conf-override-root.js' }
+in child { include: 'conf-override-module.js' }
+in module { include: 'conf-override-module.js' }
+-------------------------|----------|----------|----------|----------|-------------------|
+File                     |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
+-------------------------|----------|----------|----------|----------|-------------------|
+All files                |    77.78 |       50 |      100 |    77.78 |                   |
+ conf-override-module.js |      100 |      100 |      100 |      100 |                   |
+ conf-override-root.js   |    71.43 |       50 |      100 |    71.43 |             22,23 |
+-------------------------|----------|----------|----------|----------|-------------------|
+
+`

test/config-override.js

@@ -0,0 +1,33 @@
+const { spawn } = require('child_process')
+const { resolve } = require('path')
+const t = require('tap')
+const node = process.execPath
+const fixturesCLI = resolve(__dirname, './fixtures/cli')
+const bin = resolve(__dirname, '../self-coverage/bin/nyc')
+const rimraf = require('rimraf').sync
+const tmp = 'conf-override-test'
+
+rimraf(resolve(fixturesCLI, tmp))
+t.teardown(() => rimraf(resolve(fixturesCLI, tmp)))
+
+t.test('spawn that does config overriding', t => {
+  const args = [
+    bin, '-t', tmp,
+    '--exclude-after-remap=false',
+    '--include=conf-override-root.js',
+    node, 'conf-override-root.js'
+  ]
+  const proc = spawn(node, args, {
+    cwd: fixturesCLI
+  })
+  const out = []
+  const err = []
+  proc.stdout.on('data', c => out.push(c))
+  proc.stderr.on('data', c => err.push(c))
+  proc.on('close', (code, signal) => {
+    t.equal(code, 0)
+    t.equal(signal, null)
+    t.matchSnapshot(Buffer.concat(out).toString(), 'stdout')
+    t.end()
+  })
+})

test/fixtures/cli/conf-override-module.js

@@ -0,0 +1,2 @@
+const config = JSON.parse(process.env.NYC_CONFIG)
+console.log('in module', {include: config.include})

test/fixtures/cli/conf-override-root.js

@@ -0,0 +1,24 @@
+const config = JSON.parse(process.env.NYC_CONFIG)
+const { include } = config
+
+if (process.argv[2] !== 'child') {
+  console.log('in parent', { include })
+  require('child_process').spawn(process.execPath, [__filename, 'child'], {
+    cwd: __dirname,
+    env: Object.assign(
+      {},
+      process.env,
+      {
+        NYC_CONFIG_OVERRIDE: JSON.stringify({
+          include: 'conf-override-module.js'
+        })
+      }
+    ),
+    stdio: 'inherit',
+  })
+} else {
+  // this should run, but not be covered, even though the shebang says to
+  // the child run ONLY covers the child file, not the dump-root.js
+  console.log('in child', { include })
+  require('./conf-override-module.js')
+}