Log one-time warning when not specifying a partitioner

This is to hopefully avoid people mistakenly using the wrong partitioner
because they're relying on the default. This warning will be removed in
a future version.

src/index.js

@@ -11,6 +11,8 @@ const createConsumer = require('./consumer')
 const createAdmin = require('./admin')
 const ISOLATION_LEVEL = require('./protocol/isolationLevel')
 const defaultSocketFactory = require('./network/socketFactory')
+const once = require('./utils/once')
+const websiteUrl = require('./utils/websiteUrl')
 
 const PRIVATE = {
   CREATE_CLUSTER: Symbol('private:Kafka:createCluster'),
@@ -20,6 +22,15 @@ const PRIVATE = {
 }
 
 const DEFAULT_METADATA_MAX_AGE = 300000
+const warnOfDefaultPartitioner = once(logger => {
+  if (process.env.KAFKAJS_NO_PARTITIONER_WARNING == null) {
+    logger.warn(
+      `KafkaJS v2.0.0 switched default partitioner. To retain the same partitioning behavior as in previous versions, create the producer with the option "createPartitioner: Partitioners.LegacyPartitioner". See ${websiteUrl(
+        '/docs/2.0.0/migration-guide-v2.0.0'
+      )} for details. Silence this warning by setting the environment variable "KAFKAJS_NO_PARTITIONER_WARNING=1"`
+    )
+  }
+})
 
 module.exports = class Client {
   /**
@@ -104,6 +115,10 @@ module.exports = class Client {
       instrumentationEmitter,
     })
 
+    if (createPartitioner == null) {
+      warnOfDefaultPartitioner(this[PRIVATE.LOGGER])
+    }
+
     return createProducer({
       retry: { ...this[PRIVATE.CLUSTER_RETRY], ...retry },
       logger: this[PRIVATE.LOGGER],

src/utils/once.js

@@ -0,0 +1,10 @@
+module.exports = fn => {
+  let called = false
+
+  return (...args) => {
+    if (!called) {
+      called = true
+      return fn(...args)
+    }
+  }
+}

src/utils/once.spec.js

@@ -0,0 +1,14 @@
+const once = require('./once')
+
+describe('Utils > once', () => {
+  it('should call the wrapped function only once', () => {
+    const original = jest.fn().mockReturnValue('foo')
+    const wrapped = once(original)
+
+    expect(wrapped('hello')).toEqual('foo')
+    expect(wrapped('hello')).toBeUndefined()
+
+    expect(original).toHaveBeenCalledTimes(1)
+    expect(original).toHaveBeenCalledWith('hello')
+  })
+})

src/utils/websiteUrl.js

@@ -1,3 +1,7 @@
 const BASE_URL = 'https://kafka.js.org'
+const stripLeading = char => str => (str.charAt(0) === char ? str.substring(1) : str)
+const stripLeadingSlash = stripLeading('/')
+const stripLeadingHash = stripLeading('#')
 
-module.exports = (path, hash) => `${BASE_URL}/${path}${hash ? '#' + hash : ''}`
+module.exports = (path, hash) =>
+  `${BASE_URL}/${stripLeadingSlash(path)}${hash ? '#' + stripLeadingHash(hash) : ''}`

src/utils/websiteUrl.spec.js

@@ -1,14 +1,15 @@
 const websiteUrl = require('./websiteUrl')
 
 describe('Utils > websiteUrl', () => {
-  it('generates links to the website', () => {
-    expect(websiteUrl('docs/faq')).toEqual('https://kafka.js.org/docs/faq')
+  it.each([['docs/faq'], ['/docs/faq']])('generates links to the website', path => {
+    expect(websiteUrl(path)).toEqual('https://kafka.js.org/docs/faq')
   })
 
-  it('allows specifying a hash', () => {
-    expect(
-      websiteUrl('docs/faq', 'why-am-i-receiving-messages-for-topics-i-m-not-subscribed-to')
-    ).toEqual(
+  it.each([
+    ['why-am-i-receiving-messages-for-topics-i-m-not-subscribed-to'],
+    ['#why-am-i-receiving-messages-for-topics-i-m-not-subscribed-to'],
+  ])('allows specifying an anchor', anchor => {
+    expect(websiteUrl('docs/faq', anchor)).toEqual(
       'https://kafka.js.org/docs/faq#why-am-i-receiving-messages-for-topics-i-m-not-subscribed-to'
     )
   })