Trace upload: only send traces for current session

The `.next/trace` file contains trace events spanning multiple traces from different sessions. This changes our upload process to only send trace events for the current session that’s ending.

Before this change, we would send a single set of metadata for many traces, but these older session traces could have been invoked with different parameters like `mode`, `isTurboSession`, etc. Because of this, we started including `isTurboSession` as a span attribute on spans, but this value was dependent on an environment variable that wasn’t always set. In this case, the top-level `next-dev` span never had the correct value.

This makes it so the metadata, now always determined at upload time, always reflects the session sent, and never any others.

Other bugs fixed:

- Trace events within a single logical session had different trace ids because of worker threads. This aligns them.
- Because we upload traces out-of-process, the Telemetry `sessionId` did not reflect the real session id of other analytics sent. This passes it along to the external process.

Test Plan:

In a test app, using `NEXT_TRACE_UPLOAD_DEBUG=1` and a running Otel server:
- Start a non-turbopack dev session and end it. Verify trace events are sent with `isTurboSession: false` and note the trace id.
- Start a turbopack dev session and end it. Verify trace events are sent with `isTurboSession: true` and note events are sent for this trace, and not the previous one.
- Start a build and verify trace events are sent with `mode: build` and `isTurboSession: false` and that no older trace events are sent.

packages/next/src/build/index.ts

@@ -3865,6 +3865,7 @@ export default async function build(
         mode: 'build',
         projectDir: dir,
         distDir: loadedConfig.distDir,
+        isTurboSession: turboNextBuild,
         sync: true,
       })
     }

packages/next/src/cli/next-dev.ts

@@ -37,6 +37,7 @@ import os from 'os'
 import { once } from 'node:events'
 import { clearTimeout } from 'timers'
 import { flushAllTraces, trace } from '../trace'
+import { traceId } from '../trace/shared'
 
 export type NextDevOptions = {
   turbo?: boolean
@@ -137,6 +138,7 @@ const handleSessionStop = async (signal: NodeJS.Signals | number | null) => {
       mode: 'dev',
       projectDir: dir,
       distDir: config.distDir,
+      isTurboSession,
     })
   }
 
@@ -277,6 +279,7 @@ const nextDev = async (
           ...defaultEnv,
           TURBOPACK: process.env.TURBOPACK,
           NEXT_PRIVATE_WORKER: '1',
+          NEXT_PRIVATE_TRACE_ID: traceId,
           NODE_EXTRA_CA_CERTS: startServerOptions.selfSignedCertificate
             ? startServerOptions.selfSignedCertificate.rootCA
             : defaultEnv.NODE_EXTRA_CA_CERTS,
@@ -309,6 +312,7 @@ const nextDev = async (
               mode: 'dev',
               projectDir: dir,
               distDir: config.distDir,
+              isTurboSession,
               sync: true,
             })
           }

packages/next/src/trace/report/to-json.ts

@@ -1,5 +1,4 @@
-import { randomBytes } from 'crypto'
-import { traceGlobals } from '../shared'
+import { traceGlobals, traceId } from '../shared'
 import fs from 'fs'
 import path from 'path'
 import { PHASE_DEVELOPMENT_SERVER } from '../../shared/lib/constants'
@@ -44,7 +43,6 @@ export function batcher(reportEvents: (evts: Event[]) => Promise<void>) {
 }
 
 let writeStream: RotatingWriteStream
-let traceId: string
 let batch: ReturnType<typeof batcher> | undefined
 
 const writeStreamOptions = {
@@ -117,10 +115,6 @@ const reportToLocalHost = (event: TraceEvent) => {
     return
   }
 
-  if (!traceId) {
-    traceId = process.env.TRACE_ID || randomBytes(8).toString('hex')
-  }
-
   if (!batch) {
     batch = batcher(async (events: Event[]) => {
       if (!writeStream) {

packages/next/src/trace/shared.ts

@@ -1,3 +1,5 @@
+import { randomBytes } from 'node:crypto'
+
 let _traceGlobals: Map<any, any> = (global as any)._traceGlobals
 
 if (!_traceGlobals) {
@@ -9,3 +11,8 @@ export const traceGlobals: Map<any, any> = _traceGlobals
 export const setGlobal = (key: any, val: any) => {
   traceGlobals.set(key, val)
 }
+
+export const traceId =
+  process.env.TRACE_ID ||
+  process.env.NEXT_PRIVATE_TRACE_ID ||
+  randomBytes(8).toString('hex')

packages/next/src/trace/trace-uploader.ts

@@ -7,7 +7,6 @@ import os from 'os'
 import { createInterface } from 'readline'
 import { createReadStream } from 'fs'
 import path from 'path'
-import { Telemetry } from '../telemetry/storage'
 
 const COMMON_ALLOWED_EVENTS = ['memory-usage']
 
@@ -66,7 +65,19 @@ const {
 const isDebugEnabled = !!NEXT_TRACE_UPLOAD_DEBUG || !!NEXT_TRACE_UPLOAD_FULL
 const shouldUploadFullTrace = !!NEXT_TRACE_UPLOAD_FULL
 
-const [, , traceUploadUrl, mode, projectDir, distDir] = process.argv
+const [
+  ,
+  ,
+  traceUploadUrl,
+  mode,
+  projectDir,
+  distDir,
+  _isTurboSession,
+  traceId,
+  anonymousId,
+  sessionId,
+] = process.argv
+const isTurboSession = _isTurboSession === 'true'
 
 type TraceRequestBody = {
   metadata: TraceMetadata
@@ -105,8 +116,6 @@ interface TraceMetadata {
     )
   ).version
 
-  const telemetry = new Telemetry({ distDir })
-
   const projectPkgJsonPath = await findUp('package.json')
   assert(projectPkgJsonPath)
 
@@ -129,11 +138,15 @@ interface TraceMetadata {
     crlfDelay: Infinity,
   })
 
-  let isTurboSession = false
-  const traces = new Map<string, TraceEvent[]>()
+  const sessionTrace = []
   for await (const line of readLineInterface) {
     const lineEvents: TraceEvent[] = JSON.parse(line)
     for (const event of lineEvents) {
+      if (event.traceId !== traceId) {
+        // Only consider events for the current session
+        continue
+      }
+
       if (
         // Always include root spans
         event.parentId === undefined ||
@@ -142,22 +155,14 @@ interface TraceMetadata {
           ? DEV_ALLOWED_EVENTS.has(event.name)
           : BUILD_ALLOWED_EVENTS.has(event.name))
       ) {
-        let trace = traces.get(event.traceId)
-        if (trace === undefined) {
-          trace = []
-          traces.set(event.traceId, trace)
-        }
-        if (typeof event.tags.isTurbopack === 'boolean') {
-          isTurboSession = event.tags.isTurbopack
-        }
-        trace.push(event)
+        sessionTrace.push(event)
       }
     }
   }
 
   const body: TraceRequestBody = {
     metadata: {
-      anonymousId: telemetry.anonymousId,
+      anonymousId,
       arch: os.arch(),
       commit,
       cpus: os.cpus().length,
@@ -166,9 +171,12 @@ interface TraceMetadata {
       nextVersion,
       pkgName,
       platform: os.platform(),
-      sessionId: telemetry.sessionId,
+      sessionId,
     },
-    traces: [...traces.values()],
+    // The trace file can contain events spanning multiple sessions.
+    // Only submit traces for the current session, as the metadata we send is
+    // intended for this session only.
+    traces: [sessionTrace],
   }
 
   if (isDebugEnabled) {

packages/next/src/trace/trace.ts

@@ -52,10 +52,6 @@ export class Span {
     this.name = name
     this.parentId = parentId ?? defaultParentSpanId
     this.attrs = attrs ? { ...attrs } : {}
-    if (this.parentId === undefined) {
-      // Attach additional information to root spans
-      this.attrs.isTurbopack = Boolean(process.env.TURBOPACK)
-    }
 
     this.status = SpanStatus.Started
     this.id = getId()

packages/next/src/trace/upload-trace.ts

@@ -1,17 +1,23 @@
+import { traceId } from './shared'
+import { Telemetry } from '../telemetry/storage'
+
 export default function uploadTrace({
   traceUploadUrl,
   mode,
   projectDir,
   distDir,
+  isTurboSession,
   sync,
 }: {
   traceUploadUrl: string
   mode: 'dev' | 'build'
   projectDir: string
   distDir: string
+  isTurboSession: boolean
   sync?: boolean
 }) {
   const { NEXT_TRACE_UPLOAD_DEBUG } = process.env
+  const telemetry = new Telemetry({ distDir })
 
   // Note: cross-spawn is not used here as it causes
   // a new command window to appear when we don't want it to
@@ -33,6 +39,10 @@ export default function uploadTrace({
       mode,
       projectDir,
       distDir,
+      String(isTurboSession),
+      traceId,
+      telemetry.anonymousId,
+      telemetry.sessionId,
     ],
     {
       detached: !NEXT_TRACE_UPLOAD_DEBUG,