feat: keep parent formatters bindings function option

docs/api.md

@@ -378,12 +378,31 @@ const hooks = {
 <a id=opt-formatters></a>
 #### `formatters` (Object)
 
-An object containing functions for formatting the shape of the log lines.
+An object containing an `options` object and functions for formatting
+the shape of the log lines.
 These functions should return a JSONifiable object and
 should never throw. These functions allow for full customization of
 the resulting log lines. For example, they can be used to change
 the level key name or to enrich the default metadata.
 
+##### `options`
+
+The formatters `options` object is passed to the child loggers, except if a new formatters `options` object is passed to the `child()` method.
+
+###### `keepParentBindings`
+
+If `true`, child loggers will keep their parent `bindings` formatters function.
+
+If `false` or `undefined`, child loggers will have their `bindings` formatters function reset to a "no-op" formatter function.
+
+```js
+const formatters = {
+  options: {
+    keepParentBindings: true
+  }
+}
+```
+
 ##### `level`
 
 Changes the shape of the log level. The default shape is `{ level: number }`.

lib/proto.js

@@ -78,7 +78,7 @@ module.exports = function () {
   return Object.create(prototype)
 }
 
-const resetChildingsFormatter = bindings => bindings
+const noOpFormatter = bindings => bindings
 function child (bindings, options) {
   if (!bindings) {
     throw Error('missing bindings for child Pino')
@@ -110,17 +110,20 @@ function child (bindings, options) {
       instance[serializersSym][bks] = options.serializers[bks]
     }
   } else instance[serializersSym] = serializers
+  const keepParentBindings = typeof formatters.options === 'object' && formatters.options.keepParentBindings
   if (options.hasOwnProperty('formatters')) {
-    const { level, bindings: chindings, log } = options.formatters
+    const { options: formattersOptions, level, bindings: chindings, log } = options.formatters
     instance[formattersSym] = buildFormatters(
+      formattersOptions || formatters.options,
       level || formatters.level,
-      chindings || resetChildingsFormatter,
+      chindings || (keepParentBindings ? formatters.bindings || noOpFormatter : noOpFormatter),
       log || formatters.log
     )
   } else {
     instance[formattersSym] = buildFormatters(
+      formatters.options,
       formatters.level,
-      resetChildingsFormatter,
+      keepParentBindings ? formatters.bindings || noOpFormatter : noOpFormatter,
       formatters.log
     )
   }

lib/tools.js

@@ -352,8 +352,9 @@ function stringify (obj, stringifySafeFn) {
   }
 }
 
-function buildFormatters (level, bindings, log) {
+function buildFormatters (options, level, bindings, log) {
   return {
+    options,
     level,
     bindings,
     log

pino.js

@@ -118,6 +118,7 @@ function pino (...args) {
   })
 
   const allFormatters = buildFormatters(
+    formatters.options,
     formatters.level,
     formatters.bindings,
     formatters.log

test/formatters.test.js

@@ -200,6 +200,45 @@ test('Formatters in child logger', async ({ match }) => {
   })
 })
 
+test('Formatters with keepParentBindings option', async ({ match }) => {
+  const stream = sink()
+
+  const logger = pino({
+    formatters: {
+      options: {
+        keepParentBindings: true
+      },
+      bindings (bindings) {
+        if (bindings.foobar) bindings.foobar = bindings.foobar.toUpperCase()
+        if (bindings.faz) bindings.faz = bindings.faz.toUpperCase()
+        if (bindings.foobat) bindings.foobat = bindings.foobat.toUpperCase()
+        if (bindings.foobaz) bindings.foobaz = bindings.foobaz.toUpperCase()
+        return bindings
+      }
+    }
+  }, stream)
+  logger.setBindings({ foobar: 'foobar' })
+
+  const child = logger.child({
+    faz: 'baz',
+    nested: { object: true }
+  })
+  child.setBindings({ foobat: 'foobat' })
+
+  const childChild = child.child({ foobaz: 'foobaz' })
+
+  const o = once(stream, 'data')
+  childChild.info({ foo: 'bar', nested: { object: true } }, 'hello world')
+  match(await o, {
+    foobar: 'FOOBAR',
+    faz: 'BAZ',
+    foobat: 'FOOBAT',
+    foobaz: 'FOOBAZ',
+    foo: 'bar',
+    nested: { object: true }
+  })
+})
+
 test('Formatters without bindings in child logger', async ({ match }) => {
   const stream = sink()
   const logger = pino({