mpi_f08: fix Fortran-8-byte-INTEGER vs. C-4-byte-int issue

It is important to have the mpi_f08 Type(MPI_Status) be the same
length (in bytes) as the mpif.h status (which is an array of
MPI_STATUS_SIZE INTEGERs).  The reason is because MPI_Status_ctof()
basically does the following:

MPI_Fint *f_status = ...;
int *s = (int*) &c_status;
for i=0..sizeof(MPI_Status)/sizeof(int)
   f_status[i] = c_status[i];

Meaning: the Fortran status needs to be able to hold as many INTEGERs
are there are C int's that can fit in sizeof(MPI_Status) bytes.

This is because a Fortran INTEGER may be larger than a C int (e.g.,
Fortran 8 bytes vs. C 4 bytes).  Hence, the assignment on the Fortran
side will take sizeof(INTEGER) bytes for each sizeof(int) bytes in the
C MPI_Status.

This commit pads out the mpi_f08 Type(MPI_Status) with enough INTEGERs
to make it the same size as an array of MPI_TYPE_SIZE INTEGERs.
Hence, MPI_Status_ctof() will work properly, regardless of whether it
is assinging to an mpi_f08 Type(MPI_Status) or an mpif.h array of
MPI_STATUS_SIZE INTEGERs.

Thanks to @ahaichen for reporting the issue.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

config/ompi_setup_mpi_fortran.m4

@@ -244,12 +244,28 @@ AC_DEFUN([OMPI_SETUP_MPI_FORTRAN],[
 
     # How big should MPI_STATUS_SIZE be?  (i.e., the size of
     # MPI_STATUS, expressed in units of Fortran INTEGERs).  The C
-    # equivalent of MPI_Status contains 4 C ints and a size_t.
+    # MPI_Status struct contains 4 C ints and a size_t.
     OMPI_FORTRAN_STATUS_SIZE=0
-    AC_MSG_CHECKING([for the value of MPI_STATUS_SIZE])
+
+    # Calculate how many C int's can fit in sizeof(MPI_Status).  Yes,
+    # I do mean C ints -- not Fortran INTEGERS.  The reason is because
+    # an mpif.h MPI_Status is an array of INTEGERS.  But these
+    # sizeof(INTEGER) may be larger than sizeof(int).  Hence,
+    # MPI_Status_ctof() basically does this:
+    #
+    # MPI_Fint *f_status = ...;
+    # int *s = (int*) &c_status;
+    # for i=0..sizeof(MPI_Status)/sizeof(int)
+    #    f_status[i] = c_status[i];
+    #
+    # Meaning: we have to have as many Fortran INTEGERs in the array
+    # as int's will fit in a C MPI_Status (vs. just having a Fortran
+    # array of INTEGERs that has enough bytes to hold a C MPI_Status).
     bytes=`expr 4 \* $ac_cv_sizeof_int + $ac_cv_sizeof_size_t`
-    num_integers=`expr $bytes / $ac_cv_sizeof_int`
-    sanity=`expr $num_integers \* $ac_cv_sizeof_int`
+    AC_MSG_NOTICE([C MPI_Status is $bytes bytes long])
+    AC_MSG_CHECKING([for the value of MPI_STATUS_SIZE])
+    num_ints=`expr $bytes / $ac_cv_sizeof_int`
+    sanity=`expr $num_ints \* $ac_cv_sizeof_int`
     AS_IF([test "$sanity" != "$bytes"],
           [AC_MSG_RESULT([unknown!])
            AC_MSG_WARN([WARNING: Size of C int: $ac_cv_sizeof_int])
@@ -257,7 +273,7 @@ AC_DEFUN([OMPI_SETUP_MPI_FORTRAN],[
            AC_MSG_WARN([WARNING: Size of Fortran INTEGER: $OMPI_SIZEOF_FORTRAN_INTEGER])
            AC_MSG_WARN([Could not make this work out evenly...!])
            AC_MSG_ERROR([Cannot continue])])
-    OMPI_FORTRAN_STATUS_SIZE=$num_integers
+    OMPI_FORTRAN_STATUS_SIZE=$num_ints
     AC_MSG_RESULT([$OMPI_FORTRAN_STATUS_SIZE Fortran INTEGERs])
     AC_SUBST(OMPI_FORTRAN_STATUS_SIZE)
 

ompi/mpi/fortran/use-mpi-f08/mod/mpi-f08-types.F90

@@ -74,8 +74,18 @@ module mpi_f08_types
       integer :: MPI_SOURCE
       integer :: MPI_TAG
       integer :: MPI_ERROR
-      integer(C_INT)    OMPI_PRIVATE :: c_cancelled
-      integer(C_SIZE_T) OMPI_PRIVATE :: c_count
+      ! The mpif.h interface uses MPI_STATUS_SIZE to know how long of
+      ! an array of INTEGERs is necessary to hold a C MPI_Status.
+      ! Effectively do the same thing here: pad out this datatype with
+      ! as many INTEGERs as there are C int's can fit in
+      ! sizeof(MPI_Status) bytes -- see MPI_Status_ctof() for an
+      ! explanation why.
+      !
+      ! This padding makes this F08 Type(MPI_Status) be the same size
+      ! as the mpif.h status (i.e., an array of MPI_STATUS_SIZE
+      ! INTEGERs), which is critical for MPI_Status_ctof() to not
+      ! overwrite memory.
+      integer OMPI_PRIVATE :: internal(MPI_STATUS_SIZE - 3)
    end type MPI_Status
 
   !