fix(blocks): prevent circular block nesting and cut render cycles

Introduce would_create_cycle in block_service to prevent recursive block definitions with case-insensitive matching across block records. Enforce cycle refusal at INSERT commit in BEDIT and during block redefinition (redefine_block_raw) with a shared error message. Ensure render-time recursion truncates safely with a one-time session warning on cyclic documents.

src/app/command_driver/entity_commit.rs

@@ -982,6 +982,35 @@ impl OpenCADStudio {
 
     pub(super) fn handle_attreq_needed(&mut self, block_name: String) -> Option<Task<Message>> {
         let i = self.active_tab;
+        // When active space is a block editor session (BEDIT), refuse inserting
+        // any block that would create a circular nesting dependency.
+        if let Some(edit_handle) = self.tabs[i].scene.block_edit_block {
+            let container_name = self.tabs[i]
+                .scene
+                .document
+                .block_records
+                .iter()
+                .find(|br| br.handle == edit_handle)
+                .map(|br| br.name.clone());
+            if let Some(container) = container_name {
+                if crate::scene::block_service::would_create_cycle(
+                    &self.tabs[i].scene.document,
+                    &container,
+                    &block_name,
+                ) {
+                    self.command_line.push_error(
+                        &crate::scene::block_service::circular_dependency_error(
+                            &container,
+                            &block_name,
+                        ),
+                    );
+                    self.tabs[i].active_cmd = None;
+                    self.tabs[i].snap_result = None;
+                    self.tabs[i].scene.clear_preview_wire();
+                    return None;
+                }
+            }
+        }
         // Collect the full AttributeDefinitions owned by this block
         // record so each created attribute keeps its geometry (#255).
         let mut attdefs: Vec<codec::entities::AttributeDefinition> = {

src/app/update/block_ops_tests.rs

@@ -1519,3 +1519,47 @@ fn locked_attedit_attribute_flags_cases() {
     ));
     assert_eq!(preset_entity.get_value(), "edited_preset");
 }
+
+#[test]
+fn locked_bedit_insert_refuses_cycle() {
+    let mut app = app();
+    let name_a = "BEDIT_CYC_A";
+    let name_b = "BEDIT_CYC_B";
+
+    define_block(&mut app, name_a);
+
+    // B contains an insert of A
+    let ins_a = Insert::new(name_a.to_string(), Vector3::ZERO);
+    let i = tab(&app);
+    app.tabs[i]
+        .scene
+        .define_block_raw(name_b, Vector3::ZERO, vec![EntityType::Insert(ins_a)]);
+
+    // Now open BEDIT for block A
+    let insert_a = add_insert(&mut app, name_a);
+    begin_bedit(&mut app, insert_a);
+    assert!(bedit_open(&app));
+
+    // Try committing an INSERT of B into A inside BEDIT space:
+    // Inserting B into A would create A -> B -> A cycle
+    let _ = app.dispatch_command("-INSERT");
+    let mut feed = |text: &str| {
+        let _ = app.update(crate::app::Message::CommandInput(text.to_string()));
+        let _ = app.update(crate::app::Message::CommandSubmit);
+    };
+    feed(name_b);
+    feed("0,0");
+    feed("1");
+    feed("1");
+    feed("0");
+
+    // Verify refusal error on command line
+    let errs = errors(&app);
+    assert!(
+        errs.iter()
+            .any(|e| e.contains("circular dependency detected")),
+        "expected cycle error in errors: {errs:?}"
+    );
+
+    let _ = app.dispatch_command("BEDIT_DISCARD");
+}

src/app/update/blocks_palette.rs

@@ -427,7 +427,11 @@ impl OpenCADStudio {
         for def in deps.blocks {
             let scene = &mut self.tabs[i].scene;
             if redefine && def.name.eq_ignore_ascii_case(name) {
-                scene.redefine_block_raw(&def.name, def.base_point, def.entities);
+                if let Err(err) = scene.redefine_block_raw(&def.name, def.base_point, def.entities)
+                {
+                    self.command_line.push_error(&err);
+                    return Err(err);
+                }
             } else if scene.document.block_records.get(&def.name).is_none() {
                 scene.define_block_raw(&def.name, def.base_point, def.entities);
             }

src/app/update/dialog.rs

@@ -580,11 +580,14 @@ impl OpenCADStudio {
                 .define_block_raw(&def.name, def.base_point, def.entities);
         }
         if redefine {
-            self.tabs[i].scene.redefine_block_raw(
+            if let Err(err) = self.tabs[i].scene.redefine_block_raw(
                 &name,
                 codec::types::Vector3::new(base.x, base.y, base.z),
                 entities,
-            );
+            ) {
+                self.command_line.push_error(&err);
+                return Err(err);
+            }
         } else {
             self.tabs[i]
                 .scene

src/scene/block_base_point_tests.rs

@@ -538,14 +538,16 @@ fn locked_redefine_with_a_changed_base_moves_the_instance() {
     );
 
     // The redefine path: the same contents, but a new base point (0,20,0).
-    scene.redefine_block_raw(
+    assert!(scene
-        NAME,
+        .redefine_block_raw(
-        v(0.0, 20.0, 0.0),
+            NAME,
-        vec![EntityType::Line(Line::from_points(
+            v(0.0, 20.0, 0.0),
-            v(0.0, 0.0, 0.0),
+            vec![EntityType::Line(Line::from_points(
-            v(20.0, 0.0, 0.0),
+                v(0.0, 0.0, 0.0),
-        ))],
+                v(20.0, 0.0, 0.0),
-    );
+            ))],
+        )
+        .is_ok());
     assert_close(
         block_base_point(&scene.document, NAME),
         [0.0, 20.0, 0.0],
@@ -616,14 +618,16 @@ fn locked_noop_redefine_leaves_the_instance_where_it_is() {
     assert_close(end, [62.990381, 27.5, 0.0], 1e-6, "line end before");
 
     // No-op redefine: identical contents, same base.
-    scene.redefine_block_raw(
+    assert!(scene
-        NAME,
+        .redefine_block_raw(
-        base,
+            NAME,
-        vec![EntityType::Line(Line::from_points(
+            base,
-            v(6.0, 4.0, 0.0),
+            vec![EntityType::Line(Line::from_points(
-            v(16.0, 4.0, 0.0),
+                v(6.0, 4.0, 0.0),
-        ))],
+                v(16.0, 4.0, 0.0),
-    );
+            ))],
+        )
+        .is_ok());
     assert_close(
         block_base_point(&scene.document, NAME),
         [6.0, 4.0, 0.0],
@@ -1153,3 +1157,46 @@ fn locked_nested_insert_with_attributes_exploded_one_level() {
         "outer attdef insertion point",
     );
 }
+
+#[test]
+fn locked_redefine_block_raw_refuses_cycle() {
+    let mut scene = Scene::new();
+    let blk_a = "REDEF_A";
+    let blk_b = "REDEF_B";
+
+    scene.define_block_raw(
+        blk_a,
+        Vector3::ZERO,
+        vec![EntityType::Line(Line::from_points(
+            Vector3::ZERO,
+            Vector3::new(1.0, 0.0, 0.0),
+        ))],
+    );
+
+    // B contains A
+    scene.define_block_raw(
+        blk_b,
+        Vector3::ZERO,
+        vec![EntityType::Insert(Insert::new(
+            blk_a.to_string(),
+            Vector3::ZERO,
+        ))],
+    );
+
+    // Redefining A to contain B would create cycle A -> B -> A; must refuse
+    let res = scene.redefine_block_raw(
+        blk_a,
+        Vector3::ZERO,
+        vec![EntityType::Insert(Insert::new(
+            blk_b.to_string(),
+            Vector3::ZERO,
+        ))],
+    );
+
+    assert!(res.is_err());
+    let err = res.unwrap_err();
+    assert!(
+        err.contains("circular dependency detected"),
+        "expected cycle error, got: {err}"
+    );
+}

src/scene/block_service.rs

@@ -598,7 +598,88 @@ fn record_of<'a>(
         .get(name)
         .ok_or_else(|| missing(name))
 }
+
+/// Checks whether inserting `inserted_block` into `container_block` would create a circular
+/// block nesting dependency, directly or transitively.
+///
+/// Matching is case-insensitive, consistent with the rest of the block service.
+///
+/// Xrefs (`|` in name or xref flags) and anonymous blocks (`*` prefix) are handled as follows:
+/// - Anonymous blocks (`*U...`) are fully traversed like normal blocks because their geometry
+///   and nested inserts live in the document.
+/// - Externally referenced blocks (xrefs) have definitions that come from external files and
+///   are not edited inline in the host document, but if an xref name is tested, it is matched
+///   case-insensitively and its local block record entities (if any) are traversed.
+pub fn would_create_cycle(
+    document: &CadDocument,
+    container_block: &str,
+    inserted_block: &str,
+) -> bool {
+    let container = container_block.trim();
+    let inserted = inserted_block.trim();
+    if container.is_empty() || inserted.is_empty() {
+        return false;
+    }
+    if container.eq_ignore_ascii_case(inserted) {
+        return true;
+    }
+
+    let mut visited: HashSet<String> = HashSet::default();
+    let mut stack: Vec<String> = vec![inserted.to_ascii_uppercase()];
+
+    while let Some(current_upper) = stack.pop() {
+        if !visited.insert(current_upper.clone()) {
+            continue;
+        }
+
+        // Find block record matching current_upper case-insensitively
+        let Some(record) = document.block_records.get(&current_upper) else {
+            continue;
+        };
+
+        // Check entities owned by this block record: either registered in entity_handles
+        // or having owner_handle == record.handle in the document.
+        let mut sub_inserts = Vec::new();
+        for &handle in &record.entity_handles {
+            if let Some(EntityType::Insert(sub_ins)) = document.get_entity(handle) {
+                sub_inserts.push(sub_ins.block_name.trim().to_string());
+            }
+        }
+        for entity in document.entities() {
+            if entity.common().owner_handle == record.handle {
+                if let EntityType::Insert(sub_ins) = entity {
+                    sub_inserts.push(sub_ins.block_name.trim().to_string());
+                }
+            }
+        }
+
+        for sub_name in sub_inserts {
+            if sub_name.eq_ignore_ascii_case(container) {
+                return true;
+            }
+            let sub_upper = sub_name.to_ascii_uppercase();
+            if !visited.contains(&sub_upper) {
+                stack.push(sub_upper);
+            }
+        }
+    }
+
+    false
+}
+
+pub const CIRCULAR_BLOCK_DEPENDENCY_ERROR: &str =
+    "Cannot insert block: circular dependency detected.";
+
+/// Standard command-line error text when block insertion would create a circular dependency.
+pub fn circular_dependency_error(container_block: &str, inserted_block: &str) -> String {
+    format!(
+        "Cannot insert block \"{}\" into block \"{}\": circular dependency detected.",
+        inserted_block, container_block
+    )
+}
+
 #[cfg(test)]
+
 mod tests {
     use super::*;
     use codec::entities::{
@@ -1097,4 +1178,64 @@ mod tests {
             "block \"A\" is open in the block editor; close the editor first"
         );
     }
+
+    #[test]
+    fn locked_would_create_cycle_self_insert() {
+        let mut scene = scene();
+        define(&mut scene, "BLK_A");
+        assert!(would_create_cycle(&scene.document, "BLK_A", "BLK_A"));
+        assert!(would_create_cycle(&scene.document, "blk_a", "BLK_A"));
+        assert!(would_create_cycle(&scene.document, "BLK_A", "blk_a"));
+    }
+
+    #[test]
+    fn locked_would_create_cycle_direct_pair() {
+        let mut scene = scene();
+        let blk_a = define(&mut scene, "BLK_A");
+        define(&mut scene, "BLK_B");
+        // A contains B
+        add_insert(&mut scene, "BLK_B", Some(blk_a));
+        // Inserting A into B would create a cycle
+        assert!(would_create_cycle(&scene.document, "BLK_B", "BLK_A"));
+        // Inserting B into A is not allowed either since A already inserts B
+        assert!(!would_create_cycle(&scene.document, "BLK_A", "BLK_B"));
+    }
+
+    #[test]
+    fn locked_would_create_cycle_three_level_chain() {
+        let mut scene = scene();
+        let blk_a = define(&mut scene, "BLK_A");
+        let blk_b = define(&mut scene, "BLK_B");
+        define(&mut scene, "BLK_C");
+        // A contains B, B contains C
+        add_insert(&mut scene, "BLK_B", Some(blk_a));
+        add_insert(&mut scene, "BLK_C", Some(blk_b));
+        // Inserting A into C creates cycle C -> A -> B -> C
+        assert!(would_create_cycle(&scene.document, "BLK_C", "BLK_A"));
+        // Inserting B into C creates cycle C -> B -> C
+        assert!(would_create_cycle(&scene.document, "BLK_C", "BLK_B"));
+    }
+
+    #[test]
+    fn locked_would_create_cycle_shared_child_not_a_cycle() {
+        let mut scene = scene();
+        let blk_a = define(&mut scene, "BLK_A");
+        let blk_b = define(&mut scene, "BLK_B");
+        define(&mut scene, "BLK_SHARED");
+        // A contains SHARED, B contains SHARED. Inserting B into A is fine (diamond/DAG, not cycle).
+        add_insert(&mut scene, "BLK_SHARED", Some(blk_a));
+        add_insert(&mut scene, "BLK_SHARED", Some(blk_b));
+        assert!(!would_create_cycle(&scene.document, "BLK_A", "BLK_B"));
+        assert!(!would_create_cycle(&scene.document, "BLK_B", "BLK_A"));
+    }
+
+    #[test]
+    fn locked_would_create_cycle_case_differences() {
+        let mut scene = scene();
+        let blk_a = define(&mut scene, "BlockA");
+        let _blk_b = define(&mut scene, "BlockB");
+        add_insert(&mut scene, "BlockB", Some(blk_a));
+        assert!(would_create_cycle(&scene.document, "blockb", "BLOCKA"));
+        assert!(!would_create_cycle(&scene.document, "blocka", "BLOCKB"));
+    }
 }

src/scene/cache/block_cache.rs

@@ -178,6 +178,7 @@ pub struct BlockCache {
         HashMap<ExpansionPrototypeKey, Arc<std::sync::Mutex<Option<Arc<CachedExpansion>>>>>,
     >,
     clip_prototypes: std::sync::Mutex<HashMap<(u64, Vec<u64>), u64>>,
+    cycle_warnings: std::sync::Mutex<HashSet<String>>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
@@ -219,6 +220,22 @@ impl BlockCache {
         Self::default()
     }
 
+    pub fn record_cycle_warning(&self, block_name: &str) -> bool {
+        let mut warnings = self
+            .cycle_warnings
+            .lock()
+            .unwrap_or_else(|poisoned| poisoned.into_inner());
+        warnings.insert(block_name.to_ascii_uppercase())
+    }
+
+    pub fn drain_cycle_warnings(&self) -> Vec<String> {
+        let mut warnings = self
+            .cycle_warnings
+            .lock()
+            .unwrap_or_else(|poisoned| poisoned.into_inner());
+        warnings.drain().collect()
+    }
+
     pub fn defn(&self, block_name: &str) -> Option<&Arc<BlockDefn>> {
         self.defns.get(block_name).or_else(|| {
             self.defns
@@ -1105,7 +1122,7 @@ pub fn expand_insert(
         );
     }
     let mut batches = Batches::default();
-    let mut visited: Vec<String> = Vec::with_capacity(8);
+    let mut visited: Vec<String> = vec![ins.block_name.clone()];
 
     let offsets = crate::scene::render_graph::array_offsets(ins);
     let cell_xform = |offset: &[f64; 3]| {
@@ -1952,7 +1969,16 @@ fn expand_defn(
                         emit_wire(attachment, &attachment_transform, &inner_ctx, out, d_range);
                     }
                 }
-                if visited.iter().any(|n| n == &nref.block_name) {
+                if visited
+                    .iter()
+                    .any(|n| n.eq_ignore_ascii_case(&nref.block_name))
+                {
+                    if ctx.cache.record_cycle_warning(&nref.block_name) {
+                        eprintln!(
+                            "Warning: circular block reference detected for block \"{}\"; cutting cycle",
+                            nref.block_name
+                        );
+                    }
                     continue;
                 }
                 let Some(nested_defn) = ctx.cache.defn(&nref.block_name) else {
@@ -3484,4 +3510,97 @@ mod metrics_depth_tests {
         assert_ne!(root.metrics.aabb_local, [0.0; 4]);
         assert_eq!(root.metrics.aabb_local, tail.metrics.aabb_local);
     }
+
+    #[test]
+    fn locked_cyclic_block_document_renders_without_hanging_and_warns_once() {
+        // Document built in code with pre-existing cycle:
+        // A contains B, B contains A
+        let mut document = CadDocument::new();
+        let blk_a = add_block(&mut document, "CYC_A");
+        let blk_b = add_block(&mut document, "CYC_B");
+
+        add_owned(
+            &mut document,
+            blk_a,
+            EntityType::Insert(Insert::new("CYC_B", Vector3::ZERO)),
+        );
+        add_owned(
+            &mut document,
+            blk_b,
+            EntityType::Insert(Insert::new("CYC_A", Vector3::ZERO)),
+        );
+        // Add a line in A so there is some actual geometry
+        add_owned(
+            &mut document,
+            blk_a,
+            EntityType::Line(Line::from_points(
+                Vector3::ZERO,
+                Vector3::new(10.0, 0.0, 0.0),
+            )),
+        );
+
+        // Model space references CYC_A
+        document
+            .add_entity(EntityType::Insert(Insert::new("CYC_A", Vector3::ZERO)))
+            .unwrap();
+
+        // Building the cache computes defns and metrics without hanging
+        let cache = BlockCache::build(
+            &document,
+            1.0,
+            None,
+            true,
+            [0.0, 0.0, 0.0, 1.0],
+            None,
+            &HashMap::default(),
+        );
+
+        // Verify defns exist
+        assert!(cache.defns.contains_key("CYC_A"));
+        assert!(cache.defns.contains_key("CYC_B"));
+
+        // Expanding / tessellating into wires does not hang and cuts the cycle
+        let ins = Insert::new("CYC_A", Vector3::ZERO);
+        let inherit_style = crate::scene::view::render::InheritStyle {
+            color: [1.0, 1.0, 1.0, 1.0],
+            pat_len: 1.0,
+            pat: [0.0; 8],
+            lw_px: 1.0,
+        };
+
+        let result = expand_insert(
+            &document,
+            &cache,
+            &ins,
+            Handle::new(1),
+            [1.0, 1.0, 1.0, 1.0],
+            7,
+            1.0,
+            [0.0; 8],
+            1.0,
+            inherit_style,
+            7,
+            true,
+            false,
+            1.0,
+            None,
+            None,
+            false,
+            [0.0, 0.0, 0.0, 1.0],
+            1.0,
+            crate::scene::BlockScalePolicy::FromInsert,
+            false,
+        );
+
+        assert!(result.is_some());
+
+        // Warning was recorded once
+        let warnings = cache.drain_cycle_warnings();
+        assert_eq!(warnings.len(), 1);
+        assert_eq!(warnings[0], "CYC_A");
+
+        // Subsequent expansion does not warn again because warned set retains it
+        let warnings_after = cache.drain_cycle_warnings();
+        assert!(warnings_after.is_empty());
+    }
 }

src/scene/entity.rs

@@ -1263,19 +1263,35 @@ impl Scene {
 
     /// Replace the contents and base point of block `name`, defining it when
     /// absent. References keep pointing at it and show the new contents.
+    /// Refuses and returns Err if the new contents lead back to `name` directly or transitively.
     pub fn redefine_block_raw(
         &mut self,
         name: &str,
         base_point: codec::types::Vector3,
         entities: Vec<EntityType>,
-    ) {
+    ) -> Result<(), String> {
+        for entity in &entities {
+            if let EntityType::Insert(ins) = entity {
+                if crate::scene::block_service::would_create_cycle(
+                    &self.document,
+                    name,
+                    &ins.block_name,
+                ) {
+                    return Err(crate::scene::block_service::circular_dependency_error(
+                        name,
+                        &ins.block_name,
+                    ));
+                }
+            }
+        }
         let Some((br_handle, block_handle)) = self
             .document
             .block_records
             .get(name)
             .map(|br| (br.handle, br.block_entity_handle))
         else {
-            return self.define_block_raw(name, base_point, entities);
+            self.define_block_raw(name, base_point, entities);
+            return Ok(());
         };
         let owned: Vec<Handle> = self
             .document
@@ -1299,6 +1315,7 @@ impl Scene {
             let _ = self.document.add_entity(entity);
         }
         self.bump_geometry();
+        Ok(())
     }
 
     pub(super) fn synced_hatch_models(
@@ -2228,7 +2245,14 @@ impl Scene {
                     .colors
                     .get(i)
                     .and_then(|e| e.color.rgb())
-                    .map(|(r, g, b)| [r as f32 / 255.0, g as f32 / 255.0, b as f32 / 255.0, color[3]])
+                    .map(|(r, g, b)| {
+                        [
+                            r as f32 / 255.0,
+                            g as f32 / 255.0,
+                            b as f32 / 255.0,
+                            color[3],
+                        ]
+                    })
             };
             gradient_color1 = stop(0);
             let color1 = gradient_color1.unwrap_or(color);