A follow-up fix for potentially missing OpenSSL symbol resolving

This patch also adds 'isValid()' to know if a backend from a plugin
is in working condition (say, there is 'openssl' plugin but no or
old openssl libraries in some system).

Pick-to: dev
Task-number: QTBUG-65922
Change-Id: I0b846536a069ca8c5a94e7191f11c81bac6ad527
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/ssl/qsslsocket.cpp

@@ -2844,6 +2844,11 @@ QTlsBackend *QSslSocketPrivate::tlsBackendInUse()
     if (!activeBackendName.size())
         activeBackendName = QTlsBackend::defaultBackendName();
 
+    if (!activeBackendName.size()) {
+        qCWarning(lcSsl, "No functional TLS backend was found");
+        return nullptr;
+    }
+
     return tlsBackend = QTlsBackend::findBackend(activeBackendName);
 }
 

src/network/ssl/qtlsbackend.cpp

@@ -110,8 +110,10 @@ public:
             return names;
 
         names.reserve(backends.size());
-        for (const auto *factory : backends)
-            names.append(factory->backendName());
+        for (const auto *backend : backends) {
+            if (backend->isValid())
+                names.append(backend->backendName());
+        }
 
         return names;
     }
@@ -198,6 +200,11 @@ QTlsBackend::~QTlsBackend()
         backends->removeBackend(this);
 }
 
+bool QTlsBackend::isValid() const
+{
+    return true;
+}
+
 QString QTlsBackend::backendName() const
 {
     return QStringLiteral("dummyTLS");

src/network/ssl/qtlsbackend_openssl.cpp

@@ -41,6 +41,10 @@
 #include "qtlskey_openssl_p.h"
 #include "qx509_openssl_p.h"
 
+// TLSTODO: Later, this code (ensure initialised, etc.)
+// must move from the socket to backend.
+#include "qsslsocket_p.h"
+//
 #include "qsslsocket_openssl_symbols_p.h"
 
 #include <qssl.h>
@@ -83,6 +87,13 @@ QString QTlsBackendOpenSSL::backendName() const
     return builtinBackendNames[nameIndexOpenSSL];
 }
 
+bool QTlsBackendOpenSSL::isValid() const
+{
+    // TLSTODO: backend should do initialization,
+    // not socket.
+    return QSslSocket::supportsSsl();
+}
+
 QList<QSsl::SslProtocol> QTlsBackendOpenSSL::supportedProtocols() const
 {
     QList<QSsl::SslProtocol> protocols;

src/network/ssl/qtlsbackend_openssl_p.h

@@ -68,6 +68,7 @@ public:
     static void clearErrorQueue();
 private:
     QString backendName() const override;
+    bool isValid() const override;
 
     QList<QSsl::SslProtocol> supportedProtocols() const override;
     QList<QSsl::SupportedFeature> supportedFeatures() const override;

src/network/ssl/qtlsbackend_p.h

@@ -195,6 +195,8 @@ public:
     QTlsBackend();
     ~QTlsBackend() override;
 
+    virtual bool isValid() const;
+
     virtual QString backendName() const = 0;
     virtual QList<QSsl::SslProtocol> supportedProtocols() const = 0;
     virtual QList<QSsl::SupportedFeature> supportedFeatures() const = 0;