[Bug fortran/100300] New: compile-time infinite loop if using execute_command_line

[Bug fortran/100300] New: compile-time infinite loop if using execute_command_line

[john.harper at vuw dot ac.nz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100300

            Bug ID: 100300
           Summary: compile-time infinite loop if using
                    execute_command_line
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john.harper at vuw dot ac.nz
  Target Milestone: ---

The program writetest.f90 below writes a file and then uses
execute_command_line to compile readtest.f90, which was supposed to read the
file, but its syntax error drove gfortran into an infinite loop pointing the
error out. Linux system evidence:

cayley[~/Jfh] % gfortran -v
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)

cayley[~/Jfh] % cat writetest.f90
! Use execute_command_line to start a second program readtest.f90 that
! reads a file, testfile.txt, written by this program.
  implicit none
  integer u,i
  open(newunit=u,file='testfile.txt',action='WRITE',position='REWIND',&
       form='FORMATTED',access='SEQUENTIAL')
  write(u,"(A,I0)") ('line ',i,i=1,2)
  call execute_command_line('gfortran readtest.f90 ; ./a.out')
end program

cayley[~/Jfh] % cat readtest.f90
  implicit none
  character(40) lines(2)
  integer u
  open(newunit=u,,file='testfile.txt')
  read(u,"(A)") lines(:)
  print  "(A)",'readtest gave '//lines(:)
end program

The second program has a syntax error (,,) correctly found by gfortran but 
compiling it by calling execute_command_line in the first program gave an
infinite loop repeating this 5-line message until I stopped it manually:
----quote--------------------------------------
readtest.f90:4:18:

    4 |   open(newunit=u,,file='testfile.txt')
      |                  1
Error: Syntax error in OPEN statement at (1)
----unquote------------------------------------

Correcting the error by removing one of the commas made the program compile
and run.