don't clobber manually-installed tracer with SSI

init.js

@@ -1,7 +1,29 @@
 'use strict'
 
-const tracer = require('.')
+const path = require('path')
+const Module = require('module')
 
-tracer.init()
+let initBailout = false
 
-module.exports = tracer
+if (process.env.DD_INJECTION_ENABLED) {
+  // If we're running via single-step install, and we're not in the app's
+  // node_modules, then we should not initialize the tracer. This prevents
+  // single-step-installed tracer from clobbering the manually-installed tracer.
+  let resolvedInApp
+  const entrypoint = process.argv[1]
+  try {
+    resolvedInApp = Module.createRequire(entrypoint).resolve('dd-trace')
+  } catch (e) {}
+  if (resolvedInApp) {
+    const ourselves = path.join(__dirname, 'index.js')
+    if (ourselves !== resolvedInApp) {
+      initBailout = true
+    }
+  }
+}
+
+if (!initBailout) {
+  const tracer = require('.')
+  tracer.init()
+  module.exports = tracer
+}

integration-tests/init.spec.js

@@ -0,0 +1,57 @@
+const {
+  createSandbox,
+  spawnProc
+} = require('./helpers')
+const { assert } = require('chai')
+const path = require('path')
+
+const DD_INJECTION_ENABLED = 'tracing'
+
+describe('init.js', () => {
+  let cwd, proc, sandbox
+
+  async function runTest (cwd, env, expected) {
+    return new Promise((resolve, reject) => {
+      spawnProc(path.join(cwd, 'init/index.js'), { cwd, env }, data => {
+        try {
+          assert.strictEqual(data.toString(), expected)
+          resolve()
+        } catch (e) {
+          reject(e)
+        }
+      }).then(subproc => {
+        proc = subproc
+      })
+    })
+  }
+
+  before(async () => {
+    sandbox = await createSandbox()
+    cwd = sandbox.folder
+  })
+  afterEach(() => {
+    proc && proc.kill()
+  })
+  after(() => {
+    return sandbox.remove()
+  })
+
+  context('when dd-trace is not in the app dir', () => {
+    const NODE_OPTIONS = `--require ${path.join(__dirname, '..', 'init.js')}`
+    it('should initialize the tracer, if no DD_INJECTION_ENABLED', () => {
+      return runTest(cwd, { NODE_OPTIONS }, 'true\n')
+    })
+    it('should not initialize the tracer, if DD_INJECTION_ENABLED', () => {
+      return runTest(cwd, { NODE_OPTIONS, DD_INJECTION_ENABLED }, 'false\n')
+    })
+  })
+  context('when dd-trace in the app dir', () => {
+    const NODE_OPTIONS = '--require dd-trace/init.js'
+    it('should initialize the tracer, if no DD_INJECTION_ENABLED', () => {
+      return runTest(cwd, { NODE_OPTIONS }, 'true\n')
+    })
+    it('should initialize the tracer, if DD_INJECTION_ENABLED', () => {
+      return runTest(cwd, { NODE_OPTIONS, DD_INJECTION_ENABLED }, 'true\n')
+    })
+  })
+})

integration-tests/init/index.js

@@ -0,0 +1,3 @@
+// eslint-disable-next-line no-console
+console.log(!!global._ddtrace)
+process.exit()