Fix to potential leak introduced with fix to 40406.

Be conservative in handling the lifetime of the safe context created by XPConnect to execute JS Components, and save it off to be destroyed at cleanup time, even if some other safe context is registered with XPConnect via SetSafeJSContext.

r=vishy, a=brendan

js/src/xpconnect/src/xpcprivate.h

@@ -1370,8 +1370,11 @@ private:
     nsIXPCException*  mException;
     nsDeque*          mJSContextStack;
     JSContext*        mSafeJSContext;
+
+    // If if non-null, we own it; same as mSafeJSContext if SetSafeJSContext
+    // not called.
+    JSContext*        mOwnSafeJSContext;
     xpcPerThreadData* mNextThread;
-    PRBool            mSafeContextIsFromSetter;
 
     static PRLock*           gLock;
     static xpcPerThreadData* gThreads;

js/src/xpconnect/src/xpcthreadcontext.cpp

@@ -198,8 +198,8 @@ xpcPerThreadData::xpcPerThreadData()
     :   mException(nsnull),
         mJSContextStack(new nsDeque(nsnull)),
         mSafeJSContext(nsnull),
-        mNextThread(nsnull),
-        mSafeContextIsFromSetter(PR_FALSE)
+        mOwnSafeJSContext(nsnull),
+        mNextThread(nsnull)
 {
     if(gLock)
     {
@@ -220,10 +220,10 @@ xpcPerThreadData::Cleanup()
         mJSContextStack = nsnull;
     }
 
-    if(mSafeJSContext && !mSafeContextIsFromSetter)
+    if(mOwnSafeJSContext)
     {
-        JS_DestroyContext(mSafeJSContext);
-        mSafeJSContext = nsnull;
+        JS_DestroyContext(mOwnSafeJSContext);
+        mOwnSafeJSContext = nsnull;
     }
 }
 
@@ -319,6 +319,13 @@ xpcPerThreadData::GetSafeJSContext()
                         JS_DestroyContext(mSafeJSContext);
                         mSafeJSContext = nsnull;
                     }
+                    // Save it off so we can destroy it later, even if
+                    // mSafeJSContext has been set to another context
+                    // via SetSafeJSContext.  If we don't get here,
+                    // then mSafeJSContext must have been set via
+                    // SetSafeJSContext, and we're not responsible for
+                    // destroying the passed-in context.
+                    mOwnSafeJSContext = mSafeJSContext;
                 }
             }
         }
@@ -329,8 +336,11 @@ xpcPerThreadData::GetSafeJSContext()
 nsresult
 xpcPerThreadData::SetSafeJSContext(JSContext *cx)
 {
+    NS_ASSERTION(cx, "SetSafeJSContext called with a null context");
+
+    if(!cx)
+        return NS_ERROR_INVALID_ARG;
     mSafeJSContext = cx;
-    mSafeContextIsFromSetter = PR_TRUE;
     return NS_OK;
 }