pass append option on pino/file transport to pino.destination function

docs/transports.md

@@ -251,6 +251,17 @@ const transport = pino.transport({
 pino(transport)
 ```
 
+By default, the `pino/file` transport appends to the destination file if it exists. The `append` option may be set to `false` to configure the transport to truncate the file upon opening it for writing.
+
+```js
+const pino = require('pino')
+const transport = pino.transport({
+  target: 'pino/file',
+  options: { destination: '/path/to/file', append: false }
+})
+pino(transport)
+```
+
 The `options.destination` property may also be a number to represent a file descriptor. Typically this would be `1` to write to STDOUT or `2` to write to STDERR. If `options.destination` is not set, it defaults to `1` which means logs will be written to STDOUT.
 
 The difference between using the `pino/file` transport builtin and using `pino.destination` is that `pino.destination` runs in the main thread, whereas `pino/file` sets up `pino.destination` in a worker thread.

file.js

@@ -5,6 +5,7 @@ const { once } = require('events')
 
 module.exports = async function (opts = {}) {
   const destOpts = { dest: opts.destination || 1, sync: false }
+  if (opts.append === false) destOpts.append = false
   if (opts.mkdir) destOpts.mkdir = true
   const destination = pino.destination(destOpts)
   await once(destination, 'ready')

test/helper.js

@@ -3,7 +3,7 @@
 const os = require('os')
 const writer = require('flush-write-stream')
 const split = require('split2')
-const { existsSync, statSync } = require('fs')
+const { existsSync, readFileSync, statSync } = require('fs')
 const pid = process.pid
 const hostname = os.hostname()
 
@@ -77,4 +77,24 @@ function watchFileCreated (filename) {
   })
 }
 
-module.exports = { getPathToNull, sink, check, once, sleep, watchFileCreated, isWin, isYarnPnp }
+function watchForWrite (filename, testString) {
+  return new Promise((resolve, reject) => {
+    const TIMEOUT = 2000
+    const INTERVAL = 100
+    const threshold = TIMEOUT / INTERVAL
+    let counter = 0
+    const interval = setInterval(() => {
+      if (readFileSync(filename).includes(testString)) {
+        clearInterval(interval)
+        resolve()
+      } else if (counter <= threshold) {
+        counter++
+      } else {
+        clearInterval(interval)
+        reject(new Error(`'${testString}' hasn't been written to ${filename} within ${TIMEOUT} ms.`))
+      }
+    }, INTERVAL)
+  })
+}
+
+module.exports = { getPathToNull, sink, check, once, sleep, watchFileCreated, watchForWrite, isWin, isYarnPnp }

test/transport/core.test.js

@@ -3,8 +3,8 @@
 const os = require('os')
 const { join } = require('path')
 const { once } = require('events')
-const { readFile } = require('fs').promises
-const { watchFileCreated } = require('../helper')
+const { readFile, writeFile } = require('fs').promises
+const { watchFileCreated, watchForWrite } = require('../helper')
 const { test } = require('tap')
 const pino = require('../../')
 const url = require('url')
@@ -300,6 +300,30 @@ test('pino.transport with target pino/file and mkdir option', async ({ same, tea
   })
 })
 
+test('pino.transport with target pino/file and append option', async ({ same, teardown }) => {
+  const destination = join(
+    os.tmpdir(),
+    '_' + Math.random().toString(36).substr(2, 9)
+  )
+  await writeFile(destination, JSON.stringify({ pid, hostname, time: Date.now(), level: 30, msg: 'hello' }))
+  const transport = pino.transport({
+    target: 'pino/file',
+    options: { destination, append: false }
+  })
+  teardown(transport.end.bind(transport))
+  const instance = pino(transport)
+  instance.info('goodbye')
+  await watchForWrite(destination, '"goodbye"')
+  const result = JSON.parse(await readFile(destination))
+  delete result.time
+  same(result, {
+    pid,
+    hostname,
+    level: 30,
+    msg: 'goodbye'
+  })
+})
+
 test('pino.transport with target pino-pretty', async ({ match, teardown }) => {
   const destination = join(
     os.tmpdir(),