fix(hydroflow_lang): restore in-subgraph rendering of self-handoffs (#…

…935)

hydroflow/tests/snapshots/surface_codegen__covid_tracing@graphvis_dot.snap

@@ -37,6 +37,7 @@ digraph {
         fillcolor="#dddddd"
         style=filled
         label = "sg_1v1\nstratum 0"
+        n13v1
         n1v1
         n2v1
         n4v1

hydroflow/tests/snapshots/surface_codegen__covid_tracing@graphvis_mermaid.snap

@@ -37,6 +37,7 @@ linkStyle default stroke:#aaa
 13v1-->9v1
 14v1-->|1|10v1
 subgraph sg_1v1 ["sg_1v1 stratum 0"]
+    13v1
     1v1
     2v1
     4v1

hydroflow/tests/snapshots/surface_codegen__surface_syntax_reachability_generated@graphvis_dot.snap

@@ -27,6 +27,7 @@ digraph {
         fillcolor="#dddddd"
         style=filled
         label = "sg_1v1\nstratum 0"
+        n9v1
         n3v1
         n1v1
         n2v1

hydroflow/tests/snapshots/surface_codegen__surface_syntax_reachability_generated@graphvis_mermaid.snap

@@ -27,6 +27,7 @@ linkStyle default stroke:#aaa
 6v1-->|1|8v1
 9v1-->|1|1v1
 subgraph sg_1v1 ["sg_1v1 stratum 0"]
+    9v1
     3v1
     1v1
     2v1

hydroflow/tests/snapshots/surface_codegen__transitive_closure@graphvis_dot.snap

@@ -43,6 +43,7 @@ digraph {
         fillcolor="#dddddd"
         style=filled
         label = "sg_2v1\nstratum 0"
+        n10v1
         n6v1
         n5v1
         n7v1

hydroflow/tests/snapshots/surface_codegen__transitive_closure@graphvis_mermaid.snap

@@ -39,6 +39,7 @@ subgraph sg_1v1 ["sg_1v1 stratum 0"]
     end
 end
 subgraph sg_2v1 ["sg_2v1 stratum 0"]
+    10v1
     6v1
     5v1
     7v1

hydroflow/tests/snapshots/surface_examples__example_5_reachability.snap

@@ -29,6 +29,7 @@ linkStyle default stroke:#aaa
 6v1-->|print|8v1
 10v1-->|cycle|7v1
 subgraph sg_1v1 ["sg_1v1 stratum 0"]
+    10v1
     1v1
     2v1
     7v1

hydroflow/tests/snapshots/surface_examples__example_6_unreachability.snap

@@ -46,6 +46,7 @@ linkStyle default stroke:#aaa
 17v1-->|pos|11v1
 18v1--x|neg|11v1; linkStyle 18 stroke:red
 subgraph sg_1v1 ["sg_1v1 stratum 0"]
+    15v1
     1v1
     4v1
     5v1

hydroflow/tests/snapshots/surface_reduce__reduce@graphvis_dot.snap

@@ -31,6 +31,7 @@ digraph {
         fillcolor="#dddddd"
         style=filled
         label = "sg_1v1\nstratum 0"
+        n10v1
         n3v1
         n1v1
         n2v1

hydroflow/tests/snapshots/surface_reduce__reduce@graphvis_mermaid.snap

@@ -31,6 +31,7 @@ linkStyle default stroke:#aaa
 10v1-->|1|1v1
 11v1--x8v1; linkStyle 10 stroke:red
 subgraph sg_1v1 ["sg_1v1 stratum 0"]
+    10v1
     3v1
     1v1
     2v1

hydroflow/tests/snapshots/surface_stratum__surface_syntax_graph_unreachability@graphvis_dot.snap

@@ -56,6 +56,7 @@ digraph {
         fillcolor="#dddddd"
         style=filled
         label = "sg_2v1\nstratum 0"
+        n15v1
         n3v1
         n1v1
         n2v1

hydroflow/tests/snapshots/surface_stratum__surface_syntax_graph_unreachability@graphvis_mermaid.snap

@@ -52,6 +52,7 @@ subgraph sg_1v1 ["sg_1v1 stratum 0"]
     end
 end
 subgraph sg_2v1 ["sg_2v1 stratum 0"]
+    15v1
     3v1
     1v1
     2v1

hydroflow_lang/src/graph/hydroflow_graph.rs

@@ -1175,10 +1175,23 @@ impl HydroflowGraph {
 
         // Write nodes.
         let mut skipped_handoffs = BTreeSet::new();
+        let mut subgraph_handoffs = <BTreeMap<GraphSubgraphId, Vec<GraphNodeId>>>::new();
         for (node_id, node) in self.nodes() {
-            if write_config.no_handoffs && matches!(node, Node::Handoff { .. }) {
-                skipped_handoffs.insert(node_id);
-                continue;
+            if matches!(node, Node::Handoff { .. }) {
+                if write_config.no_handoffs {
+                    skipped_handoffs.insert(node_id);
+                    continue;
+                } else {
+                    let pred_node = self.node_predecessor_nodes(node_id).next().unwrap();
+                    let pred_sg = self.node_subgraph(pred_node);
+                    let succ_node = self.node_successor_nodes(node_id).next().unwrap();
+                    let succ_sg = self.node_subgraph(succ_node);
+                    if let Some((pred_sg, succ_sg)) = pred_sg.zip(succ_sg) {
+                        if pred_sg == succ_sg {
+                            subgraph_handoffs.entry(pred_sg).or_default().push(node_id);
+                        }
+                    }
+                }
             }
             graph_write.write_node(
                 node_id,
@@ -1221,12 +1234,12 @@ impl HydroflowGraph {
         // Write subgraphs.
         if !write_config.no_subgraphs {
             for (subgraph_id, subgraph_node_ids) in self.subgraph_nodes.iter() {
+                let handoff_node_ids = subgraph_handoffs.get(&subgraph_id).into_iter().flatten();
+                let subgraph_node_ids = subgraph_node_ids.iter();
+                let all_node_ids = handoff_node_ids.chain(subgraph_node_ids).copied();
+
                 let stratum = self.subgraph_stratum.get(subgraph_id);
-                graph_write.write_subgraph_start(
-                    subgraph_id,
-                    *stratum.unwrap(),
-                    subgraph_node_ids.iter().copied(),
-                )?;
+                graph_write.write_subgraph_start(subgraph_id, *stratum.unwrap(), all_node_ids)?;
                 // Write out any variable names within the subgraph.
                 if !write_config.no_varnames {
                     for (varname, varname_node_ids) in