refactor: provide via "systemLog" function

packages/build/src/plugins/child/run.js

@@ -2,6 +2,7 @@ import { getNewEnvChanges, setEnvChanges } from '../../env/changes.js'
 import { logPluginMethodStart, logPluginMethodEnd } from '../../log/messages/ipc.js'
 
 import { cloneNetlifyConfig, getConfigMutations } from './diff.js'
+import { getSystemLog } from './systemLog.js'
 import { getUtils } from './utils.js'
 
 // Run a specific plugin event handler
@@ -11,9 +12,19 @@ export const run = async function (
 ) {
   const method = methods[event]
   const runState = {}
+  const systemLog = getSystemLog()
   const utils = getUtils({ event, constants, runState })
   const netlifyConfigCopy = cloneNetlifyConfig(netlifyConfig)
-  const runOptions = { utils, constants, inputs, netlifyConfig: netlifyConfigCopy, packageJson, error, featureFlags }
+  const runOptions = {
+    utils,
+    constants,
+    inputs,
+    netlifyConfig: netlifyConfigCopy,
+    packageJson,
+    error,
+    featureFlags,
+    systemLog,
+  }
 
   const envBefore = setEnvChanges(envChanges)
 

packages/build/src/plugins/child/systemLog.js

@@ -0,0 +1,15 @@
+import { appendFileSync, openSync } from 'fs'
+
+const systemLogLocation = '/dev/fd/4'
+
+export const getSystemLog = () => {
+  try {
+    // throws if system log wasn't hooked up
+    const fd = openSync(systemLogLocation, 'a')
+    return (message) => {
+      appendFileSync(fd, `${message}\n`)
+    }
+  } catch {
+    return
+  }
+}

packages/build/tests/plugins/fixtures/feature_flags/plugin.js

@@ -1,8 +1,4 @@
-import { appendFileSync } from  "fs"
-
-const systemLog = "/dev/fd/4" // 🪄 magic ✨
-
-export const onBuild = function ({ featureFlags }) {
-  appendFileSync(systemLog, "some system-facing logs\n")
+export const onBuild = function ({ featureFlags, systemLog }) {
+  systemLog("some system-facing logs")
   console.log(JSON.stringify(featureFlags))
 }

packages/build/tests/plugins/fixtures/feature_flags_untrusted/plugin.js

@@ -1,16 +1,5 @@
-import { appendFileSync } from  "fs"
-
-const isSystemLogAccessible = () => {
-  try {
-    appendFileSync("/dev/fd/4", "some system-facing logs")
-    return true
-  } catch (error) {
-    return false
-  }
-}
-
-export const onBuild = function ({ featureFlags }) {
-  if (isSystemLogAccessible()) {
+export const onBuild = function ({ featureFlags, systemLog }) {
+  if (systemLog) {
     throw new Error("System log shouldn't be acessible from untrusted plugins")
   }
   console.log("typeof featureflags:", typeof featureFlags)