MDEV-35891 mtr, Windows - fix multi-process append for stdout and stderr

This backports mysql/mysql-server@e2f685972985,
which works around perl issue Perl/perl5#17570

MySQL fix  ensures that FILE_APPEND_DATA is used in redirected
stdout/stderr open flags, rather than FILE_GENERIC_WRITE. Only this gives
lossless append behavior, if multiple processes use the same file.

Author: vaintroub

mysql-test/lib/My/SafeProcess/safe_process_win.cc

@@ -152,6 +152,24 @@ void handle_signal (int signal)
   }
 }
 
+/**
+  Sets the append flag (FILE_APPEND_DATA) so that the handle inherited by the
+  child process will be in append mode.
+  Workaround for perl bug https://github.com/Perl/perl5/issues/17570
+*/
+static void fix_file_append_flag_inheritance(DWORD std_handle)
+{
+  HANDLE old_handle = GetStdHandle(std_handle);
+  HANDLE new_handle = ReOpenFile(old_handle, FILE_APPEND_DATA,
+                                 FILE_SHARE_WRITE | FILE_SHARE_READ, 0);
+  if (new_handle != INVALID_HANDLE_VALUE)
+  {
+    SetHandleInformation(new_handle, HANDLE_FLAG_INHERIT, HANDLE_FLAG_INHERIT);
+    SetStdHandle(std_handle, new_handle);
+    CloseHandle(old_handle);
+  }
+}
+
 
 int main(int argc, const char** argv )
 {
@@ -270,6 +288,9 @@ int main(int argc, const char** argv )
     SetErrorMode(SEM_FAILCRITICALERRORS | SEM_NOGPFAULTERRORBOX
                  | SEM_NOOPENFILEERRORBOX);
 
+  fix_file_append_flag_inheritance(STD_OUTPUT_HANDLE);
+  fix_file_append_flag_inheritance(STD_ERROR_HANDLE);
+
 #if 0
   /* Setup stdin, stdout and stderr redirect */
   si.dwFlags= STARTF_USESTDHANDLES;