Merge pull request #2817 from adri326/fix-2815-run_module_predicate-backtrack

Fix backtracking on the topmost predicate triggering UB in run_module_predicate

src/machine/lib_machine/mod.rs

@@ -568,7 +568,7 @@ impl Machine {
         self.run_module_predicate(atom!("loader"), (atom!("consult_stream"), 2));
     }
 
-    fn allocate_stub_choice_point(&mut self) {
+    pub(crate) fn allocate_stub_choice_point(&mut self) {
         // NOTE: create a choice point to terminate the dispatch_loop
         // if an exception is thrown.
 

src/machine/mod.rs

@@ -271,6 +271,11 @@ impl Machine {
             .unwrap()
     }
 
+    /// Runs the predicate `key` in `module_name` until completion.
+    /// Siltently ignores failure, thrown errors and choice points.
+    ///
+    /// Consider using [`Machine::run_query`] if you wish to handle
+    /// predicates that may fail, leave a choice point or throw.
     pub(crate) fn run_module_predicate(
         &mut self,
         module_name: Atom,
@@ -279,6 +284,8 @@ impl Machine {
         if let Some(module) = self.indices.modules.get(&module_name) {
             if let Some(code_index) = module.code_dir.get(&key) {
                 let p = code_index.local().unwrap();
+                // Leave a halting choice point to backtrack to in case the predicate fails or throws.
+                self.allocate_stub_choice_point();
 
                 self.machine_st.cp = BREAK_FROM_DISPATCH_LOOP_LOC;
                 self.machine_st.p = p;
@@ -1248,3 +1255,24 @@ impl Machine {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::config::*;
+    use super::*;
+
+    #[test]
+    #[cfg_attr(miri, ignore)]
+    fn test_run_module_predicate_throw() {
+        let mut machine = MachineBuilder::default()
+            .with_toplevel(
+                r#"
+                    :- module('$toplevel', []).
+                    repl :- throw(kaboom).
+                "#,
+            )
+            .build();
+
+        machine.run_module_predicate(atom!("$toplevel"), (atom!("repl"), 0));
+    }
+}

src/toplevel.pl

@@ -29,6 +29,19 @@ load_scryerrc :-
     ).
 
 '$repl' :-
+    catch(
+        start_repl,
+        _,
+        % Something bad enough happened that the REPL itself threw an error.
+        % This can be caused by a broken user_output stream, so we cannot
+        % print an error.
+        %
+        % The best we can do now is halt with an error code,
+        % so that users can try to diagnose the issue:
+        halt(99)
+    ).
+
+start_repl :-
     asserta('$toplevel':started),
     raw_argv(Args0),
     (   append(Args1, ["--"|_], Args0) ->