bug#79808: Possible missing fclose() in tac_nonseekable (src/tac.c)

tag 79808 notabug
close 79808
stop

details below...

On 10/11/2025 09:45, Ray steven wrote:
> Hello coreutils maintainers,
> 
> While reviewing the source code of GNU coreutils (tac.c), I noticed a small
> resource management
> issue in the function `tac_nonseekable()`:
> 
> After calling `copy_to_temp(&tmp_stream, &tmp_file, input_fd, file)`, the
> function uses `tmp_stream`
> in `tac_seekable(fileno(tmp_stream), tmp_file, bytes_copied)` but never
> calls `fclose(tmp_stream)` afterwards.
> As a result, the FILE stream and its underlying file descriptor remain open
> until process termination.
> 
> Although this does not cause user-visible problems for short-lived `tac`
> executions, adding an explicit
> `fclose(tmp_stream)` (and perhaps removing the temporary file) would
> improve resource hygiene and
> avoid potential descriptor exhaustion if the function were reused in a
> long-running context.
> 
> Environment:
> - Observed in current `src/tac.c` (line ~423 in GNU coreutils latest)
> 
> Suggested fix (simplified):
> ```c
> bool ok = tac_seekable(fileno(tmp_stream), tmp_file, bytes_copied);
> fclose(tmp_stream);
> unlink(tmp_file);
> return ok;

The temp-stream module manages a single global resource per process,
so this would be incorrect.
I verified that there is no leak with `tac - - - - -`.

thanks,
Padraig