aep export: note what the concurrency test covers and why a failed template build is not kept (#3277)

crates/aep/src/export/emit.rs

@@ -930,8 +930,10 @@ mod tests {
         }
     }
 
-    /// Exports running at the same time on several threads, the first of them possibly building
-    /// the shared template, all give the single-threaded bytes.
+    /// Exports running at the same time on several threads all give the single-threaded bytes.
+    /// This checks concurrent exports from a template that is already built: other tests in this
+    /// binary usually fill the shared template first, so the race on the first build is not
+    /// reliably exercised here (the `static OnceLock` makes that race safe by construction).
     #[test]
     fn concurrent_exports_give_the_same_bytes() {
         let plans: Vec<ProjectPlan> = ["", " x", " y", " z"].iter().map(|t| plan_of(&varied(t))).collect();

crates/aep/src/export/template.rs

@@ -94,6 +94,7 @@ pub(crate) fn shared() -> R<&'static Template> {
     if let Some(t) = SHARED.get() {
         return Ok(t);
     }
+    // A failed build returns here, before `get_or_init`, so it is never kept.
     let t = Template::load()?;
     Ok(SHARED.get_or_init(|| t))
 }