[DI] Add a global max snapshot sample rate of 25/second

Each enhanced log probe has a sample rate of one second. However, too
many individual probes might still overload the system, so a global
snapshot sample rate across all enhanced log probes is required.

integration-tests/debugger/snapshot-global-sample-rate.spec.js

@@ -0,0 +1,91 @@
+'use strict'
+
+const { assert } = require('chai')
+const { setup } = require('./utils')
+
+describe('Dynamic Instrumentation', function () {
+  const t = setup({
+    testApp: 'target-app/basic.js'
+  })
+
+  describe('input messages', function () {
+    describe('with snapshot', function () {
+      beforeEach(t.triggerBreakpoint)
+
+      it('should respect global max snapshot sampling rate', function (_done) {
+        const MAX_SNAPSHOTS_PER_SECOND_GLOBALLY = 25
+        const snapshotsPerSecond = MAX_SNAPSHOTS_PER_SECOND_GLOBALLY * 2
+        const probeConf = { captureSnapshot: true, sampling: { snapshotsPerSecond } }
+        let start = 0
+        let hitBreakpoints = 0
+        let isDone = false
+        let prevTimestamp
+
+        const rcConfig1 = t.breakpoints[0].generateRemoteConfig(probeConf)
+        const rcConfig2 = t.breakpoints[1].generateRemoteConfig(probeConf)
+
+        // Two breakpoints, each triggering a request every 10ms, so we should get 200 requests per second
+        const state = {
+          [rcConfig1.config.id]: {
+            tiggerBreakpointContinuously () {
+              t.axios.get(t.breakpoints[0].url).catch(done)
+              this.timer = setTimeout(this.tiggerBreakpointContinuously.bind(this), 10)
+            }
+          },
+          [rcConfig2.config.id]: {
+            tiggerBreakpointContinuously () {
+              t.axios.get(t.breakpoints[1].url).catch(done)
+              this.timer = setTimeout(this.tiggerBreakpointContinuously.bind(this), 10)
+            }
+          }
+        }
+
+        t.agent.on('debugger-diagnostics', ({ payload }) => {
+          payload.forEach((event) => {
+            const { probeId, status } = event.debugger.diagnostics
+            if (status === 'INSTALLED') {
+              state[probeId].tiggerBreakpointContinuously()
+            }
+          })
+        })
+
+        t.agent.on('debugger-input', ({ payload }) => {
+          payload.forEach(({ 'debugger.snapshot': { timestamp } }) => {
+            if (isDone) return
+            if (start === 0) start = timestamp
+            if (++hitBreakpoints <= MAX_SNAPSHOTS_PER_SECOND_GLOBALLY) {
+              prevTimestamp = timestamp
+            } else {
+              const duration = timestamp - start
+              const timeSincePrevTimestamp = timestamp - prevTimestamp
+
+              // Allow for a variance of +50ms (time will tell if this is enough)
+              assert.isAtLeast(duration, 1000)
+              assert.isBelow(duration, 1050)
+
+              // A sanity check to make sure we're not saturating the event loop. We expect a lot of snapshots to be
+              // sampled in the beginning of the sample window and then once the threshold is hit, we expect a "quiet"
+              // period until the end of the window. If there's no "quiet" period, then we're saturating the event loop
+              // and this test isn't really testing anything.
+              assert.isAtLeast(timeSincePrevTimestamp, 250)
+
+              clearTimeout(state[rcConfig1.config.id].timer)
+              clearTimeout(state[rcConfig2.config.id].timer)
+
+              done()
+            }
+          })
+        })
+
+        t.agent.addRemoteConfig(rcConfig1)
+        t.agent.addRemoteConfig(rcConfig2)
+
+        function done (err) {
+          if (isDone) return
+          isDone = true
+          _done(err)
+        }
+      })
+    })
+  })
+})

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

@@ -1,6 +1,7 @@
 'use strict'
 
 module.exports = {
+  MAX_SNAPSHOTS_PER_SECOND_GLOBALLY: 25,
   MAX_SNAPSHOTS_PER_SECOND_PER_PROBE: 1,
   MAX_NON_SNAPSHOTS_PER_SECOND_PER_PROBE: 5_000
 }

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

@@ -8,6 +8,7 @@ const send = require('./send')
 const { getStackFromCallFrames } = require('./state')
 const { ackEmitting, ackError } = require('./status')
 const { parentThreadId } = require('./config')
+const { MAX_SNAPSHOTS_PER_SECOND_GLOBALLY } = require('./defaults')
 const log = require('../../log')
 const { version } = require('../../../../../package.json')
 
@@ -24,11 +25,14 @@ const expression = `
 const threadId = parentThreadId === 0 ? `pid:${process.pid}` : `pid:${process.pid};tid:${parentThreadId}`
 const threadName = parentThreadId === 0 ? 'MainThread' : `WorkerThread:${parentThreadId}`
 
+const oneSecondNs = BigInt(1_000_000_000)
+let globalSnapshotSamplingRateWindowStart = BigInt(0)
+let snapshotsSampledWithinTheLastSecond = 0
+
 // WARNING: The code above the line `await session.post('Debugger.resume')` is highly optimized. Please edit with care!
 session.on('Debugger.paused', async ({ params }) => {
   const start = process.hrtime.bigint()
 
-  let captureSnapshotForProbe = null
   let maxReferenceDepth, maxCollectionSize, maxFieldCount, maxLength
 
   // V8 doesn't allow seting more than one breakpoint at a specific location, however, it's possible to set two
@@ -38,6 +42,9 @@ session.on('Debugger.paused', async ({ params }) => {
   let sampled = false
   const length = params.hitBreakpoints.length
   let probes = new Array(length)
+  // TODO: Consider reusing this array between pauses and only recreating it if it needs to grow
+  const snapshotProbeIndex = new Uint8Array(length) // TODO: Is a limit of 256 probes ever going to be a problem?
+  let numberOfProbesWithSnapshots = 0
   for (let i = 0; i < length; i++) {
     const id = params.hitBreakpoints[i]
     const probe = breakpoints.get(id)
@@ -46,17 +53,28 @@ session.on('Debugger.paused', async ({ params }) => {
       continue
     }
 
-    sampled = true
-    probe.lastCaptureNs = start
-
     if (probe.captureSnapshot === true) {
-      captureSnapshotForProbe = probe
+      // This algorithm to calculate number of sampled snapshots within the last second is not perfect, as it's not a
+      // sliding window. But it's quick and easy :)
+      if (i === 0 && start - globalSnapshotSamplingRateWindowStart > oneSecondNs) {
+        snapshotsSampledWithinTheLastSecond = 1
+        globalSnapshotSamplingRateWindowStart = start
+      } else if (snapshotsSampledWithinTheLastSecond >= MAX_SNAPSHOTS_PER_SECOND_GLOBALLY) {
+        continue
+      } else {
+        snapshotsSampledWithinTheLastSecond++
+      }
+
+      snapshotProbeIndex[numberOfProbesWithSnapshots++] = i
       maxReferenceDepth = highestOrUndefined(probe.capture.maxReferenceDepth, maxReferenceDepth)
       maxCollectionSize = highestOrUndefined(probe.capture.maxCollectionSize, maxCollectionSize)
       maxFieldCount = highestOrUndefined(probe.capture.maxFieldCount, maxFieldCount)
       maxLength = highestOrUndefined(probe.capture.maxLength, maxLength)
     }
 
+    sampled = true
+    probe.lastCaptureNs = start
+
     probes[i] = probe
   }
 
@@ -68,17 +86,17 @@ session.on('Debugger.paused', async ({ params }) => {
   const dd = await getDD(params.callFrames[0].callFrameId)
 
   let processLocalState
-  if (captureSnapshotForProbe !== null) {
+  if (numberOfProbesWithSnapshots !== 0) {
     try {
       // TODO: Create unique states for each affected probe based on that probes unique `capture` settings (DEBUG-2863)
       processLocalState = await getLocalStateForCallFrame(
         params.callFrames[0],
         { maxReferenceDepth, maxCollectionSize, maxFieldCount, maxLength }
       )
     } catch (err) {
-      // TODO: This error is not tied to a specific probe, but to all probes with `captureSnapshot: true`.
-      // However, in 99,99% of cases, there will be just a single probe, so I guess this simplification is ok?
-      ackError(err, captureSnapshotForProbe) // TODO: Ok to continue after sending ackError?
+      for (let i = 0; i < numberOfProbesWithSnapshots; i++) {
+        ackError(err, probes[snapshotProbeIndex[i]]) // TODO: Ok to continue after sending ackError?
+      }
     }
   }