[DI] Adhere to maxFieldCount limit in snapshots

The limit controls the maximum number of properties collected on an
object. The default is 20.

It's also applied on each scope when collecting properties. If there's
for example more than maxFieldCount properties in the current scope,
they are not all collected.

integration-tests/debugger/snapshot.spec.js

@@ -96,8 +96,8 @@ describe('Dynamic Instrumentation', function () {
 
           // from closure scope
           // There's no reason to test the `fastify` object 100%, instead just check its fingerprint
-          assert.deepEqual(Object.keys(fastify), ['type', 'fields'])
           assert.equal(fastify.type, 'Object')
+          assert.typeOf(fastify.fields, 'Object')
 
           assert.deepEqual(getSomeData, {
             type: 'Function',
@@ -186,6 +186,54 @@ describe('Dynamic Instrumentation', function () {
 
         t.agent.addRemoteConfig(t.generateRemoteConfig({ captureSnapshot: true, capture: { maxCollectionSize: 3 } }))
       })
+
+      it('should respect maxFieldCount', (done) => {
+        const maxFieldCount = 3
+
+        function assertMaxFieldCount (prop) {
+          if ('fields' in prop) {
+            if (prop.notCapturedReason === 'fieldCount') {
+              assert.strictEqual(Object.keys(prop.fields).length, maxFieldCount)
+              assert.isAbove(prop.size, maxFieldCount)
+            } else {
+              assert.isBelow(Object.keys(prop.fields).length, maxFieldCount)
+            }
+          }
+
+          for (const value of Object.values(prop.fields || prop.elements || prop.entries || {})) {
+            assertMaxFieldCount(value)
+          }
+        }
+
+        t.agent.on('debugger-input', ({ payload: { 'debugger.snapshot': { captures } } }) => {
+          const { locals } = captures.lines[t.breakpoint.line]
+
+          assert.deepEqual(Object.keys(locals), [
+            // Up to 3 properties from the local scope
+            'request', 'nil', 'undef',
+            // Up to 3 properties from the closure scope
+            'fastify', 'getSomeData'
+          ])
+
+          assert.strictEqual(locals.request.type, 'Request')
+          assert.strictEqual(Object.keys(locals.request.fields).length, maxFieldCount)
+          assert.strictEqual(locals.request.notCapturedReason, 'fieldCount')
+          assert.isAbove(locals.request.size, maxFieldCount)
+
+          assert.strictEqual(locals.fastify.type, 'Object')
+          assert.strictEqual(Object.keys(locals.fastify.fields).length, maxFieldCount)
+          assert.strictEqual(locals.fastify.notCapturedReason, 'fieldCount')
+          assert.isAbove(locals.fastify.size, maxFieldCount)
+
+          for (const value of Object.values(locals)) {
+            assertMaxFieldCount(value)
+          }
+
+          done()
+        })
+
+        t.agent.addRemoteConfig(t.generateRemoteConfig({ captureSnapshot: true, capture: { maxFieldCount } }))
+      })
     })
   })
 })

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

@@ -23,13 +23,14 @@ session.on('Debugger.paused', async ({ params }) => {
   const timestamp = Date.now()
 
   let captureSnapshotForProbe = null
-  let maxReferenceDepth, maxCollectionSize, maxLength
+  let maxReferenceDepth, maxCollectionSize, maxFieldCount, maxLength
   const probes = params.hitBreakpoints.map((id) => {
     const probe = breakpoints.get(id)
     if (probe.captureSnapshot) {
       captureSnapshotForProbe = probe
       maxReferenceDepth = highestOrUndefined(probe.capture.maxReferenceDepth, maxReferenceDepth)
       maxCollectionSize = highestOrUndefined(probe.capture.maxCollectionSize, maxCollectionSize)
+      maxFieldCount = highestOrUndefined(probe.capture.maxFieldCount, maxFieldCount)
       maxLength = highestOrUndefined(probe.capture.maxLength, maxLength)
     }
     return probe
@@ -41,7 +42,7 @@ session.on('Debugger.paused', async ({ params }) => {
       // 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, maxLength }
+        { maxReferenceDepth, maxCollectionSize, maxFieldCount, maxLength }
       )
     } catch (err) {
       // TODO: This error is not tied to a specific probe, but to all probes with `captureSnapshot: true`.

packages/dd-trace/src/debugger/devtools_client/snapshot/collector.js

@@ -1,6 +1,6 @@
 'use strict'
 
-const { collectionSizeSym } = require('./symbols')
+const { collectionSizeSym, fieldCountSym } = require('./symbols')
 const session = require('../session')
 
 const LEAF_SUBTYPES = new Set(['date', 'regexp'])
@@ -30,6 +30,11 @@ async function getObject (objectId, opts, depth = 0, collection = false) {
       result.splice(opts.maxCollectionSize)
       result[collectionSizeSym] = size
     }
+  } else if (result.length > opts.maxFieldCount) {
+    // Trim the number of properties on the object if there's too many.
+    const size = result.length
+    result.splice(opts.maxFieldCount)
+    result[fieldCountSym] = size
   } else if (privateProperties) {
     result.push(...privateProperties)
   }

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

@@ -5,6 +5,7 @@ const { processRawState } = require('./processor')
 
 const DEFAULT_MAX_REFERENCE_DEPTH = 3
 const DEFAULT_MAX_COLLECTION_SIZE = 100
+const DEFAULT_MAX_FIELD_COUNT = 20
 const DEFAULT_MAX_LENGTH = 255
 
 module.exports = {
@@ -16,6 +17,7 @@ async function getLocalStateForCallFrame (
   {
     maxReferenceDepth = DEFAULT_MAX_REFERENCE_DEPTH,
     maxCollectionSize = DEFAULT_MAX_COLLECTION_SIZE,
+    maxFieldCount = DEFAULT_MAX_FIELD_COUNT,
     maxLength = DEFAULT_MAX_LENGTH
   } = {}
 ) {
@@ -24,7 +26,10 @@ async function getLocalStateForCallFrame (
 
   for (const scope of callFrame.scopeChain) {
     if (scope.type === 'global') continue // The global scope is too noisy
-    rawState.push(...await getRuntimeObject(scope.object.objectId, { maxReferenceDepth, maxCollectionSize }))
+    rawState.push(...await getRuntimeObject(
+      scope.object.objectId,
+      { maxReferenceDepth, maxCollectionSize, maxFieldCount }
+    ))
   }
 
   // Deplay calling `processRawState` so the caller gets a chance to resume the main thread before processing `rawState`

packages/dd-trace/src/debugger/devtools_client/snapshot/processor.js

@@ -1,6 +1,6 @@
 'use strict'
 
-const { collectionSizeSym } = require('./symbols')
+const { collectionSizeSym, fieldCountSym } = require('./symbols')
 
 module.exports = {
   processRawState: processProperties
@@ -139,7 +139,18 @@ function toString (str, maxLength) {
 
 function toObject (type, props, maxLength) {
   if (props === undefined) return notCapturedDepth(type)
-  return { type, fields: processProperties(props, maxLength) }
+
+  const result = {
+    type,
+    fields: processProperties(props, maxLength)
+  }
+
+  if (fieldCountSym in props) {
+    result.notCapturedReason = 'fieldCount'
+    result.size = props[fieldCountSym]
+  }
+
+  return result
 }
 
 function toArray (type, elements, maxLength) {

packages/dd-trace/src/debugger/devtools_client/snapshot/symbols.js

@@ -1,5 +1,6 @@
 'use stict'
 
 module.exports = {
-  collectionSizeSym: Symbol('datadog.collectionSize')
+  collectionSizeSym: Symbol('datadog.collectionSize'),
+  fieldCountSym: Symbol('datadog.fieldCount')
 }

packages/dd-trace/test/debugger/devtools_client/snapshot/complex-types.spec.js

@@ -23,7 +23,7 @@ describe('debugger -> devtools client -> snapshot.getLocalStateForCallFrame', fu
       session.once('Debugger.paused', async ({ params }) => {
         expect(params.hitBreakpoints.length).to.eq(1)
 
-        resolve((await getLocalStateForCallFrame(params.callFrames[0]))())
+        resolve((await getLocalStateForCallFrame(params.callFrames[0], { maxFieldCount: 999 }))())
       })
 
       await setAndTriggerBreakpoint(target, 10)

packages/dd-trace/test/debugger/devtools_client/snapshot/max-field-count.spec.js

@@ -0,0 +1,49 @@
+'use strict'
+
+require('../../../setup/mocha')
+
+const { getTargetCodePath, enable, teardown, assertOnBreakpoint, setAndTriggerBreakpoint } = require('./utils')
+
+const DEFAULT_MAX_FIELD_COUNT = 20
+const target = getTargetCodePath(__filename)
+
+describe('debugger -> devtools client -> snapshot.getLocalStateForCallFrame', function () {
+  describe('maxFieldCount', function () {
+    beforeEach(enable(__filename))
+
+    afterEach(teardown)
+
+    describe('shold respect the default maxFieldCount if not set', generateTestCases())
+
+    describe('shold respect maxFieldCount if set to 10', generateTestCases({ maxFieldCount: 10 }))
+  })
+})
+
+function generateTestCases (config) {
+  const maxFieldCount = config?.maxFieldCount ?? DEFAULT_MAX_FIELD_COUNT
+  let state
+
+  const expectedFields = {}
+  for (let i = 1; i <= maxFieldCount; i++) {
+    expectedFields[`field${i}`] = { type: 'number', value: i.toString() }
+  }
+
+  return function () {
+    beforeEach(function (done) {
+      assertOnBreakpoint(done, config, (_state) => {
+        state = _state
+      })
+      setAndTriggerBreakpoint(target, 11)
+    })
+
+    it('should capture expected snapshot', function () {
+      expect(state).to.have.keys(['obj'])
+      expect(state).to.have.deep.property('obj', {
+        type: 'Object',
+        fields: expectedFields,
+        notCapturedReason: 'fieldCount',
+        size: 40
+      })
+    })
+  }
+}

packages/dd-trace/test/debugger/devtools_client/snapshot/target-code/max-field-count.js

@@ -0,0 +1,14 @@
+'use stict'
+
+function run () {
+  const obj = {}
+
+  // 40 is larger the default maxFieldCount of 20
+  for (let i = 1; i <= 40; i++) {
+    obj[`field${i}`] = i
+  }
+
+  return 'my return value' // breakpoint at this line
+}
+
+module.exports = { run }