Don't re-capture backtraces when propagating traps through host frames (#5049)

* Add a benchmark for traps with many Wasm<-->host calls on the stack

* Add a test for expected Wasm stack traces with Wasm<--host calls on the stack when we trap

* Don't re-capture backtraces when propagating traps through host frames

This fixes some accidentally quadratic code where we would re-capture a Wasm
stack trace (takes `O(n)` time) every time we propagated a trap through a host
frame back to Wasm (can happen `O(n)` times). And `O(n) * O(n) = O(n^2)`, of
course. Whoops. After this commit, it trapping with a call stack that is `n`
frames deep of Wasm-to-host-to-Wasm calls just captures a single backtrace and
is therefore just a proper `O(n)` time operation, as it is intended to be.

Now we explicitly track whether we need to capture a Wasm backtrace or not when
raising a trap. This unfortunately isn't as straightforward as one might hope,
however, because of the split between `wasmtime::Trap` and
`wasmtime_runtime::Trap`. We need to decide whether or not to capture a Wasm
backtrace inside `wasmtime_runtime` but in order to determine whether to do that
or not we need to reflect on the `anyhow::Error` and see if it is a
`wasmtime::Trap` that already has a backtrace or not. This can't be done the
straightforward way because it would introduce a cyclic dependency between the
`wasmtime` and `wasmtime-runtime` crates. We can't merge those two `Trap`
types-- at least not without effectively merging the whole `wasmtime` and
`wasmtime-runtime` crates together, which would be a good idea in a perfect
world but would be a *ton* of ocean boiling from where we currently are --
because `wasmtime::Trap` does symbolication of stack traces which relies on
module registration information data that resides inside the `wasmtime` crate
and therefore can't be moved into `wasmtime-runtime`. We resolve this problem by
adding a boolean to `wasmtime_runtime::raise_user_trap` that controls whether we
should capture a Wasm backtrace or not, and then determine whether we need a
backtrace or not at each of that function's call sites, which are in `wasmtime`
and therefore can do the reflection to determine whether the user trap already
has a backtrace or not. Phew!

Fixes #5037

* debug assert that we don't record unnecessary backtraces for traps

* Add assertions around `needs_backtrace`

Unfortunately we can't do

    debug_assert_eq!(needs_backtrace, trap.inner.backtrace.get().is_some());

because `needs_backtrace` doesn't consider whether Wasm backtraces have been
disabled via config.

* Consolidate `needs_backtrace` calculation followed by calling `raise_user_trap` into one place

tests/all/traps.rs

@@ -69,6 +69,72 @@ fn test_trap_trace() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_trap_through_host() -> Result<()> {
+    let wat = r#"
+        (module $hello_mod
+            (import "" "" (func $host_func_a))
+            (import "" "" (func $host_func_b))
+            (func $a (export "a")
+                call $host_func_a
+            )
+            (func $b (export "b")
+                call $host_func_b
+            )
+            (func $c (export "c")
+                unreachable
+            )
+        )
+    "#;
+
+    let engine = Engine::default();
+    let module = Module::new(&engine, wat)?;
+    let mut store = Store::<()>::new(&engine, ());
+
+    let host_func_a = Func::new(
+        &mut store,
+        FuncType::new(vec![], vec![]),
+        |mut caller, _args, _results| {
+            caller
+                .get_export("b")
+                .unwrap()
+                .into_func()
+                .unwrap()
+                .call(caller, &[], &mut [])?;
+            Ok(())
+        },
+    );
+    let host_func_b = Func::new(
+        &mut store,
+        FuncType::new(vec![], vec![]),
+        |mut caller, _args, _results| {
+            caller
+                .get_export("c")
+                .unwrap()
+                .into_func()
+                .unwrap()
+                .call(caller, &[], &mut [])?;
+            Ok(())
+        },
+    );
+
+    let instance = Instance::new(
+        &mut store,
+        &module,
+        &[host_func_a.into(), host_func_b.into()],
+    )?;
+    let a = instance
+        .get_typed_func::<(), (), _>(&mut store, "a")
+        .unwrap();
+    let err = a.call(&mut store, ()).unwrap_err();
+    let trace = err.trace().expect("backtrace is available");
+    assert_eq!(trace.len(), 3);
+    assert_eq!(trace[0].func_name(), Some("c"));
+    assert_eq!(trace[1].func_name(), Some("b"));
+    assert_eq!(trace[2].func_name(), Some("a"));
+    Ok(())
+}
+
 #[test]
 #[allow(deprecated)]
 fn test_trap_backtrace_disabled() -> Result<()> {