[MERGE #5582 @irinayat-MS] Replace literal nullptr with null var object in JavascriptStackWalker::GetThis

Merge pull request #5582 from irinayat-MS:nullThis

Fixes part of OS#18397663. We might get nullptr for "this" argument from DocumentContextBridge::_CreateBrowserObject and possibly in other cases. We now translate nullptr into a proper JS null object.

Also removed semantically unclear return bool (nobody was checking it anyway).

Author: Irina Yatsenko

lib/Runtime/Language/JavascriptStackWalker.cpp

@@ -187,21 +187,20 @@ namespace Js
         return FALSE;
     }
 
-    bool JavascriptStackWalker::GetThis(Var* pVarThis, int moduleId) const
+    void JavascriptStackWalker::GetThis(Var* pVarThis, int moduleId) const
     {
 #if ENABLE_NATIVE_CODEGEN
         if (inlinedFramesBeingWalked)
         {
             if (inlinedFrameWalker.GetArgc() == 0)
             {
                 *pVarThis = JavascriptOperators::OP_GetThis(this->scriptContext->GetLibrary()->GetUndefined(), moduleId, scriptContext);
-                return false;
             }
-
-            *pVarThis = inlinedFrameWalker.GetThisObject();
-            Assert(*pVarThis);
-
-            return true;
+            else
+            {
+                *pVarThis = inlinedFrameWalker.GetThisObject();
+                Assert(*pVarThis);
+            }
         }
         else
 #endif
@@ -210,11 +209,16 @@ namespace Js
             if (callInfo.Count == 0)
             {
                 *pVarThis = JavascriptOperators::OP_GetThis(scriptContext->GetLibrary()->GetUndefined(), moduleId, scriptContext);
-                return false;
             }
+            else
+            {
+                *pVarThis = this->GetThisFromFrame();
+            }
+        }
 
-            *pVarThis = this->GetThisFromFrame();
-            return (*pVarThis) != nullptr;
+        if (*pVarThis == nullptr)
+        {
+            *pVarThis = this->scriptContext->GetLibrary()->GetNull();
         }
     }
 
@@ -1188,14 +1192,17 @@ namespace Js
         return FALSE;
     }
 
-    bool JavascriptStackWalker::GetThis(Var* pThis, int moduleId, ScriptContext* scriptContext)
+    void JavascriptStackWalker::GetThis(Var* pThis, int moduleId, ScriptContext* scriptContext)
     {
         JavascriptStackWalker walker(scriptContext);
         JavascriptFunction* caller;
-        return walker.GetCaller(&caller) && walker.GetThis(pThis, moduleId);
+        if (walker.GetCaller(&caller))
+        {
+            walker.GetThis(pThis, moduleId);
+        }
     }
 
-    bool JavascriptStackWalker::GetThis(Var* pThis, int moduleId, JavascriptFunction* func, ScriptContext* scriptContext)
+    void JavascriptStackWalker::GetThis(Var* pThis, int moduleId, JavascriptFunction* func, ScriptContext* scriptContext)
     {
         JavascriptStackWalker walker(scriptContext);
         JavascriptFunction* caller;
@@ -1204,10 +1211,9 @@ namespace Js
             if (caller == func)
             {
                 walker.GetThis(pThis, moduleId);
-                return true;
+                return;
             }
         }
-        return false;
     }
 
     // Try to see whether there is a top-most javascript frame, and if there is return true if it's native.

lib/Runtime/Language/JavascriptStackWalker.h

@@ -217,7 +217,7 @@ namespace Js
         void SetCurrentFunction(JavascriptFunction *  function);
         CallInfo GetCallInfo(bool includeInlinedFrames = true) const;
         CallInfo GetCallInfoFromPhysicalFrame() const;
-        bool GetThis(Var *pThis, int moduleId) const;
+        void GetThis(Var *pThis, int moduleId) const;
         Js::Var * GetJavascriptArgs(bool boxArgsAndDeepCopy) const;
         void **GetCurrentArgv() const;
 
@@ -244,8 +244,8 @@ namespace Js
         // noinline, we want to use own stack frame.
         static _NOINLINE BOOL GetCaller(_Out_opt_ JavascriptFunction** ppFunc, ScriptContext* scriptContext);
         static _NOINLINE BOOL GetCaller(_Out_opt_ JavascriptFunction** ppFunc, uint32* byteCodeOffset, ScriptContext* scriptContext);
-        static _NOINLINE bool GetThis(Var* pThis, int moduleId, ScriptContext* scriptContext);
-        static _NOINLINE bool GetThis(Var* pThis, int moduleId, JavascriptFunction* func, ScriptContext* scriptContext);
+        static _NOINLINE void GetThis(Var* pThis, int moduleId, ScriptContext* scriptContext);
+        static _NOINLINE void GetThis(Var* pThis, int moduleId, JavascriptFunction* func, ScriptContext* scriptContext);
 
         static bool IsDisplayCaller(JavascriptFunction* func);
         bool GetDisplayCaller(_Out_opt_ JavascriptFunction ** ppFunc);