Make osl_getLocalHostname thread-safe

Change-Id: I82b8c49fcbbec161bf968573e28992fa5737b45b
Reviewed-on: https://gerrit.libreoffice.org/67508
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

sal/osl/w32/socket.cxx

@@ -17,6 +17,10 @@
  *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
  */
 
+#include <sal/config.h>
+
+#include <utility>
+
 #include "system.h"
 
 #include <osl/socket.h>
@@ -596,10 +600,9 @@ void SAL_CALL osl_destroyHostAddr(oslHostAddr pAddr)
 
 oslSocketResult SAL_CALL osl_getLocalHostname (rtl_uString **strLocalHostname)
 {
-    static sal_Unicode LocalHostname[256] = {0};
+    static auto const init = []() -> std::pair<oslSocketResult, OUString> {
+            sal_Unicode LocalHostname[256] = {0};
 
-    if (rtl_ustr_getLength(LocalHostname) == 0)
-    {
             sal_Char Host[256]= "";
             if (gethostname(Host, sizeof(Host)) == 0)
             {
@@ -638,15 +641,18 @@ oslSocketResult SAL_CALL osl_getLocalHostname (rtl_uString **strLocalHostname)
                     }
                 }
             }
-    }
 
             if (rtl_ustr_getLength(LocalHostname) > 0)
             {
-        rtl_uString_newFromStr (strLocalHostname, LocalHostname);
-        return osl_Socket_Ok;
+                return {osl_Socket_Ok, LocalHostname};
             }
 
-    return osl_Socket_Error;
+            return {osl_Socket_Error, OUString()};
+        }();
+
+    rtl_uString_assign (strLocalHostname, init.second.pData);
+
+    return init.first;
 }
 
 oslSocketAddr SAL_CALL osl_resolveHostname(rtl_uString* strHostname)