fix(scopes): Walk parent chain iteratively in isAncestorOf

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Fixes
+
+- Avoid `StackOverflowError` in `Scopes.isAncestorOf` on deep `Scopes` chains ([#6249](https://github.com/getsentry/sentry-java/pull/6249))
+
 ## 8.60.0
 
 ### Features

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

@@ -96,18 +96,13 @@ private Scopes(
   @Override
   @ApiStatus.Internal
   public boolean isAncestorOf(final @Nullable IScopes otherScopes) {
-    if (otherScopes == null) {
-      return false;
-    }
-
-    if (this == otherScopes) {
-      return true;
-    }
-
-    if (otherScopes.getParentScopes() != null) {
-      return isAncestorOf(otherScopes.getParentScopes());
+    @Nullable IScopes candidate = otherScopes;
+    while (candidate != null) {
+      if (this == candidate) {
+        return true;
+      }
+      candidate = candidate.getParentScopes();
     }
-
     return false;
   }
 

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

@@ -169,6 +169,38 @@ class ScopesTest {
     assertEquals("b", cloneScope!!.tags["scopes"])
   }
 
+  @Test
+  fun `isAncestorOf does not overflow the stack on a deep chain`() {
+    val options = SentryOptions()
+    options.cacheDirPath = file.absolutePath
+    options.dsn = "https://key@sentry.io/proj"
+    options.setSerializer(mock())
+    val root = createScopes(options)
+    var deepest: IScopes = root
+    repeat(20_000) { deepest = deepest.forkedCurrentScope("deep") }
+
+    val result = AtomicReference<Any>()
+    val thread =
+      Thread(
+        null,
+        {
+          result.set(
+            try {
+              root.isAncestorOf(deepest)
+            } catch (e: StackOverflowError) {
+              e
+            }
+          )
+        },
+        "small-stack",
+        256 * 1024,
+      )
+    thread.start()
+    thread.join()
+
+    assertEquals(true, result.get())
+  }
+
   @Test
   fun `when scopes is initialized, breadcrumbs are capped as per options`() {
     val options = SentryOptions()