QHostInfo: use dlsym() with RTLD_DEFAULT in case libs cannot be loaded

The current code only tries to load the required functions from
LIBRESOLV_SO (if defined) and resolv, but on FreeBSD they are in libc:
https://www.freebsd.org/cgi/man.cgi?query=res_query&sektion=3&apropos=0&manpath=freebsd

This commit changes the code so that, after failing to load the
non-existent libraries, it attempts to load the functions with dlsym()
using the special handle RTLD_DEFAULT, which searches for the specified
symbol in the loaded libraries.

This is a follow-up to 8eeb5150

.

Change-Id: I19d90b0ca8703398bf4f5f4edd5ae31e346ef251
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/network/kernel/kernel.pri

@@ -38,13 +38,12 @@ qtConfig(dnslookup) {
 }
 
 unix {
-    !integrity:qtConfig(dnslookup) {
-        SOURCES += kernel/qdnslookup_unix.cpp
-        qtConfig(dlopen): QMAKE_USE_PRIVATE += libdl
-    }
+    !integrity:qtConfig(dnslookup): SOURCES += kernel/qdnslookup_unix.cpp
 
     SOURCES += kernel/qhostinfo_unix.cpp
 
+    qtConfig(dlopen): QMAKE_USE_PRIVATE += libdl
+
     qtConfig(linux-netlink): SOURCES += kernel/qnetworkinterface_linux.cpp
     else: SOURCES += kernel/qnetworkinterface_unix.cpp
 }

src/network/kernel/qhostinfo_unix.cpp

@@ -66,6 +66,10 @@
 #  include <gnu/lib-names.h>
 #endif
 
+#if defined(Q_OS_FREEBSD) || QT_CONFIG(dlopen)
+#  include <dlfcn.h>
+#endif
+
 QT_BEGIN_NAMESPACE
 
 // Almost always the same. If not, specify in qplatformdefs.h.
@@ -115,6 +119,18 @@ struct LibResolv
 };
 }
 
+static QFunctionPointer resolveSymbol(QLibrary &lib, const char *sym)
+{
+    if (lib.isLoaded())
+        return lib.resolve(sym);
+
+#if defined(RTLD_DEFAULT) && (defined(Q_OS_FREEBSD) || QT_CONFIG(dlopen))
+    return reinterpret_cast<QFunctionPointer>(dlsym(RTLD_DEFAULT, sym));
+#else
+    return nullptr;
+#endif
+}
+
 LibResolv::LibResolv()
 {
     QLibrary lib;
@@ -124,31 +140,30 @@ LibResolv::LibResolv()
 #endif
     {
         lib.setFileName(QLatin1String("resolv"));
-        if (!lib.load())
-            return;
+        lib.load();
     }
 
     // res_ninit is required for localDomainName()
-    local_res_ninit = res_ninit_proto(lib.resolve("__res_ninit"));
+    local_res_ninit = res_ninit_proto(resolveSymbol(lib, "__res_ninit"));
     if (!local_res_ninit)
-        local_res_ninit = res_ninit_proto(lib.resolve("res_ninit"));
+        local_res_ninit = res_ninit_proto(resolveSymbol(lib, "res_ninit"));
     if (local_res_ninit) {
         // we must now find res_nclose
-        local_res_nclose = res_nclose_proto(lib.resolve("res_nclose"));
+        local_res_nclose = res_nclose_proto(resolveSymbol(lib, "res_nclose"));
         if (!local_res_nclose)
-            local_res_nclose = res_nclose_proto(lib.resolve("__res_nclose"));
+            local_res_nclose = res_nclose_proto(resolveSymbol(lib, "__res_nclose"));
         if (!local_res_nclose)
             local_res_ninit = nullptr;
     }
 
     if (ReinitNecessary || !local_res_ninit) {
-        local_res_init = res_init_proto(lib.resolve("__res_init"));
+        local_res_init = res_init_proto(resolveSymbol(lib, "__res_init"));
         if (!local_res_init)
-            local_res_init = res_init_proto(lib.resolve("res_init"));
+            local_res_init = res_init_proto(resolveSymbol(lib, "res_init"));
 
         if (local_res_init && !local_res_ninit) {
             // if we can't get a thread-safe context, we have to use the global _res state
-            local_res = res_state_ptr(lib.resolve("_res"));
+            local_res = res_state_ptr(resolveSymbol(lib, "_res"));
         }
     }
 }