Fix panic in multi_buffer::Anchor::is_valid (#55644) (cherry-pick to preview)

crates/multi_buffer/src/anchor.rs

@@ -232,6 +232,9 @@ impl ExcerptAnchor {
         let Some(excerpt) = cursor.item() else {
             return false;
         };
+        if excerpt.buffer_id != buffer_snapshot.remote_id() {
+            return false;
+        }
         let is_valid = self.text_anchor == excerpt.range.context.start
             || self.text_anchor == excerpt.range.context.end
             || self.text_anchor.is_valid(&buffer_snapshot);

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -6206,3 +6206,54 @@ fn test_resolving_max_anchor_for_buffer(cx: &mut TestAppContext) {
         assert_eq!(point, Point::new(10, 0));
     })
 }
+
+#[gpui::test]
+fn test_is_valid_anchor_past_last_excerpt_for_buffer(cx: &mut TestAppContext) {
+    let buffer_a = cx.new(|cx| Buffer::local("aaa\nbbb\nccc\n", cx));
+    buffer_a.update(cx, |buffer, cx| {
+        let len = buffer.len();
+        buffer.edit([(len..len, "ddd\neee\n")], None, cx);
+    });
+    let buffer_b = cx.new(|cx| Buffer::local("xxx\n", cx));
+    for line in ["yyy\n", "zzz\n", "www\n", "vvv\n"] {
+        buffer_b.update(cx, |buffer, cx| {
+            let len = buffer.len();
+            buffer.edit([(len..len, line)], None, cx);
+        });
+    }
+
+    let path_a = PathKey::with_sort_prefix(0, rel_path("aaa.rs").into_arc());
+    let path_b = PathKey::with_sort_prefix(1, rel_path("bbb.rs").into_arc());
+
+    let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
+
+    multibuffer.update(cx, |multibuffer, cx| {
+        multibuffer.set_excerpts_for_path(
+            path_a.clone(),
+            buffer_a.clone(),
+            vec![Point::new(1, 0)..Point::new(2, 3)],
+            0,
+            cx,
+        );
+        multibuffer.set_excerpts_for_path(
+            path_b.clone(),
+            buffer_b.clone(),
+            vec![Point::new(1, 0)..Point::new(3, 3)],
+            0,
+            cx,
+        );
+    });
+
+    multibuffer.read_with(cx, |multibuffer, cx| {
+        let snapshot = multibuffer.snapshot(cx);
+
+        let buffer_a_snapshot = buffer_a.read(cx).snapshot();
+        let anchor_past_excerpt = buffer_a_snapshot.anchor_after(Point::new(4, 0));
+        let mb_anchor = snapshot.anchor_in_buffer(anchor_past_excerpt).unwrap();
+
+        assert!(
+            !mb_anchor.is_valid(&snapshot),
+            "anchor past the last excerpt for its buffer should not be valid"
+        );
+    });
+}