feat: pass logger/child logger as param to mixin

docs/api.md

@@ -121,6 +121,8 @@ Default: `undefined`
 
 If provided, the `mixin` function is called each time one of the active
 logging methods is called. The first parameter is the value `mergeObject` or an empty object. The second parameter is the log level number.
+The third parameter is the logger or child logger itself, which can be used to
+retrieve logger-specific context from within the `mixin` function.
 The function must synchronously return an object. The properties of the returned object will be added to the
 logged JSON.
 
@@ -177,7 +179,40 @@ logger.error('Message 2')
 ```
 
 If the `mixin` feature is being used merely to add static metadata to each log message,
-then a [child logger ⇗](/docs/child-loggers.md) should be used instead.
+then a [child logger ⇗](/docs/child-loggers.md) should be used instead. Unless your application
+needs to concatenate values for a specific key multiple times, in which case `mixin` can be
+used to avoid the [duplicate keys caveat](/docs/child-loggers.md#duplicate-keys-caveat):
+
+```js
+const logger = pino({
+  mixin (obj, num, logger) {
+    return {
+      tags: logger.tags
+    }
+  }
+})
+logger.tags = {}
+
+logger.addTag = function (key, value) {
+  logger.tags[key] = value
+}
+
+function createChild (parent, ...context) {
+  const newChild = logger.child(...context)
+  newChild.tags = { ...logger.tags }
+  newChild.addTag = function (key, value) {
+    newChild.tags[key] = value
+  }
+  return newChild
+}
+
+logger.addTag('foo', 1)
+const child = createChild(logger, {})
+child.addTag('bar', 2)
+logger.info('this will only have `foo: 1`')
+child.info('this will have both `foo: 1` and `bar: 2`')
+logger.info('this will still only have `foo: 1`')
+```
 
 As of pino 7.x, when the `mixin` is used with the [`nestedKey` option](#opt-nestedkey), 
 the object returned from the `mixin` method will also be nested. Prior versions would mix 

lib/proto.js

@@ -194,7 +194,7 @@ function write (_obj, msg, num) {
   }
 
   if (mixin) {
-    obj = mixinMergeStrategy(obj, mixin(obj, num))
+    obj = mixinMergeStrategy(obj, mixin(obj, num, this))
   }
 
   const s = this[asJsonSym](obj, msg, num, t)

test/mixin.test.js

@@ -160,3 +160,59 @@ test('mixin can use level number', async ({ ok, same }) => {
     stack: 'stack'
   }, 'test')
 })
+
+test('mixin receives logger as third parameter', async ({ ok, same }) => {
+  const stream = sink()
+  const instance = pino({
+    mixin (context, num, logger) {
+      ok(logger !== null, 'logger should be defined')
+      ok(logger !== undefined, 'logger should be defined')
+      same(logger, instance)
+      return { ...context, num }
+    }
+  }, stream)
+  instance.level = name
+  instance[name]({
+    message: '123'
+  }, 'test')
+})
+
+test('mixin receives child logger', async ({ ok, same }) => {
+  const stream = sink()
+  let child = null
+  const instance = pino({
+    mixin (context, num, logger) {
+      ok(logger !== null, 'logger should be defined')
+      ok(logger !== undefined, 'logger should be defined')
+      same(logger.expected, child.expected)
+      return { ...context, num }
+    }
+  }, stream)
+  instance.level = name
+  instance.expected = false
+  child = instance.child({})
+  child.expected = true
+  child[name]({
+    message: '123'
+  }, 'test')
+})
+
+test('mixin receives logger even if child exists', async ({ ok, same }) => {
+  const stream = sink()
+  let child = null
+  const instance = pino({
+    mixin (context, num, logger) {
+      ok(logger !== null, 'logger should be defined')
+      ok(logger !== undefined, 'logger should be defined')
+      same(logger.expected, instance.expected)
+      return { ...context, num }
+    }
+  }, stream)
+  instance.level = name
+  instance.expected = false
+  child = instance.child({})
+  child.expected = true
+  instance[name]({
+    message: '123'
+  }, 'test')
+})