[DI] Use custom error property for errors reporting state snapshot (#5731)

If for whatever reason the probe state snapshot cannot be captured or
included in the payload, an error message should be included instead.
The backend doesn’t support a root `notCapturedReason`, so instead a new
`debugger.snapshot.captureError` should be introduced to hold a string
error message.

Author: watson

integration-tests/debugger/snapshot-pruning.spec.js

@@ -13,16 +13,13 @@ describe('Dynamic Instrumentation', function () {
       it('should prune snapshot if payload is too large', function (done) {
         t.agent.on('debugger-input', ({ payload: [payload] }) => {
           assert.isBelow(Buffer.byteLength(JSON.stringify(payload)), 1024 * 1024) // 1MB
-          assert.deepEqual(payload.debugger.snapshot.captures, {
-            lines: {
-              [t.breakpoint.line]: {
-                locals: {
-                  notCapturedReason: 'Snapshot was too large',
-                  size: 6
-                }
-              }
-            }
-          })
+          assert.notProperty(payload.debugger.snapshot, 'captures')
+          assert.strictEqual(
+            payload.debugger.snapshot.captureError,
+            'Snapshot was too large (max allowed size is 1 MiB). ' +
+            'Consider reducing the capture depth or turn off "Capture Variables" completely, ' +
+            'and instead include the variables of interest directly in the message template.'
+          )
           done()
         })
 

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

@@ -13,7 +13,8 @@ const { version } = require('../../../../../package.json')
 module.exports = send
 
 const MAX_MESSAGE_LENGTH = 8 * 1024 // 8KB
-const MAX_LOG_PAYLOAD_SIZE = 1024 * 1024 // 1MB
+const MAX_LOG_PAYLOAD_SIZE_MB = 1
+const MAX_LOG_PAYLOAD_SIZE_BYTES = MAX_LOG_PAYLOAD_SIZE_MB * 1024 * 1024
 
 const ddsource = 'dd_debugger'
 const hostname = getHostname()
@@ -48,13 +49,13 @@ function send (message, logger, dd, snapshot) {
   let json = JSON.stringify(payload)
   let size = Buffer.byteLength(json)
 
-  if (size > MAX_LOG_PAYLOAD_SIZE) {
+  if (size > MAX_LOG_PAYLOAD_SIZE_BYTES) {
     // TODO: This is a very crude way to handle large payloads. Proper pruning will be implemented later (DEBUG-2624)
-    const line = Object.values(payload.debugger.snapshot.captures.lines)[0]
-    line.locals = {
-      notCapturedReason: 'Snapshot was too large',
-      size: Object.keys(line.locals).length
-    }
+    delete payload.debugger.snapshot.captures
+    payload.debugger.snapshot.captureError =
+      `Snapshot was too large (max allowed size is ${MAX_LOG_PAYLOAD_SIZE_MB} MiB). ` +
+      'Consider reducing the capture depth or turn off "Capture Variables" completely, ' +
+      'and instead include the variables of interest directly in the message template.'
     json = JSON.stringify(payload)
     size = Buffer.byteLength(json)
   }