[PATCH] Fix PR preprocessor/15167

[Eric Botcazou]

> I see nothing that would cause it to become true once again.  It is
> only set to true in read_file_guts, which has already happened at the
> point where _cpp_stack_file clears buffer_valid.

But read_file is invoked in the comparison routine for each new instance that 
is tentatively pushed!

  /* We may have read the file under a different name.  Look
     for likely candidates and compare file contents to be sure.  */
  for (f = pfile->all_files; f; f = f->next_file)
    {
      if (f == file)
	continue;

      if ((import || f->once_only)
	  && f->err_no == 0
	  && f->st.st_mtime == file->st.st_mtime
	  && f->st.st_size == file->st.st_size
	  && read_file (pfile, f)
	  /* Size might have changed in read_file().  */
	  && f->st.st_size == file->st.st_size
	  && !memcmp (f->buffer, file->buffer, f->st.st_size))
	break;
    }

b.h (the pragma-onced header) is first pushed, then another instance is 
tentatively pushed.  Then the first instance is popped and a new instance is 
tentatively pushed again.

> Neil is not being as picky as me.  The primary problem is that the
> #ifndef wrapper idiom has not been reliably optimized for some time -
> I'm not sure whether it broke with the big PCH patch, or when the
> "missing-include" hook was introduced.  Also, the sheer
> incomprehensibility of this logic is itself a problem.  And in my
> ideal world we would never have to read and memcmp() the files to
> determine whether #pragma once applies.

OK.  I'm actually only interested in fixing the problem on the 3.4 branch.

-- 
Eric Botcazou