QHostInfo: adapt to glibc 2.26 no longer requiring us to res_init

From the announcement[1]
  - The GNU C Library will now detect when /etc/resolv.conf has been
    modified and reload the changed configuration.  The new resolver
    option “no-reload” (RES_NORELOAD) disables this behavior.

Since glibc can do it, we don't have to call res_init before every
single getaddrinfo() call. And since we don't need to call res_init, we
don't need to load libresolv.so at all, until we need res_ninit.

We won't do it even if the user configured "no-reload" in
/etc/resolv.conf or RES_NORELOAD in the res variable -- let's assume
that it is the intent, like when /etc/resolv.conf is known to never
change.

[1] https://sourceware.org/ml/libc-alpha/2017-08/msg00010.html

Change-Id: I3868166e5efc45538544fffd14d738d40c375fd1
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/kernel/qhostinfo_unix.cpp

@@ -83,6 +83,11 @@ QT_BEGIN_NAMESPACE
 #  define Q_ADDRCONFIG          AI_ADDRCONFIG
 #endif
 
+enum LibResolvFeature {
+    NeedResInit,
+    NeedResNInit
+};
+
 typedef struct __res_state *res_state_ptr;
 
 typedef int (*res_init_proto)(void);
@@ -93,9 +98,29 @@ typedef void (*res_nclose_proto)(res_state_ptr);
 static res_nclose_proto local_res_nclose = 0;
 static res_state_ptr local_res = 0;
 
-static bool resolveLibraryInternal()
-{
 #if QT_CONFIG(library) && !defined(Q_OS_QNX)
+namespace {
+struct LibResolv
+{
+    enum {
+#ifdef RES_NORELOAD
+        // If RES_NORELOAD is defined, then the libc is capable of watching
+        // /etc/resolv.conf for changes and reloading as necessary. So accept
+        // whatever is configured.
+        ReinitNecessary = false
+#else
+        ReinitNecessary = true
+#endif
+    };
+
+    QLibrary lib;
+    LibResolv();
+    ~LibResolv() { lib.unload(); }
+};
+}
+
+LibResolv::LibResolv()
+{
     QLibrary lib;
 #ifdef LIBRESOLV_SO
     lib.setFileName(QStringLiteral(LIBRESOLV_SO));
@@ -104,32 +129,45 @@ static bool resolveLibraryInternal()
     {
         lib.setFileName(QLatin1String("resolv"));
         if (!lib.load())
-            return false;
+            return;
     }
 
-    local_res_init = res_init_proto(lib.resolve("__res_init"));
+    // res_ninit is required for localDomainName()
-    if (!local_res_init)
-        local_res_init = res_init_proto(lib.resolve("res_init"));
-
     local_res_ninit = res_ninit_proto(lib.resolve("__res_ninit"));
     if (!local_res_ninit)
         local_res_ninit = res_ninit_proto(lib.resolve("res_ninit"));
-
+    if (local_res_ninit) {
-    if (!local_res_ninit) {
+        // we must now find res_nclose
-        // 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"));
-    } else {
         local_res_nclose = res_nclose_proto(lib.resolve("res_nclose"));
         if (!local_res_nclose)
             local_res_nclose = res_nclose_proto(lib.resolve("__res_nclose"));
         if (!local_res_nclose)
-            local_res_ninit = 0;
+            local_res_ninit = nullptr;
     }
-#endif
 
-    return true;
+    if (ReinitNecessary || !local_res_ninit) {
+        local_res_init = res_init_proto(lib.resolve("__res_init"));
+        if (!local_res_init)
+            local_res_init = res_init_proto(lib.resolve("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"));
+        }
+    }
 }
-Q_GLOBAL_STATIC_WITH_ARGS(bool, resolveLibrary, (resolveLibraryInternal()))
+Q_GLOBAL_STATIC(LibResolv, libResolv)
+
+static void resolveLibrary(LibResolvFeature f)
+{
+    if (LibResolv::ReinitNecessary || f == NeedResNInit)
+        libResolv();
+}
+#else // QT_CONFIG(library) || Q_OS_QNX
+static void resolveLibrary(LibResolvFeature)
+{
+}
+#endif // QT_CONFIG(library) || Q_OS_QNX
 
 QHostInfo QHostInfoAgent::fromName(const QString &hostName)
 {
@@ -141,7 +179,7 @@ QHostInfo QHostInfoAgent::fromName(const QString &hostName)
 #endif
 
     // Load res_init on demand.
-    resolveLibrary();
+    resolveLibrary(NeedResInit);
 
     // If res_init is available, poll it.
     if (local_res_init)
@@ -320,7 +358,7 @@ QHostInfo QHostInfoAgent::fromName(const QString &hostName)
 QString QHostInfo::localDomainName()
 {
 #if !defined(Q_OS_VXWORKS) && !defined(Q_OS_ANDROID)
-    resolveLibrary();
+    resolveLibrary(NeedResNInit);
     if (local_res_ninit) {
         // using thread-safe version
         res_state_ptr state = res_state_ptr(malloc(sizeof(*state)));