QLibrary: introduce a mutex to protect non-atomic internals

And make pHnd atomic.

The majority of the variables is updated in QLibraryPrivate::load_sys
and updatePluginState(), which get the mutex protection.
QLibraryPrivate::unload_sys() doesn't need a mutex protection because we
have the refcounting.

[ChangeLog][QtCore][QLibrary & QPluginLoader] Fixed a number of race
conditions caused by having two QLibrary objects pointing to the same
library being operated in different threads.

Fixes: QTBUG-39642
Change-Id: I46bf1f65e8db46afbde5fffd15e1a5b3f5e74ea4
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 13 insertions, 11 deletions

diff --git a/src/corelib/plugin/qlibrary_unix.cpp b/src/corelib/plugin/qlibrary_unix.cpp
index 23f96dbdaf..29813e5863 100644
--- a/src/corelib/plugin/qlibrary_unix.cpp
+++ b/src/corelib/plugin/qlibrary_unix.cpp
@@ -214,8 +214,9 @@ bool QLibraryPrivate::load_sys()
 #endif
 
     bool retry = true;
-    for(int prefix = 0; retry && !pHnd && prefix < prefixes.size(); prefix++) {
-        for(int suffix = 0; retry && !pHnd && suffix < suffixes.size(); suffix++) {
+    Handle hnd = nullptr;
+    for (int prefix = 0; retry && !hnd && prefix < prefixes.size(); prefix++) {
+        for (int suffix = 0; retry && !hnd && suffix < suffixes.size(); suffix++) {
             if (!prefixes.at(prefix).isEmpty() && name.startsWith(prefixes.at(prefix)))
                 continue;
             if (path.isEmpty() && prefixes.at(prefix).contains(QLatin1Char('/')))
@@ -232,7 +233,7 @@ bool QLibraryPrivate::load_sys()
                 attempt = path + prefixes.at(prefix) + name + suffixes.at(suffix);
             }
 
-            pHnd = dlopen(QFile::encodeName(attempt), dlFlags);
+            hnd = dlopen(QFile::encodeName(attempt), dlFlags);
 #ifdef Q_OS_ANDROID
             if (!pHnd) {
                 auto attemptFromBundle = attempt;
@@ -248,7 +249,7 @@ bool QLibraryPrivate::load_sys()
             }
 #endif
 
-            if (!pHnd && fileName.startsWith(QLatin1Char('/')) && QFile::exists(attempt)) {
+            if (!hnd && fileName.startsWith(QLatin1Char('/')) && QFile::exists(attempt)) {
                 // We only want to continue if dlopen failed due to that the shared library did not exist.
                 // However, we are only able to apply this check for absolute filenames (since they are
                 // not influenced by the content of LD_LIBRARY_PATH, /etc/ld.so.cache, DT_RPATH etc...)
@@ -259,7 +260,7 @@ bool QLibraryPrivate::load_sys()
     }
 
 #ifdef Q_OS_MAC
-    if (!pHnd) {
+    if (!hnd) {
         QByteArray utf8Bundle = fileName.toUtf8();
         QCFType<CFURLRef> bundleUrl = CFURLCreateFromFileSystemRepresentation(NULL, reinterpret_cast<const UInt8*>(utf8Bundle.data()), utf8Bundle.length(), true);
         QCFType<CFBundleRef> bundle = CFBundleCreate(NULL, bundleUrl);
@@ -268,23 +269,24 @@ bool QLibraryPrivate::load_sys()
             char executableFile[FILENAME_MAX];
             CFURLGetFileSystemRepresentation(url, true, reinterpret_cast<UInt8*>(executableFile), FILENAME_MAX);
             attempt = QString::fromUtf8(executableFile);
-            pHnd = dlopen(QFile::encodeName(attempt), dlFlags);
+            hnd = dlopen(QFile::encodeName(attempt), dlFlags);
         }
     }
 #endif
-    if (!pHnd) {
+    if (!hnd) {
         errorString = QLibrary::tr("Cannot load library %1: %2").arg(fileName, qdlerror());
     }
-    if (pHnd) {
+    if (hnd) {
         qualifiedFileName = attempt;
         errorString.clear();
     }
-    return (pHnd != 0);
+    pHnd.storeRelaxed(hnd);
+    return (hnd != nullptr);
 }
 
 bool QLibraryPrivate::unload_sys()
 {
-    if (dlclose(pHnd)) {
+    if (dlclose(pHnd.loadAcquire())) {
 #if defined (Q_OS_QNX)                // Workaround until fixed in QNX; fixes crash in
         char *error = dlerror();      // QtDeclarative auto test "qqmlenginecleanup" for instance
         if (!qstrcmp(error, "Shared objects still referenced")) // On QNX that's only "informative"
@@ -316,7 +318,7 @@ Q_CORE_EXPORT QFunctionPointer qt_mac_resolve_sys(void *handle, const char *symb
 
 QFunctionPointer QLibraryPrivate::resolve_sys(const char* symbol)
 {
-    QFunctionPointer address = QFunctionPointer(dlsym(pHnd, symbol));
+    QFunctionPointer address = QFunctionPointer(dlsym(pHnd.loadAcquire(), symbol));
     return address;
 }