[Ada] Don't correct socket timeout on Windows Server 2019

[Pierre-Marie de Rodat]

Windows Server 2019 do not need 500ms socket timeout correction. The
problem is that Windows API recommended function
IsWindowsVersionOrGreater is not comparing the build part which is only
difference between Windows 2016 and 2019 servers.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-12  Dmitriy Anisimkov  <anisi...@adacore.com>

gcc/ada/

        * socket.c (__gnat_minus_500ms): Use GetVersionEx to detect
        Windows Server version.
        * libgnat/g-sothco.ads (Minus_500ms_Windows_Timeout): Remade to
        Boolean constant.
        * libgnat/g-socket.adb (Set_Socket_Option): Use
        Minus_500ms_Windows_Timeout constant instead of function call.
--- gcc/ada/libgnat/g-socket.adb
+++ gcc/ada/libgnat/g-socket.adb
@@ -2722,7 +2722,7 @@ package body GNAT.Sockets is
                --  Old windows versions actual timeout is 500 ms + the given
                --  value (unless it is 0).
 
-               if Minus_500ms_Windows_Timeout /= 0 then
+               if Minus_500ms_Windows_Timeout then
                   if U4 > 500 then
                      U4 := U4 - 500;
 

--- gcc/ada/libgnat/g-sothco.ads
+++ gcc/ada/libgnat/g-sothco.ads
@@ -451,12 +451,19 @@ package GNAT.Sockets.Thin_Common is
    renames Short_To_Network;
    --  Symmetric operation
 
-   function Minus_500ms_Windows_Timeout return C.int;
+   Minus_500ms_Windows_Timeout : constant Boolean;
    --  Microsoft Windows desktop older then 8.0 and Microsoft Windows Server
    --  older than 2019 need timeout correction for 500 milliseconds. This
-   --  routine returns 1 for such versions.
+   --  constant is True for such versions.
 
 private
+
+   function Get_Minus_500ms_Timeout return C.int
+     with Import, Convention => C, External_Name => "__gnat_minus_500ms";
+
+   Minus_500ms_Windows_Timeout : constant Boolean :=
+                                   Get_Minus_500ms_Timeout /= 0;
+
    pragma Import (C, Get_Socket_From_Set, "__gnat_get_socket_from_set");
    pragma Import (C, Is_Socket_In_Set, "__gnat_is_socket_in_set");
    pragma Import (C, Last_Socket_In_Set, "__gnat_last_socket_in_set");
@@ -488,6 +495,4 @@ private
    pragma Import (C, Hostent_H_Length,   "__gnat_hostent_h_length");
    pragma Import (C, Hostent_H_Addr,     "__gnat_hostent_h_addr");
 
-   pragma Import (C, Minus_500ms_Windows_Timeout, "__gnat_minus_500ms");
-
 end GNAT.Sockets.Thin_Common;

--- gcc/ada/socket.c
+++ gcc/ada/socket.c
@@ -801,14 +801,26 @@ const char * __gnat_gai_strerror(int errcode) {
 
 int __gnat_minus_500ms() {
 #if defined (_WIN32)
-  // Windows 8.0 and newer do not need 500 millisecond socket timeout
-  // correction.
-  // We do not know the Windows server version without socket timeout
-  // correction for now. When we know, we can add the call for
-  // IsWindowsVersionOrGreater(10, 0, ????) into condition.
-  return !IsWindows8OrGreater() || IsWindowsServer();
+  // Windows Server 2019 and Windows 8.0 do not need 500 millisecond socket
+  // timeout correction.
+  if (IsWindowsServer()) {
+    OSVERSIONINFO osvi;
+    ZeroMemory(&osvi, sizeof(OSVERSIONINFO));
+    osvi.dwOSVersionInfoSize = sizeof(OSVERSIONINFO);
+    // Documentation proposes to use IsWindowsVersionOrGreater(10, 0, 17763)
+    // but it does not compare by the build number (last parameter). See
+    // regression test for RC03-012 in fixedbugs, there are some code to
+    // investigate Windows version API behavior.
+    GetVersionEx(&osvi);
+    return osvi.dwMajorVersion < 10
+        || osvi.dwMajorVersion == 10
+        && osvi.dwMinorVersion == 0
+        && osvi.dwBuildNumber < 17763;
+  } else {
+    return !IsWindows8OrGreater();
+  }
 #else
-   return 0;
+  return 0;
 #endif
 }