Serialize err.cause in a nice way when possible (#78)

* Serialize `err.cause` in a nice way when possible

* Self-host pony-cause helpers

* Add pony-cause banner

lib/err-helpers.js

@@ -0,0 +1,113 @@
+'use strict'
+
+// **************************************************************
+// * Code initially copied/adapted from "pony-cause" npm module *
+// * Please upstream improvements there                         *
+// **************************************************************
+
+/**
+ * @param {Error|{ cause?: unknown|(()=>err)}} err
+ * @returns {Error|undefined}
+ */
+const getErrorCause = (err) => {
+  if (!err) return
+
+  /** @type {unknown} */
+  // @ts-ignore
+  const cause = err.cause
+
+  // VError / NError style causes
+  if (typeof cause === 'function') {
+    // @ts-ignore
+    const causeResult = err.cause()
+
+    return causeResult instanceof Error
+      ? causeResult
+      : undefined
+  } else {
+    return cause instanceof Error
+      ? cause
+      : undefined
+  }
+}
+
+/**
+ * Internal method that keeps a track of which error we have already added, to avoid circular recursion
+ *
+ * @private
+ * @param {Error} err
+ * @param {Set<Error>} seen
+ * @returns {string}
+ */
+const _stackWithCauses = (err, seen) => {
+  if (!(err instanceof Error)) return ''
+
+  const stack = err.stack || ''
+
+  // Ensure we don't go circular or crazily deep
+  if (seen.has(err)) {
+    return stack + '\ncauses have become circular...'
+  }
+
+  const cause = getErrorCause(err)
+
+  if (cause) {
+    seen.add(err)
+    return (stack + '\ncaused by: ' + _stackWithCauses(cause, seen))
+  } else {
+    return stack
+  }
+}
+
+/**
+ * @param {Error} err
+ * @returns {string}
+ */
+const stackWithCauses = (err) => _stackWithCauses(err, new Set())
+
+/**
+ * Internal method that keeps a track of which error we have already added, to avoid circular recursion
+ *
+ * @private
+ * @param {Error} err
+ * @param {Set<Error>} seen
+ * @param {boolean} [skip]
+ * @returns {string}
+ */
+const _messageWithCauses = (err, seen, skip) => {
+  if (!(err instanceof Error)) return ''
+
+  const message = skip ? '' : (err.message || '')
+
+  // Ensure we don't go circular or crazily deep
+  if (seen.has(err)) {
+    return message + ': ...'
+  }
+
+  const cause = getErrorCause(err)
+
+  if (cause) {
+    seen.add(err)
+
+    // @ts-ignore
+    const skipIfVErrorStyleCause = typeof err.cause === 'function'
+
+    return (message +
+      (skipIfVErrorStyleCause ? '' : ': ') +
+      _messageWithCauses(cause, seen, skipIfVErrorStyleCause))
+  } else {
+    return message
+  }
+}
+
+/**
+ * @param {Error} err
+ * @returns {string}
+ */
+const messageWithCauses = (err) => _messageWithCauses(err, new Set())
+
+module.exports = {
+  getErrorCause,
+  stackWithCauses,
+  messageWithCauses
+}

lib/err.js

@@ -2,6 +2,8 @@
 
 module.exports = errSerializer
 
+const { messageWithCauses, stackWithCauses } = require('./err-helpers')
+
 const { toString } = Object.prototype
 const seen = Symbol('circular-ref-tag')
 const rawSymbol = Symbol('pino-raw-err-ref')
@@ -46,12 +48,12 @@ function errSerializer (err) {
   _err.type = toString.call(err.constructor) === '[object Function]'
     ? err.constructor.name
     : err.name
-  _err.message = err.message
-  _err.stack = err.stack
+  _err.message = messageWithCauses(err)
+  _err.stack = stackWithCauses(err)
   for (const key in err) {
     if (_err[key] === undefined) {
       const val = err[key]
-      if (val instanceof Error) {
+      if (val instanceof Error && key !== 'cause') {
         /* eslint-disable no-prototype-builtins */
         if (!val.hasOwnProperty(seen)) {
           _err[key] = errSerializer(val)

test/err.test.js

@@ -46,6 +46,38 @@ test('serializes nested errors', function (t) {
   t.match(serialized.inner.stack, /err\.test\.js:/)
 })
 
+test('serializes error causes', function (t) {
+  t.plan(6)
+  const err = Error('foo')
+  err.cause = Error('bar')
+  err.cause.cause = Error('abc')
+  const serialized = serializer(err)
+  t.is(serialized.type, 'Error')
+  t.is(serialized.message, 'foo: bar: abc')
+  t.match(serialized.stack, /err\.test\.js:/)
+  t.match(serialized.stack, /Error: foo/)
+  t.match(serialized.stack, /Error: bar/)
+  t.match(serialized.stack, /Error: abc/)
+})
+
+test('serializes error causes with VError support', function (t) {
+  t.plan(6)
+  // Fake VError-style setyp
+  const err = Error('foo: bar')
+  err.cause = () => {
+    const err = Error('bar')
+    err.cause = Error('abc')
+    return err
+  }
+  const serialized = serializer(err)
+  t.is(serialized.type, 'Error')
+  t.is(serialized.message, 'foo: bar: abc')
+  t.match(serialized.stack, /err\.test\.js:/)
+  t.match(serialized.stack, /Error: foo/)
+  t.match(serialized.stack, /Error: bar/)
+  t.match(serialized.stack, /Error: abc/)
+})
+
 test('prevents infinite recursion', function (t) {
   t.plan(4)
   const err = Error('foo')