feat(core): Capture # of dropped spans through beforeSendTransaction (#13022)

Fixes #12849.

This is a bit tricky because `beforeSendTransaction` can return a
promise, so in order to avoid dealing with this I put the # of spans on
the sdk metadata, and then compare that afterwards.

Author: mydea

.size-limit.js

@@ -48,14 +48,14 @@ module.exports = [
     path: 'packages/browser/build/npm/esm/index.js',
     import: createImport('init', 'browserTracingIntegration', 'replayIntegration', 'replayCanvasIntegration'),
     gzip: true,
-    limit: '76 KB',
+    limit: '78 KB',
   },
   {
     name: '@sentry/browser (incl. Tracing, Replay, Feedback)',
     path: 'packages/browser/build/npm/esm/index.js',
     import: createImport('init', 'browserTracingIntegration', 'replayIntegration', 'feedbackIntegration'),
     gzip: true,
-    limit: '89 KB',
+    limit: '90 KB',
   },
   {
     name: '@sentry/browser (incl. Tracing, Replay, Feedback, metrics)',

packages/core/src/baseclient.ts

@@ -371,20 +371,21 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
   /**
    * @inheritDoc
    */
-  public recordDroppedEvent(reason: EventDropReason, category: DataCategory, _event?: Event): void {
-    // Note: we use `event` in replay, where we overwrite this hook.
-
+  public recordDroppedEvent(reason: EventDropReason, category: DataCategory, eventOrCount?: Event | number): void {
     if (this._options.sendClientReports) {
+      // TODO v9: We do not need the `event` passed as third argument anymore, and can possibly remove this overload
+      // If event is passed as third argument, we assume this is a count of 1
+      const count = typeof eventOrCount === 'number' ? eventOrCount : 1;
+
       // We want to track each category (error, transaction, session, replay_event) separately
       // but still keep the distinction between different type of outcomes.
       // We could use nested maps, but it's much easier to read and type this way.
       // A correct type for map-based implementation if we want to go that route
       // would be `Partial<Record<SentryRequestType, Partial<Record<Outcome, number>>>>`
       // With typescript 4.1 we could even use template literal types
       const key = `${reason}:${category}`;
-      DEBUG_BUILD && logger.log(`Adding outcome: "${key}"`);
-
-      this._outcomes[key] = (this._outcomes[key] || 0) + 1;
+      DEBUG_BUILD && logger.log(`Recording outcome: "${key}"${count > 1 ? ` (${count} times)` : ''}`);
+      this._outcomes[key] = (this._outcomes[key] || 0) + count;
     }
   }
 
@@ -794,11 +795,11 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
       .then(processedEvent => {
         if (processedEvent === null) {
           this.recordDroppedEvent('before_send', dataCategory, event);
-          if (isTransactionEvent(event)) {
+          if (isTransaction) {
             const spans = event.spans || [];
             // the transaction itself counts as one span, plus all the child spans that are added
             const spanCount = 1 + spans.length;
-            this._outcomes['span'] = (this._outcomes['span'] || 0) + spanCount;
+            this.recordDroppedEvent('before_send', 'span', spanCount);
           }
           throw new SentryError(`${beforeSendLabel} returned \`null\`, will not send event.`, 'log');
         }
@@ -808,6 +809,18 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
           this._updateSessionFromEvent(session, processedEvent);
         }
 
+        if (isTransaction) {
+          const spanCountBefore =
+            (processedEvent.sdkProcessingMetadata && processedEvent.sdkProcessingMetadata.spanCountBeforeProcessing) ||
+            0;
+          const spanCountAfter = processedEvent.spans ? processedEvent.spans.length : 0;
+
+          const droppedSpanCount = spanCountBefore - spanCountAfter;
+          if (droppedSpanCount > 0) {
+            this.recordDroppedEvent('before_send', 'span', droppedSpanCount);
+          }
+        }
+
         // None of the Sentry built event processor will update transaction name,
         // so if the transaction name has been changed by an event processor, we know
         // it has to come from custom event processor added by a user
@@ -973,6 +986,15 @@ function processBeforeSend(
     }
 
     if (beforeSendTransaction) {
+      if (event.spans) {
+        // We store the # of spans before processing in SDK metadata,
+        // so we can compare it afterwards to determine how many spans were dropped
+        const spanCountBefore = event.spans.length;
+        event.sdkProcessingMetadata = {
+          ...event.sdkProcessingMetadata,
+          spanCountBeforeProcessing: spanCountBefore,
+        };
+      }
       return beforeSendTransaction(event, hint);
     }
   }

packages/core/test/lib/base.test.ts

@@ -1042,6 +1042,30 @@ describe('BaseClient', () => {
       expect(TestClient.instance!.event!.transaction).toBe('/adopt/dont/shop');
     });
 
+    test('calls `beforeSendTransaction` and drops spans', () => {
+      const beforeSendTransaction = jest.fn(event => {
+        event.spans = [{ span_id: 'span5', trace_id: 'trace1', start_timestamp: 1234 }];
+        return event;
+      });
+      const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSendTransaction });
+      const client = new TestClient(options);
+
+      client.captureEvent({
+        transaction: '/dogs/are/great',
+        type: 'transaction',
+        spans: [
+          { span_id: 'span1', trace_id: 'trace1', start_timestamp: 1234 },
+          { span_id: 'span2', trace_id: 'trace1', start_timestamp: 1234 },
+          { span_id: 'span3', trace_id: 'trace1', start_timestamp: 1234 },
+        ],
+      });
+
+      expect(beforeSendTransaction).toHaveBeenCalled();
+      expect(TestClient.instance!.event!.spans?.length).toBe(1);
+
+      expect(client['_outcomes']).toEqual({ 'before_send:span': 2 });
+    });
+
     test('calls `beforeSendSpan` and uses the modified spans', () => {
       expect.assertions(3);
 
@@ -1120,8 +1144,6 @@ describe('BaseClient', () => {
     });
 
     test('calls `beforeSendSpan` and discards the span', () => {
-      expect.assertions(2);
-
       const beforeSendSpan = jest.fn(() => null);
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSendSpan });
       const client = new TestClient(options);
@@ -1149,6 +1171,7 @@ describe('BaseClient', () => {
       expect(beforeSendSpan).toHaveBeenCalledTimes(2);
       const capturedEvent = TestClient.instance!.event!;
       expect(capturedEvent.spans).toHaveLength(0);
+      expect(client['_outcomes']).toEqual({ 'before_send:span': 2 });
     });
 
     test('calls `beforeSend` and logs info about invalid return value', () => {
@@ -2017,6 +2040,7 @@ describe('BaseClient', () => {
 
   describe('hook removal with `on`', () => {
     it('should return a cleanup function that, when executed, unregisters a hook', async () => {
+      jest.useFakeTimers();
       expect.assertions(8);
 
       const client = new TestClient(