fix: prevent duplicate debug logs when multiple undici instances exist (#4630)

Author: mcollina

lib/core/diagnostics.js

@@ -36,6 +36,14 @@ function trackClientEvents (debugLog = undiciDebugLog) {
     return
   }
 
+  // Check if any of the channels already have subscribers to prevent duplicate subscriptions
+  // This can happen when both Node.js built-in undici and undici as a dependency are present
+  if (channels.beforeConnect.hasSubscribers || channels.connected.hasSubscribers ||
+      channels.connectError.hasSubscribers || channels.sendHeaders.hasSubscribers) {
+    isTrackingClientEvents = true
+    return
+  }
+
   isTrackingClientEvents = true
 
   diagnosticsChannel.subscribe('undici:client:beforeConnect',
@@ -98,6 +106,14 @@ function trackRequestEvents (debugLog = undiciDebugLog) {
     return
   }
 
+  // Check if any of the channels already have subscribers to prevent duplicate subscriptions
+  // This can happen when both Node.js built-in undici and undici as a dependency are present
+  if (channels.headers.hasSubscribers || channels.trailers.hasSubscribers ||
+      channels.error.hasSubscribers) {
+    isTrackingRequestEvents = true
+    return
+  }
+
   isTrackingRequestEvents = true
 
   diagnosticsChannel.subscribe('undici:request:headers',
@@ -146,6 +162,15 @@ function trackWebSocketEvents (debugLog = websocketDebuglog) {
     return
   }
 
+  // Check if any of the channels already have subscribers to prevent duplicate subscriptions
+  // This can happen when both Node.js built-in undici and undici as a dependency are present
+  if (channels.open.hasSubscribers || channels.close.hasSubscribers ||
+      channels.socketError.hasSubscribers || channels.ping.hasSubscribers ||
+      channels.pong.hasSubscribers) {
+    isTrackingWebSocketEvents = true
+    return
+  }
+
   isTrackingWebSocketEvents = true
 
   diagnosticsChannel.subscribe('undici:websocket:open',

test/fixtures/duplicate-debug.js

@@ -0,0 +1,24 @@
+'use strict'
+
+const { createServer } = require('node:http')
+const { request } = require('../..')
+
+// Simulate the scenario where diagnostics module is loaded multiple times
+// This mimics having both Node.js built-in undici and undici as dependency
+delete require.cache[require.resolve('../../lib/core/diagnostics.js')]
+require('../../lib/core/diagnostics.js')
+
+const server = createServer({ joinDuplicateHeaders: true }, (_req, res) => {
+  res.writeHead(200, { 'Content-Type': 'text/plain' })
+  res.end('hello world')
+})
+
+server.listen(0, () => {
+  const { port, address, family } = server.address()
+  const hostname = family === 'IPv6' ? `[${address}]` : address
+  request(`http://${hostname}:${port}`)
+    .then(res => res.body.dump())
+    .then(() => {
+      server.close()
+    })
+})

test/node-test/debug.js

@@ -126,6 +126,46 @@ test('debug#undici', { skip: isCITGM || isNode23Plus }, async t => {
   await assert.completed
 })
 
+test('debug#undici no duplicates', { skip: isCITGM || isNode23Plus }, async t => {
+  const assert = tspl(t, { plan: 7 })
+  const child = spawn(
+    process.execPath,
+    [
+      '--no-experimental-fetch',
+      join(__dirname, '../fixtures/duplicate-debug.js')
+    ],
+    {
+      env: {
+        NODE_DEBUG: 'undici'
+      }
+    }
+  )
+  const chunks = []
+  const assertions = [
+    /(UNDICI [0-9]+:) (connecting to)/,
+    /(UNDICI [0-9]+:) (connected to)/,
+    /(UNDICI [0-9]+:) (sending request)/,
+    /(UNDICI [0-9]+:) (received response)/,
+    /(UNDICI [0-9]+:) (trailers received)/,
+    /^$/
+  ]
+
+  child.stderr.setEncoding('utf8')
+  child.stderr.on('data', chunk => {
+    chunks.push(chunk)
+  })
+  child.stderr.on('end', () => {
+    const lines = extractLines(chunks)
+    // Should have exactly the expected number of lines, no duplicates
+    assert.strictEqual(lines.length, assertions.length, 'Should not have duplicate log lines')
+    for (let i = 0; i < lines.length; i++) {
+      assert.match(lines[i], assertions[i])
+    }
+  })
+
+  await assert.completed
+})
+
 function extractLines (chunks) {
   return chunks
     .join('')