Short live qt_unique_lock/qt_scoped_lock! (until C++17)

Now that QRecursiveMutex is getting split off of QMutex, QMutexLocker
will stop working on QRecursiveMutex once the split has been finalized
in Qt 6.

Even today, QMutexLocker contains casts from QBasicMutex to QMutex
that some reviewers are uncomfortable with. One way to carry
QMutexLocker forward is to template it on the mutex type, possibly
with aliases like QBasicMutexLocker and QRecursiveMutexLocker. C++17
code would then not require a port, thanks to CTAD.

But we have the problem now, and we can't template QMutexLocker in Qt 5.

The alternative is to look at std and realize that they have surpassed
QMutexLocker in expressiveness already. A scoped_lock cannot be
unlocked again, a unique_lock can be moved around. QMutexLocker
doesn't do either. The only "problem" is that the std lock classes are
already templates, but we can't, yet, rely on C++17 CTAD to make them
look as if they weren't.

So, prepare for a future with C++17 CTAD by writing factory functions,
qt_scoped_lock and qt_unique_lock, which will later port mechanically
to their C++17 equivalents (mostly).

The functions are added to a new private qlocking_p.h becauee we don't
want to make them public. These are for use in Qt's own implementation,
or for users that don't care about compatibility and will not mind them
to be removed once we depend on C++17. Originally, I planned to use
qmutex_p.h instead, but that header is not self-contained and causes
build errors when we started to include it into libraries other than
QtCore.

Regarding the return value of qt_scoped_lock: Ideally, we'd like to
return a std::scoped_lock, but two things stand in the way: First,
scoped_lock was only added in C++17 (we fall back to lock_guard if
scoped_lock is not available). Second, returning one from a function
requires C++17 guaranteed copy elision, because neither scoped_lock
not lock_guard have a copy ctor. In order for code not to come to
depend on a particular lock class, we return any of lock_guard,
unique_lock or scoped_guard, depending on what the compiler supports,
and therefore wrap the functions in the unnamed namespace to avoid
running into ODR if (private) headers are used from different projects
(autotests, e.g.). By the time we can drop them, however, qt_*_lock
will be semantically 100% identical to their replacements.

Port some initial users.

Change-Id: I2a208ef2a4a533ee8e675812273986460e6b4d00
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/corelib/thread/thread.pri b/src/corelib/thread/thread.pri
index d11e6500ff..9fc9af0e65 100644
--- a/src/corelib/thread/thread.pri
+++ b/src/corelib/thread/thread.pri
@@ -28,6 +28,7 @@ qtConfig(thread) {
         thread/qbasicatomic.h \
         thread/qfutex_p.h \
         thread/qgenericatomic.h \
+        thread/qlocking_p.h \
         thread/qmutex_p.h \
         thread/qorderedmutexlocker_p.h \
         thread/qreadwritelock_p.h \