[Stack Switching] Throw on trying to suspend through JS (#7825)

Avoid asserting there.

Author: kripken

scripts/test/fuzzing.py

@@ -116,6 +116,7 @@
     'cont_simple.wast',
     'gufa-cont.wast',
     'cont_many_unhandled.wast',
+    'cont_export.wast',
     # TODO: fix split_wast() on tricky escaping situations like a string ending
     #       in \\" (the " is not escaped - there is an escaped \ before it)
     'string-lifting-section.wast',

src/tools/execution-results.h

@@ -255,7 +255,10 @@ struct LoggingExternalInterface : public ShellExternalInterface {
 
     // Call the function.
     auto flow = instance->callFunction(func->name, arguments);
-    assert(!flow.suspendTag);
+    // Suspending through JS is not valid.
+    if (flow.suspendTag) {
+      throwJSException();
+    }
     return flow.values;
   }
 

test/lit/exec/cont_export.wast

@@ -0,0 +1,53 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --output=fuzz-exec and should not be edited.
+
+;; RUN: foreach %s %t wasm-opt -all --fuzz-exec-before -q -o /dev/null 2>&1 | filecheck %s
+
+(module
+  (type $none (func))
+  (type $cont (cont $none))
+
+  (import "fuzzing-support" "log" (func $log (param i32)))
+
+  (import "fuzzing-support" "call-export" (func $call-export (param i32 i32)))
+
+  (tag $tag (type $none))
+
+  ;; CHECK:      [fuzz-exec] calling suspend
+  ;; CHECK-NEXT: [LoggingExternalInterface logging 10]
+  ;; CHECK-NEXT: [exception thrown: unhandled suspend]
+  (func $suspend (export "suspend")
+    ;; Helper for below.
+    (call $log (i32.const 10))
+    (suspend $tag)
+    (call $log (i32.const 20))
+  )
+
+  ;; CHECK:      [fuzz-exec] calling call-call-export
+  ;; CHECK-NEXT: [LoggingExternalInterface logging 10]
+  ;; CHECK-NEXT: [exception thrown: __private externref]
+  (func $call-call-export (export "call-call-export")
+    ;; Call suspend as an export. We cannot suspend through JS, so we throw.
+    (call $call-export
+      (i32.const 0)
+      (i32.const 0)
+    )
+  )
+
+  ;; CHECK:      [fuzz-exec] calling handled
+  ;; CHECK-NEXT: [LoggingExternalInterface logging 10]
+  ;; CHECK-NEXT: [exception thrown: __private externref]
+  (func $handled (export "handled")
+    ;; As above, but inside a continuation, so it would be handled - if we could
+    ;; suspend though JS. But we can't, so we throw.
+    (drop
+      (block $inner (result (ref $cont))
+        (resume $cont (on $tag $inner)
+          (cont.new $cont
+            (ref.func $call-call-export)
+          )
+        )
+        (unreachable)
+      )
+    )
+  )
+)