feat: Pick up sentry-trace in JS <meta/> tag (#2703)

* feat: Pick up sentry-trace in frontend

* meta: Add changelog

* ref: Code review

Author: HazAT

CHANGELOG.md

@@ -6,6 +6,9 @@
 - [tracing] fix: Add manual Location typing (#2700)
 - [react] feat: Expose eventId on ErrorBoundary component
 
+- [tracing] feat: Pick up sentry-trace in JS <meta/> tag (#2703)
+- [tracing] fix: Respect sample decision when continuing trace from header in node (#2703)
+
 ## 5.18.1
 
 - [react] feat: Update peer dependencies for `react` and `react-dom` (#2694)

packages/apm/src/integrations/tracing.ts

@@ -285,6 +285,44 @@ export class Tracing implements Integration {
     });
   }
 
+  /**
+   * Returns a new Transaction either continued from sentry-trace meta or a new one
+   */
+  private static _getNewTransaction(hub: Hub, transactionContext: TransactionContext): Transaction {
+    let traceId;
+    let parentSpanId;
+    let sampled;
+
+    const header = Tracing._getMeta('sentry-trace');
+    if (header) {
+      const span = SpanClass.fromTraceparent(header);
+      if (span) {
+        traceId = span.traceId;
+        parentSpanId = span.parentSpanId;
+        sampled = span.sampled;
+        Tracing._log(
+          `[Tracing] found 'sentry-meta' '<meta />' continuing trace with: trace_id: ${traceId} span_id: ${parentSpanId}`,
+        );
+      }
+    }
+
+    return hub.startTransaction({
+      parentSpanId,
+      sampled,
+      traceId,
+      trimEnd: true,
+      ...transactionContext,
+    }) as Transaction;
+  }
+
+  /**
+   * Returns the value of a meta tag
+   */
+  private static _getMeta(metaName: string): string | null {
+    const el = document.querySelector(`meta[name=${metaName}]`);
+    return el ? el.getAttribute('content') : null;
+  }
+
   /**
    * Pings the heartbeat
    */
@@ -456,10 +494,7 @@ export class Tracing implements Integration {
       return undefined;
     }
 
-    Tracing._activeTransaction = hub.startTransaction({
-      trimEnd: true,
-      ...transactionContext,
-    }) as Transaction;
+    Tracing._activeTransaction = Tracing._getNewTransaction(hub, transactionContext);
 
     // We set the transaction here on the scope so error events pick up the trace context and attach it to the error
     hub.configureScope(scope => scope.setSpan(Tracing._activeTransaction));

packages/node/src/handlers.ts

@@ -34,6 +34,7 @@ export function tracingHandler(): (
 
     let traceId;
     let parentSpanId;
+    let sampled;
 
     // If there is a trace header set, we extract the data from it and set the span on the scope
     // to be the origin an created transaction set the parent_span_id / trace_id
@@ -42,13 +43,15 @@ export function tracingHandler(): (
       if (span) {
         traceId = span.traceId;
         parentSpanId = span.parentSpanId;
+        sampled = span.sampled;
       }
     }
 
     const transaction = startTransaction({
       name: `${reqMethod} ${reqUrl}`,
       op: 'http.server',
       parentSpanId,
+      sampled,
       traceId,
     });