Debug: handle Store::debug_frames() with no activation. (#11899)

This was a corner case I missed in #11769. When the last-exited state on
the `Store` is all zeroes because there is no current activation, we
need to return an empty iterator rather than hit an assert in the
stackwalk.

Author: cfallin

crates/wasmtime/src/runtime/vm/traphandlers/backtrace.rs

@@ -366,11 +366,16 @@ impl<'a, T: 'static> CurrentActivationBacktrace<'a, T> {
         let exit_pc = unsafe { *(*vm_store_context).last_wasm_exit_pc.get() };
         let exit_fp = unsafe { (*vm_store_context).last_wasm_exit_fp() };
         let trampoline_fp = unsafe { *(*vm_store_context).last_wasm_entry_fp.get() };
-        let unwind = store.0.unwinder();
-        // Establish the iterator.
-        let inner = Box::new(unsafe {
-            wasmtime_unwinder::frame_iterator(unwind, exit_pc, exit_fp, trampoline_fp)
-        });
+        let inner: Box<dyn Iterator<Item = Frame>> = if exit_fp == 0 {
+            // No activations on this Store; return an empty iterator.
+            Box::new(core::iter::empty())
+        } else {
+            let unwind = store.0.unwinder();
+            // Establish the iterator.
+            Box::new(unsafe {
+                wasmtime_unwinder::frame_iterator(unwind, exit_pc, exit_fp, trampoline_fp)
+            })
+        };
 
         CurrentActivationBacktrace { store, inner }
     }

tests/all/debug.rs

@@ -186,3 +186,17 @@ fn gc_access_during_call() -> anyhow::Result<()> {
         },
     )
 }
+
+#[test]
+#[cfg_attr(miri, ignore)]
+fn debug_frames_on_store_with_no_wasm_activation() -> anyhow::Result<()> {
+    let mut config = Config::default();
+    config.guest_debug(true);
+    let engine = Engine::new(&config)?;
+    let mut store = Store::new(&engine, ());
+    let frames = store
+        .debug_frames()
+        .expect("Debug frames should be available");
+    assert!(frames.done());
+    Ok(())
+}