fix(core): Require active session for session trace lifecycle

Only remap root transactions onto the session propagation context when a session is currently active. This avoids reusing the ambient scope trace when session trace lifecycle is enabled before a session starts.

Co-Authored-By: Claude <[email protected]>

Author: adinauer

sentry/src/main/java/io/sentry/Scopes.java

@@ -1024,10 +1024,11 @@ && getOptions().getProfileLifecycle() == ProfileLifecycle.TRACE
 
   private @NotNull TransactionContext maybeApplySessionTraceLifecycle(
       final @NotNull TransactionContext transactionContext) {
-    final @NotNull PropagationContext propagationContext =
-        getCombinedScopeView().getPropagationContext();
     if (getOptions().isEnableSessionTraceLifecycle()
-        && transactionContext.getParentSpanId() == null) {
+        && transactionContext.getParentSpanId() == null
+        && getCombinedScopeView().getSession() != null) {
+      final @NotNull PropagationContext propagationContext =
+          getCombinedScopeView().getPropagationContext();
       return TransactionContext.fromPropagationContextAsRoot(
           propagationContext, transactionContext);
     }

sentry/src/test/java/io/sentry/ScopesTest.kt

@@ -1823,10 +1823,28 @@ class ScopesTest {
   }
 
   @Test
-  fun `when session trace lifecycle is enabled, startTransaction uses session propagation context`() {
+  fun `when session trace lifecycle is enabled without active session, root transaction does not use scope propagation context`() {
     val scopes = generateScopes { it.isEnableSessionTraceLifecycle = true }
     var propagationContext: PropagationContext? = null
     scopes.configureScope { propagationContext = it.propagationContext }
+    val context = TransactionContext("name", "op")
+
+    val transaction = scopes.startTransaction(context)
+
+    assertTrue(transaction is SentryTracer)
+    assertEquals(context.traceId, transaction.root.spanContext.traceId)
+    assertNotEquals(propagationContext!!.traceId, transaction.root.spanContext.traceId)
+  }
+
+  @Test
+  fun `when session trace lifecycle is enabled, startTransaction uses session propagation context`() {
+    val scopes = generateScopes {
+      it.isEnableSessionTraceLifecycle = true
+      it.release = "1.0.0"
+    }
+    scopes.startSession()
+    var propagationContext: PropagationContext? = null
+    scopes.configureScope { propagationContext = it.propagationContext }
 
     val transaction = scopes.startTransaction(TransactionContext("name", "op"))
 
@@ -1839,7 +1857,11 @@ class ScopesTest {
 
   @Test
   fun `continued trace with parent span is not remapped to session trace`() {
-    val scopes = generateScopes { it.isEnableSessionTraceLifecycle = true }
+    val scopes = generateScopes {
+      it.isEnableSessionTraceLifecycle = true
+      it.release = "1.0.0"
+    }
+    scopes.startSession()
     val traceId = "75302ac48a024bde9a3b3734a82e36c8"
     val parentSpanId = "1000000000000000"
     val context = scopes.continueTrace("$traceId-$parentSpanId-1", emptyList())!!
@@ -1853,7 +1875,11 @@ class ScopesTest {
 
   @Test
   fun `when session trace lifecycle is enabled, root transaction uses current propagation context`() {
-    val scopes = generateScopes { it.isEnableSessionTraceLifecycle = true }
+    val scopes = generateScopes {
+      it.isEnableSessionTraceLifecycle = true
+      it.release = "1.0.0"
+    }
+    scopes.startSession()
     val traceId = "75302ac48a024bde9a3b3734a82e36c8"
     val parentSpanId = "1000000000000000"
     scopes.continueTrace("$traceId-$parentSpanId-1", emptyList())