script: Return global objects for DOM objects in the relevant realm (#37120)

DomObject::global is a tricky API because it's used pervasively but has
subtle requirements that are not documented and not yet enforced by the
type system (#36116). The method returns the relevant global object for
a given DOM object, but that operation is only meaningful if there is an
active realm. We usually, but not always, have an active realm.

This change avoids a footgun by following the principle of least
surprise. Rather than making every single caller of `something.global()`
both prove that there is an active realm and think about which realm
they want active, we implement the obvious behaviour: always activate
the realm of the callee before obtaining the relevant global.
 
Testing: Existing WPT coverage is sufficient; this method is called all
over the codebase.
Fixes: #37070 #27037

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

1 files changed, 12 insertions, 2 deletions

diff --git a/components/script/dom/bindings/reflector.rs b/components/script/dom/bindings/reflector.rs
index 0a5afbce487..c888686974e 100644
--- a/components/script/dom/bindings/reflector.rs
+++ b/components/script/dom/bindings/reflector.rs
@@ -11,7 +11,7 @@ use crate::DomTypes;
 use crate::dom::bindings::conversions::DerivedFrom;
 use crate::dom::bindings::root::DomRoot;
 use crate::dom::globalscope::GlobalScope;
-use crate::realms::InRealm;
+use crate::realms::{InRealm, enter_realm};
 use crate::script_runtime::CanGc;
 
 /// Create the reflector for a new DOM object and yield ownership to the
@@ -42,7 +42,16 @@ where
 }
 
 pub(crate) trait DomGlobal {
+    /// Returns the [relevant global] in whatever realm is currently active.
+    ///
+    /// [relevant global]: https://html.spec.whatwg.org/multipage/#concept-relevant-global
     fn global_(&self, realm: InRealm) -> DomRoot<GlobalScope>;
+
+    /// Returns the [relevant global] in the same realm as the callee object.
+    /// If you know the callee's realm is already the current realm, it is
+    /// more efficient to call [DomGlobal::global_] instead.
+    ///
+    /// [relevant global]: https://html.spec.whatwg.org/multipage/#concept-relevant-global
     fn global(&self) -> DomRoot<GlobalScope>;
 }
 
@@ -51,7 +60,8 @@ impl<T: DomGlobalGeneric<crate::DomTypeHolder>> DomGlobal for T {
         <Self as DomGlobalGeneric<crate::DomTypeHolder>>::global_(self, realm)
     }
     fn global(&self) -> DomRoot<GlobalScope> {
-        <Self as DomGlobalGeneric<crate::DomTypeHolder>>::global(self)
+        let realm = enter_realm(self);
+        <Self as DomGlobalGeneric<crate::DomTypeHolder>>::global_(self, InRealm::entered(&realm))
     }
 }