[Bug fortran/107031] New: endfile truncates file at wrong position

["kishore96 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107031

            Bug ID: 107031
           Summary: endfile truncates file at wrong position
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kishore96 at gmail dot com
  Target Milestone: ---

SUMMARY
-------
When `endfile` is used to truncate a file after a read statement, gfortran
wrongly inserts end-of-file after the next line, rather than after the current
line.

STEPS TO REPRODUCE
------------------
1. create the following program, truncate.f90:
```fortran
program test_truncate
    integer :: num_rec, tmp, i, ioerr

    open(1, file="in.dat", action='readwrite')

    num_rec = 5
    i = 1
    ioerr = 0
    do while (i <= num_rec .and. ioerr == 0)
        read(1, *, iostat=ioerr) tmp
        i = i+1
    enddo

    !backspace(1)
    endfile(1)
    close(1)
end program test_truncate
```

2. Create the following text file, `in.dat`
```
1
2
3
4
5
6
7
8
9
10
```

3. Compile the program: `gfortran truncate.f90`

4. Run the program: `./a.out`

OBSERVED RESULTS
----------------
```bash
$cat in.dat
1
2
3
4
5
6
```

EXPECTED RESULTS
----------------
```bash
$cat in.dat
1
2
3
4
5
```

Since after the end of the loop, the current record is `5`, one would expect
endfile to write an end-of-file record as the next record (e.g.
<https://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vn8v/index.html>), leading
to the result above.

SOFTWARE VERSIONS
-----------------
OS: Arch Linux
gfortran: 12.2.0


ADDITIONAL INFORMATION
----------------------
Intel ifort (version 19.0.5.281 20190815) produces the expected result.

May be related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52387, but I
think not, since my example program doesn't use nonadvancing reads anywhere.

Uncommenting the `backspace(1)` gives output
```bash
$cat in.dat
1
2
3
4
```