subplotlib: Enable catching of panics

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

2 files changed, 22 insertions, 2 deletions

diff --git a/subplotlib/Cargo.toml b/subplotlib/Cargo.toml
index 5ca2616..2e6c306 100644
--- a/subplotlib/Cargo.toml
+++ b/subplotlib/Cargo.toml
@@ -10,4 +10,4 @@ fehler = "1"
 subplotlib-derive = { version="0.1", path = "../subplotlib-derive" }
 lazy_static = "1"
 base64 = "0.13"
-state = "0.4"
\ No newline at end of file
+state = "0.4"
diff --git a/subplotlib/src/step.rs b/subplotlib/src/step.rs
index 3eb6a0c..f83b29d 100644
--- a/subplotlib/src/step.rs
+++ b/subplotlib/src/step.rs
@@ -4,6 +4,9 @@
 //! here to provide wrappers to make the scenarios easier to work with.
 //!
 
+use std::any::Any;
+use std::panic::{catch_unwind, AssertUnwindSafe};
+
 use crate::scenario::ScenarioContext;
 use crate::types::StepResult;
 
@@ -32,11 +35,28 @@ impl ScenarioStep {
         }
     }
 
+    /// Attempt to render a message.
+    /// If something panics with a type other than a static string or
+    /// a formatted string then we won't be able to render it sadly.
+    fn render_panic(name: &str, err: Box<dyn Any + Send>) -> String {
+        if let Some(msg) = err.downcast_ref::<&str>() {
+            format!("step {} panic'd: {}", name, msg)
+        } else if let Some(msg) = err.downcast_ref::<String>() {
+            format!("step {} panic'd: {}", name, msg)
+        } else {
+            format!("step {} panic'd", name)
+        }
+    }
+
     /// Call the step function
     ///
     /// This simply calls the encased step function
     pub fn call(&self, context: &ScenarioContext) -> StepResult {
-        (*self.func)(context)
+        // Note, panic here will be absorbed and so there's a risk that
+        // subsequent step calls may not be sound.  There's not a lot we can
+        // do to ensure things are good except try.
+        let func = AssertUnwindSafe(|| (*self.func)(context));
+        catch_unwind(func).map_err(|e| Self::render_panic(self.name(), e))?
     }
 
     /// Return the name of this step