QUnicodeTools: fix data race in initialization of libthai symbols

The facilities of qunicodetools.cpp are not limited to the GUI thread,
so initialization must be thread-safe.

The old code wasn't, though, and contained several data races

- non-atomic initialized was read while another thead may write it
- th_brk and th_next_cell were read while another thead may write them

Fix by using Double-Checked Locking. This also prepares the code for
an eventual port to th_brk_find_breaks() (th_brk is deprecated).

The function pointers don't need to be atomic, because all reads from
them are guaranteed to happen-after the writes to them (as long as all
users call init_libthai() and don't proceeed if it returns false; this
could be ensured by returning a struct with the function pointers from
init_libthai() instead of maintaining them as statically-visible
globals, but that's outsize the scope of this patch).

As a drive-by, remove a pointless static_cast<int>(~~int expression~~).

Fixes: QTBUG-105543
Pick-to: 6.4 6.3 6.2
Change-Id: I492acd7e9a257e5c4b91f576e9bc448b6bb96ad1
Reviewed-by: Ievgenii Meshcheriakov <ievgenii.meshcheriakov@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@gmail.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 12 insertions, 5 deletions

diff --git a/src/corelib/text/qunicodetools.cpp b/src/corelib/text/qunicodetools.cpp
index a2a080d4ae..9d6590c6b8 100644
--- a/src/corelib/text/qunicodetools.cpp
+++ b/src/corelib/text/qunicodetools.cpp
@@ -3,12 +3,15 @@
 
 #include "qunicodetools_p.h"
 
+#include "qmutex.h"
 #include "qunicodetables_p.h"
 #include "qvarlengtharray.h"
 #if QT_CONFIG(library)
 #include "qlibrary.h"
 #endif
 
+#include <mutex>
+
 #include <limits.h>
 
 #define FLAG(x) (1 << (x))
@@ -1405,11 +1408,15 @@ Q_CONSTINIT static th_next_cell_def th_next_cell = nullptr;
 
 static int init_libthai() {
 #if QT_CONFIG(library)
-    Q_CONSTINIT static bool initialized = false;
-    if (!initialized && (!th_brk || !th_next_cell)) {
-        th_brk = reinterpret_cast<th_brk_def>(QLibrary::resolve("thai"_L1, static_cast<int>(LIBTHAI_MAJOR), "th_brk"));
-        th_next_cell = (th_next_cell_def)QLibrary::resolve("thai"_L1, LIBTHAI_MAJOR, "th_next_cell");
-        initialized = true;
+    Q_CONSTINIT static QBasicAtomicInt initialized = Q_BASIC_ATOMIC_INITIALIZER(false);
+    Q_CONSTINIT static QBasicMutex mutex;
+    if (!initialized.loadAcquire()) {
+        const auto locker = std::scoped_lock(mutex);
+        if (!initialized.loadAcquire()) {
+            th_brk = reinterpret_cast<th_brk_def>(QLibrary::resolve("thai"_L1, LIBTHAI_MAJOR, "th_brk"));
+            th_next_cell = (th_next_cell_def)QLibrary::resolve("thai"_L1, LIBTHAI_MAJOR, "th_next_cell");
+            initialized.storeRelease(true);
+        }
     }
     if (th_brk && th_next_cell)
         return 1;