QSignalSpy: make sig member const

Previously, the sig member was used to indicate successful
construction of the QSignalSpy (incl. successful connection to the
monitored signal), so it was set only after all other initialization
had taken place, preventing it from being marked as const, which would
indicate that accesses to it need not be protected by the mutex.

Now that we have it, we can instead use the d_ptr's value to indicate
success, and mark sig const.

[ChangeLog][QtTest][Important Behavior Changes][QSignalSpy] The
signal() method no longer necessarily returns an empty byte array when
the connection failed. Use the existing isValid() method to determine
whether a given QSignalSpy object listens to a valid signal on a valid
object.

Change-Id: Ia08fe3b383681f3f203cf1a121c0e1ce08ad268b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/testlib/qsignalspy.cpp

@@ -250,7 +250,8 @@ public:
 };
 
 QSignalSpy::QSignalSpy(ObjectSignal os)
-    : args(os.obj ? makeArgs(os.sig, os.obj) : QList<int>{})
+    : sig(os.sig.methodSignature()),
+      args(os.obj ? makeArgs(os.sig, os.obj) : QList<int>{})
 {
     if (!os.obj)
         return;
@@ -266,8 +267,6 @@ QSignalSpy::QSignalSpy(ObjectSignal os)
     }
 
     d_ptr = std::move(i);
-
-    sig = os.sig.methodSignature();
 }
 
 /*!

src/testlib/qsignalspy.h

@@ -41,7 +41,7 @@ public:
         : QSignalSpy(verify(obj, signal)) {}
     Q_TESTLIB_EXPORT ~QSignalSpy();
 
-    inline bool isValid() const { return !sig.isEmpty(); }
+    bool isValid() const noexcept { return d_ptr != nullptr; }
     inline QByteArray signal() const { return sig; }
 
     bool wait(int timeout)
@@ -58,7 +58,7 @@ private:
     Q_TESTLIB_EXPORT void appendArgs(void **a);
 
     // the full, normalized signal name
-    QByteArray sig;
+    const QByteArray sig;
     // holds the QMetaType types for the argument list of the signal
     const QList<int> args;