fix(debugger): reuse loaded tracer for trace context in devtools client (#9765)

The devtools worker evaluates an expression on every paused call frame to read
the active trace and span IDs. That expression used global.require('dd-trace'),
which re-resolves and re-requires the tracer module on each hit.

Use globalThis._ddtrace instead, matching the singleton instance installed by
bootstrap when the application first loads dd-trace. This avoids duplicate
module resolution, works when the bare 'dd-trace' specifier is not resolvable
from the paused frame (e.g. sirun benchmarks that load the tracer by path), and
removes per-hit require overhead from the hot pause path.

Author: watson

benchmark/sirun/debugger/start-devtools-client.js

@@ -2,23 +2,16 @@
 
 const assert = require('node:assert/strict')
 const fs = require('node:fs')
-const Module = require('node:module')
 
 // Point the tracer at this variant's per-core agent (port matches `agent.js`)
 // before `getConfig()` reads the URL. Set it unconditionally so a globally
 // inherited `DD_TRACE_AGENT_URL` can't redirect us back to a shared port that
 // another parallel variant owns.
 process.env.DD_TRACE_AGENT_URL = `http://127.0.0.1:${8080 + Number(process.env.CPU_AFFINITY || 0)}`
 
-// The trace-context expression the devtools client evaluates on the paused frame
-// for every hit does `global.require('dd-trace')`. This bench loads the tracer by
-// relative path, so the bare specifier would otherwise throw MODULE_NOT_FOUND on
-// every hit and skew the measurement. Resolve it to this checkout's entry point.
-const ddTraceEntry = require.resolve('../../..')
-const originalResolveFilename = Module._resolveFilename
-Module._resolveFilename = function (request, ...rest) {
-  return originalResolveFilename.call(this, request === 'dd-trace' ? ddTraceEntry : request, ...rest)
-}
+// Production initializes the tracer before starting Dynamic Instrumentation. This benchmark imports the debugger
+// internals directly, so initialize the tracer explicitly before the paused-frame expression reads global._ddtrace.
+require('../../..')
 
 // The global snapshot cap (MAX_SNAPSHOTS_PER_SECOND_GLOBALLY) is read in the
 // devtools worker thread at module load, with no config or env path to override
@@ -27,9 +20,6 @@ Module._resolveFilename = function (request, ...rest) {
 // No-op unless the variant opts in via the env var.
 patchGlobalSnapshotCap(process.env.MAX_SNAPSHOTS_PER_SECOND_GLOBALLY)
 
-// Entry point normally primes this; bench imports src directly.
-globalThis[Symbol.for('dd-trace')] ??= { beforeExitHandlers: new Set() }
-
 const getConfig = require('../../../packages/dd-trace/src/config')
 const { start } = require('../../../packages/dd-trace/src/debugger')
 const { generateProbeConfig } = require('../../../packages/dd-trace/test/debugger/devtools_client/utils')

packages/dd-trace/src/debugger/devtools_client/index.js

@@ -23,7 +23,6 @@ require('./remote_config')
 
 /** @typedef {import('node:inspector').Debugger.EvaluateOnCallFrameReturnType} EvaluateOnCallFrameResult */
 
-// Expression to run on a call frame of the paused thread to get its active trace and span id.
 const templateExpressionSetupCode = `
   const $dd_inspect = global.require('node:util').inspect;
   const $dd_segmentInspectOptions = {
@@ -34,8 +33,10 @@ const templateExpressionSetupCode = `
     breakLength: Infinity
   };
 `
+
+// Expression to run on a call frame of the paused thread to get its active trace and span id.
 const getDDTagsExpression = `(() => {
-  const context = global.require('dd-trace').scope().active()?.context();
+  const context = globalThis._ddtrace.scope().active()?.context();
   return { trace_id: context?.toTraceId(), span_id: context?.toSpanId() }
 })()`