Re: [PATCH] Fix PR preprocessor/15167

[Eric Botcazou <ebotcazou at libertysurf dot fr>]

> The bug you've found is just the tip of the iceberg, though.  Calling
> read_file from this loop, for a file which is already on the stack,
> clobbers the buffer for that file - which must not happen, because
> that buffer is still being used by the lexer.  I suspect that we get
> away with it because the block of memory holding the file contents is
> not freed, and the lexer doesn't ever refer to file->buffer.  So it's
> "just" a memory leak.  I think.

Could you elaborate a little on the "clobber"?  Do you mean that, because 
buffer_valid is cleared in _cpp_stack_file, read_file allocates a new buffer 
even though the previous one is still alive?

If so, I agree.  However, I think the bug I'm trying to fix is orthogonal to 
this and I can't think of any reason why clearing buffer_valid in 
_cpp_pop_file_buffer can be wrong.

> You can fix _this_ bug properly, I think, by splitting out the body of
> that loop to a separate function that reads the file into a private
> block of memory (*not* using read_file_guts - copy the inner loop) and
> does the comparison from there.  I suspect we are gonna be in just as
> much trouble from plain old recursive #include of a header with no
> multiple-include guards whatsoever - but that's a very rare case, so
> let's not worry about it now.

OK, thanks for the tip.  But re-reading the pragma-onced each time could be 
unnecessarily costly though.  Should I add another buffer+flag to _cpp_file 
and use them in the new comparison function?

-- 
Eric Botcazou