Allow stack versions to be temporarily paused

This way, when detecting an error, we can defer the decision about
whether to bail or recover until all stack versions are processed.

src/runtime/stack.c

@@ -49,12 +49,19 @@ typedef struct {
 
 typedef Array(StackNode *) StackNodeArray;
 
+typedef enum {
+  StackStatusActive,
+  StackStatusPaused,
+  StackStatusHalted,
+} StackStatus;
+
 typedef struct {
   StackNode *node;
   Tree *last_external_token;
-  uint32_t push_count;
-  bool is_halted;
   StackSummary *summary;
+  uint32_t push_count;
+  TSSymbol lookahead_when_paused;
+  StackStatus status;
 } StackHead;
 
 struct Stack {
@@ -215,7 +222,8 @@ static StackVersion ts_stack__add_version(Stack *self, StackVersion original_ver
     .node = node,
     .push_count = self->heads.contents[original_version].push_count,
     .last_external_token = last_external_token,
-    .is_halted = false,
+    .status = StackStatusActive,
+    .lookahead_when_paused = 0,
   };
   array_push(&self->heads, head);
   stack_node_retain(node);
@@ -581,7 +589,8 @@ bool ts_stack_can_merge(Stack *self, StackVersion version1, StackVersion version
   StackHead *head1 = &self->heads.contents[version1];
   StackHead *head2 = &self->heads.contents[version2];
   return
-    !head1->is_halted && !head2->is_halted &&
+    head1->status == StackStatusActive &&
+    head2->status == StackStatusActive &&
     head1->node->state == head2->node->state &&
     head1->node->position.bytes == head2->node->position.bytes &&
     head1->node->depth == head2->node->depth &&
@@ -598,11 +607,34 @@ void ts_stack_force_merge(Stack *self, StackVersion version1, StackVersion versi
 }
 
 void ts_stack_halt(Stack *self, StackVersion version) {
-  array_get(&self->heads, version)->is_halted = true;
+  array_get(&self->heads, version)->status = StackStatusHalted;
 }
 
-bool ts_stack_is_halted(Stack *self, StackVersion version) {
-  return array_get(&self->heads, version)->is_halted;
+void ts_stack_pause(Stack *self, StackVersion version, TSSymbol lookahead) {
+  StackHead *head = array_get(&self->heads, version);
+  head->status = StackStatusPaused;
+  head->lookahead_when_paused = lookahead;
+}
+
+bool ts_stack_is_active(const Stack *self, StackVersion version) {
+  return array_get(&self->heads, version)->status == StackStatusActive;
+}
+
+bool ts_stack_is_halted(const Stack *self, StackVersion version) {
+  return array_get(&self->heads, version)->status == StackStatusHalted;
+}
+
+bool ts_stack_is_paused(const Stack *self, StackVersion version) {
+  return array_get(&self->heads, version)->status == StackStatusPaused;
+}
+
+TSSymbol ts_stack_resume(Stack *self, StackVersion version) {
+  StackHead *head = array_get(&self->heads, version);
+  assert(head->status == StackStatusPaused);
+  TSSymbol result = head->lookahead_when_paused;
+  head->status = StackStatusActive;
+  head->lookahead_when_paused = 0;
+  return result;
 }
 
 void ts_stack_clear(Stack *self) {
@@ -614,7 +646,8 @@ void ts_stack_clear(Stack *self) {
   array_push(&self->heads, ((StackHead){
     .node = self->base_node,
     .last_external_token = NULL,
-    .is_halted = false,
+    .status = StackStatusActive,
+    .lookahead_when_paused = 0,
   }));
 }
 
@@ -631,13 +664,18 @@ bool ts_stack_print_dot_graph(Stack *self, const char **symbol_names, FILE *f) {
 
   array_clear(&self->iterators);
   for (uint32_t i = 0; i < self->heads.size; i++) {
-    if (ts_stack_is_halted(self, i)) continue;
     StackHead *head = &self->heads.contents[i];
+    if (head->status == StackStatusHalted) continue;
+
     fprintf(f, "node_head_%u [shape=none, label=\"\"]\n", i);
-    fprintf(
-      f,
-      "node_head_%u -> node_%p [label=%u, fontcolor=blue, weight=10000, "
-      "labeltooltip=\"push_count: %u\ndepth: %u", i, head->node, i, head->push_count, head->node->depth
+    fprintf(f, "node_head_%u -> node_%p [", i, head->node);
+
+    if (head->status == StackStatusPaused) {
+      fprintf(f, "color=red ");
+    }
+    fprintf(f,
+      "label=%u, fontcolor=blue, weight=10000, labeltooltip=\"push_count: %u\ndepth: %u",
+      i, head->push_count, head->node->depth
     );
 
     if (head->last_external_token) {