Use Q_GLOBAL_STATIC for the udev symbol loading QLibrary instance

Even though this has been introduced in 5.1, officially, we can rely on it in
the previous versions even if private API that it was.

* Global variables are better avoided.

* Q_GLOBAL_STATIC is thread-safe in Qt 5.

* QLibrary depends on QCoreApplication and we need to make sure that is
initialized afterwards, respectively.

The instantiation is now moved into the corresponding source file so that if it
is included at more than one place, it will not be instantiated each time it is
used in a new source file including this header.

Task-number: QTBUG-36870

Change-Id: I96de1257e5836b69d0c48b717d7c2e708d6b0fee
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>

2 files changed, 17 insertions, 15 deletions

diff --git a/src/serialport/qserialportinfo_unix.cpp b/src/serialport/qserialportinfo_unix.cpp
index 80b2f7d..a8bb6d3 100644
--- a/src/serialport/qserialportinfo_unix.cpp
+++ b/src/serialport/qserialportinfo_unix.cpp
@@ -195,10 +195,14 @@ QList<QSerialPortInfo> availablePortsBySysfs()
     return serialPortInfoList;
 }
 
+#ifndef LINK_LIBUDEV
+    Q_GLOBAL_STATIC(QLibrary, udevLibrary)
+#endif
+
 QList<QSerialPortInfo> availablePortsByUdev()
 {
 #ifndef LINK_LIBUDEV
-    static bool symbolsResolved = resolveSymbols();
+    static bool symbolsResolved = resolveSymbols(udevLibrary());
     if (!symbolsResolved)
         return QList<QSerialPortInfo>();
 #endif
diff --git a/src/serialport/qtudev_p.h b/src/serialport/qtudev_p.h
index 1187165..434292d 100644
--- a/src/serialport/qtudev_p.h
+++ b/src/serialport/qtudev_p.h
@@ -57,7 +57,7 @@ extern "C"
     fp_##symbolName symbolName;
 
 #define RESOLVE_SYMBOL(symbolName) \
-    symbolName = (fp_##symbolName)resolveSymbol(#symbolName); \
+    symbolName = (fp_##symbolName)resolveSymbol(udevLibrary, #symbolName); \
     if (!symbolName) \
         return false;
 
@@ -89,16 +89,14 @@ GENERATE_SYMBOL_VARIABLE(void, udev_device_unref, struct udev_device *)
 GENERATE_SYMBOL_VARIABLE(void, udev_enumerate_unref, struct udev_enumerate *)
 GENERATE_SYMBOL_VARIABLE(void, udev_unref, struct udev *)
 
-QLibrary udevLibrary;
-
 #if (QT_VERSION >= QT_VERSION_CHECK(5, 0, 0))
-inline QFunctionPointer resolveSymbol(const char *symbolName)
+inline QFunctionPointer resolveSymbol(QLibrary *udevLibrary, const char *symbolName)
 {
-    QFunctionPointer symbolFunctionPointer = udevLibrary.resolve(symbolName);
+    QFunctionPointer symbolFunctionPointer = udevLibrary->resolve(symbolName);
 #else
-inline void *resolveSymbol(const char *symbolName)
+inline void *resolveSymbol(QLibrary *udevLibrary, const char *symbolName)
 {
-    void *symbolFunctionPointer = udevLibrary.resolve(symbolName);
+    void *symbolFunctionPointer = udevLibrary->resolve(symbolName);
 #endif
     if (!symbolFunctionPointer)
         qWarning("Failed to resolve the udev symbol: %s", symbolName);
@@ -106,14 +104,14 @@ inline void *resolveSymbol(const char *symbolName)
     return symbolFunctionPointer;
 }
 
-inline bool resolveSymbols()
+inline bool resolveSymbols(QLibrary *udevLibrary)
 {
-    if (!udevLibrary.isLoaded()) {
-        udevLibrary.setFileNameAndVersion(QStringLiteral("udev"), 1);
-        if (!udevLibrary.load()) {
-            udevLibrary.setFileNameAndVersion(QStringLiteral("udev"), 0);
-            if (!udevLibrary.load()) {
-                qWarning("Failed to load the library: %s, supported version(s): %i and %i", qPrintable(udevLibrary.fileName()), 1, 0);
+    if (!udevLibrary->isLoaded()) {
+        udevLibrary->setFileNameAndVersion(QStringLiteral("udev"), 1);
+        if (!udevLibrary->load()) {
+            udevLibrary->setFileNameAndVersion(QStringLiteral("udev"), 0);
+            if (!udevLibrary->load()) {
+                qWarning("Failed to load the library: %s, supported version(s): %i and %i", qPrintable(udevLibrary->fileName()), 1, 0);
                 return false;
             }
         }