fix: only activate once, when on (#1824)

While debugging the Jest memory leak, I was surprised to find the Nock
was "activating" twice when imported. This is because intercept.js
was calling `activate()` directly _AND_ back was setting the mode, which
calls `restore()` and `activate()`. There is no need for this double
initialization. Think of the CPUs we could be saving.
I also discovered that Nock "activates" even if the proc is started with
the `NOCK_OFF` env var set, which means Nock is polluting the global
scope even when callers have asked it not to.

Since it felt like the `back.setMode()` call was hiding in its module,
I've moved it to the entry index file and wrapped it in a `isOn` check.

index.js

@@ -6,6 +6,7 @@ const {
   activate,
   isActive,
   isDone,
+  isOn,
   pendingMocks,
   activeMocks,
   removeInterceptor,
@@ -31,7 +32,7 @@ Object.assign(module.exports, {
   // TODO-12.x Historically `nock.cleanAll()` has returned the nock global.
   // The other global methods do not do this, so it's not clear this was
   // deliberate or is even helpful. This shim is included for backward
-  // compatibility and shoulud be replaced with an alias to `removeAll()`.
+  // compatibility and should be replaced with an alias to `removeAll()`.
   cleanAll() {
     removeAll()
     return module.exports
@@ -49,3 +50,11 @@ Object.assign(module.exports, {
   restore: recorder.restore,
   back,
 })
+
+// We always activate Nock on import, overriding the globals.
+// Setting the Back mode "activates" Nock by overriding the global entries in the `http/s` modules.
+// If Nock Back is configured, we need to honor that setting for backward compatibility,
+// otherwise we rely on Nock Back's default initializing side effect.
+if (isOn()) {
+  back.setMode(process.env.NOCK_BACK_MODE || 'dryrun')
+}

lib/back.js

@@ -284,6 +284,5 @@ Back.setMode = function(mode) {
 
 Back.fixtures = null
 Back.currentMode = null
-Back.setMode(process.env.NOCK_BACK_MODE || 'dryrun')
 
 module.exports = Back

lib/intercept.js

@@ -428,8 +428,6 @@ function activate() {
   })
 }
 
-activate()
-
 module.exports = {
   addInterceptor,
   remove,
@@ -443,7 +441,6 @@ module.exports = {
   activeMocks,
   enableNetConnect,
   disableNetConnect,
-  overrideClientRequest,
   restoreOverriddenClientRequest,
   abortPendingRequests: common.removeAllTimers,
 }

tests/test_back.js

@@ -502,6 +502,7 @@ test('assertScopesFinished throws exception when Back still has pending scopes',
 
 test('nockBack dryrun throws the expected exception when fs is not available', t => {
   const nockBackWithoutFs = proxyquire('../lib/back', { fs: null })
+  nockBackWithoutFs.setMode('dryrun')
 
   nockBackWithoutFs.fixtures = `${__dirname}/fixtures`
   t.throws(() => nockBackWithoutFs('goodRequest.json'), { message: 'no fs' })

tests/test_nock_off.js

@@ -1,6 +1,7 @@
 'use strict'
 
 const { expect } = require('chai')
+const http = require('http')
 const nock = require('..')
 const got = require('./got_client')
 const ssl = require('./ssl')
@@ -41,4 +42,15 @@ describe('NOCK_OFF env var', () => {
     expect(body).to.equal(responseBody)
     scope.done()
   })
+
+  it('when true before import, Nock does not activate', async () => {
+    nock.restore()
+    const originalClient = http.ClientRequest
+
+    delete require.cache[require.resolve('..')]
+    const newNock = require('..')
+
+    expect(http.ClientRequest).to.equal(originalClient)
+    expect(newNock.isActive()).to.equal(false)
+  })
 })