Bug 1448880 - Part 1: Always close a generator on early forced return. r=jimb

The previous code failed to close the generator in the case where
JSOP_GENERATOR had run but JSOP_INITIAL_YIELD had not, a bit of sloppiness that
created yet another special case. Things will get more complicated when we
start keeping frames live while suspended; it's better to not have this special
case.

Differential Revision: https://phabricator.services.mozilla.com/D6982

--HG--
extra : moz-landing-system : lando

js/src/jit-test/tests/debug/Frame-onStep-generator-resumption-01.js

@@ -0,0 +1,42 @@
+// The debugger can force an early return from any instruction before the initial yield.
+
+let g = newGlobal();
+g.eval(`
+  function* f() {
+    yield 1;
+  }
+`);
+
+function test(ttl) {
+    let dbg = new Debugger(g);
+    let exiting = false;  // we ran out of time-to-live and have forced return
+    let done = false;  // we reached the initial yield without forced return
+    dbg.onEnterFrame = frame => {
+        assertEq(frame.callee.name, "f");
+        frame.onEnterFrame = undefined;
+        frame.onStep = () => {
+            if (ttl == 0) {
+                exiting = true;
+                // Forced return here causes the generator object, if any, not
+                // to be exposed.
+                return {return: "ponies"};
+            }
+            ttl--;
+        };
+        frame.onPop = completion => {
+            if (!exiting)
+                done = true;
+        };
+    };
+
+    let result = g.f();
+    if (done)
+        assertEq(result instanceof g.f, true);
+    else
+        assertEq(result, "ponies");
+
+    dbg.enabled = false;
+    return done;
+}
+
+for (let ttl = 0; !test(ttl); ttl++) {}

js/src/vm/Debugger.cpp

@@ -1543,18 +1543,20 @@ AdjustGeneratorResumptionValue(JSContext* cx, AbstractFramePtr frame,
         // on that bytecode sequence existing in the debuggee, somehow jump to
         // it, and then avoid re-entering the debugger from it.
         Rooted<GeneratorObject*> genObj(cx, GetGeneratorObjectForFrame(cx, frame));
-        if (genObj && !genObj->isBeforeInitialYield()) {
+        if (genObj) {
             // 1.  `return <value>` creates and returns a new object,
             //     `{value: <value>, done: true}`.
-            JSObject *pair = CreateIterResultObject(cx, vp, true);
-            if (!pair) {
-                // Out of memory in debuggee code. Arrange for this to propagate.
-                MOZ_ALWAYS_TRUE(cx->getPendingException(vp));
-                cx->clearPendingException();
-                resumeMode = ResumeMode::Throw;
-                return;
+            if (!genObj->isBeforeInitialYield()) {
+                JSObject *pair = CreateIterResultObject(cx, vp, true);
+                if (!pair) {
+                    // Out of memory in debuggee code. Arrange for this to propagate.
+                    MOZ_ALWAYS_TRUE(cx->getPendingException(vp));
+                    cx->clearPendingException();
+                    resumeMode = ResumeMode::Throw;
+                    return;
+                }
+                vp.setObject(*pair);
             }
-            vp.setObject(*pair);
 
             // 2.  The generator must be closed.
             genObj->setClosed();