[Bug fortran/100875] New: Spurious behavior for non-advancing user defined derived type IO

[Bug fortran/100875] New: Spurious behavior for non-advancing user defined derived type IO

[ehlert at thch dot uni-bonn.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100875

            Bug ID: 100875
           Summary: Spurious behavior for non-advancing user defined
                    derived type IO
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ehlert at thch dot uni-bonn.de
  Target Milestone: ---

Created attachment 50907
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50907&action=edit
Minimal reproducer

First reported in https://github.com/fortran-lang/stdlib/issues/354


This is an instance of user defined derived type IO using non-advancing read to
retrieve a whole line into a deferred length character component of a derived
type.

The implementation boils more or less down to the following subroutine
(complete reproducer attached):

    !> Read a character sequence from a connected formatted unit into the
string.
    subroutine read_formatted(string, unit, iotype, v_list, iostat, iomsg)
        type(string_type), intent(inout) :: string
        integer, intent(in) :: unit
        character(len=*), intent(in) :: iotype
        integer, intent(in) :: v_list(:)
        integer, intent(out) :: iostat
        character(len=*), intent(inout) :: iomsg
        character(len=:), allocatable :: line

        call read_line(unit, line, iostat, iomsg)

        string%raw = line

    contains

        !> Internal routine to read a whole record from a formatted unit
        subroutine read_line(unit, line, iostat, iomsg)
            integer, intent(in) :: unit
            character(len=:), allocatable, intent(out) :: line
            integer, intent(out) :: iostat
            character(len=*), intent(inout) :: iomsg
            integer, parameter :: buffer_size = 512
            character(len=buffer_size) :: buffer
            integer :: chunk
            line = ''
            do
                read(unit, '(a)', iostat=iostat, iomsg=iomsg, size=chunk,
advance='no') &
                    buffer
                if (iostat > 0) exit
                line = line // buffer(:chunk)
                if (iostat < 0) exit
            end do

            if (is_iostat_eor(iostat)) then
                iostat = 0
            end if
        end subroutine read_line

    end subroutine read_formatted


Reading from a file with three lines

 (1) Some value
 (2) Important saved value
 (3) Another line


will skip the second line when using the above routine through the
write(formatted) interface.


Compiling and running the minimal reproducer yields:

❯ gfortran -v -save-temps -Wall -Wextra -fno-strict-aliasing -fwrapv mwe.f90
Driving: gfortran -v -save-temps -Wall -Wextra -fno-strict-aliasing -fwrapv
mwe.f90 -l gfortran -l m -shared-libgcc
Using built-in specs.
COLLECT_GCC=gfortran
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --with-isl
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (GCC) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing'
'-fwrapv' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/f951 mwe.f90 -quiet -dumpbase mwe.f90
-mtune=generic -march=x86-64 -auxbase mwe -Wall -Wextra -version
-fno-strict-aliasing -fwrapv -fintrinsic-modules-path
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/finclude
-fpre-include=/usr/include/finclude/math-vector-fortran.h -o mwe.s
GNU Fortran (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 10.2.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.21-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU Fortran2008 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 10.2.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.21-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
mwe.f90:217:29:

  217 |     character(len=512) :: flc
      |                             1
Warning: Unused variable ‘flc’ declared at (1) [-Wunused-variable]
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing'
'-fwrapv' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 as -v --64 -o mwe.o mwe.s
GNU assembler version 2.36.1 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.36.1
Reading specs from
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/libgfortran.spec
rename spec lib to liborig
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing'
'-fwrapv' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing'
'-fwrapv' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/collect2 -plugin
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
-plugin-opt=-fresolution=mwe.res -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lquadmath
-plugin-opt=-pass-through=-lm -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --build-id
--eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0
-L/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../.. mwe.o
-lgfortran -lm -lgcc_s -lgcc -lquadmath -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing'
'-fwrapv' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
❯ ./a.out
string: "(3) Another line              "
stat: 0
is_iostat_eor(stat): F
is_iostat_end(stat): F


Which is both the wrong line and the wrong length of the raw value.