gio: add G_CREDENTIALS_TYPE_WIN32_PID

Credentials are often used to check peer processes details.

With AF_UNIX sockets on Windows, SIO_AF_UNIX_GETPEERPID can
be used to retrive the peer PID.

We will probably introduce more advanced mechanisms later on, though,
but I am not a Windows API expert.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

gio/gsocket.c

@@ -76,6 +76,10 @@
 #include "glibintl.h"
 #include "gioprivate.h"
 
+#ifdef G_OS_WIN32
+#include "giowin32-afunix.h"
+#endif
+
 /**
  * SECTION:gsocket
  * @short_description: Low-level socket object
@@ -6129,6 +6133,23 @@ g_socket_get_credentials (GSocket   *socket,
         ucred_free (ucred);
       }
   }
+#elif G_CREDENTIALS_USE_WIN32_PID
+  {
+    DWORD peerid, drc;
+
+    if (WSAIoctl (socket->priv->fd, SIO_AF_UNIX_GETPEERPID,
+                  NULL, 0U,
+                  &peerid, sizeof(peerid),
+                  /* Windows bug: always 0 https://github.com/microsoft/WSL/issues/4676 */
+                  &drc,
+                  NULL, NULL) == 0)
+      {
+        ret = g_credentials_new ();
+        g_credentials_set_native (ret,
+                                  G_CREDENTIALS_TYPE_WIN32_PID,
+                                  &peerid);
+      }
+  }
 #else
   #error "G_CREDENTIALS_SOCKET_GET_CREDENTIALS_SUPPORTED is set but this is no code for this platform"
 #endif