fix bug of zero-byte shared window allocation.

In MPIDI_CH3I_Win_allocate_shared, if the size of shared memory
segment is 0, we just set window's base to NULL, do not allocate
shared memory segment and mutex memory region.

In MPIDI_CH3I_SHM_Win_free, we detach the shared memory segment
and mutex memory region only when the size of shared memory segment
is not 0.
Signed-off-by: Pavan Balaji <balaji@mcs.anl.gov>

src/mpid/ch3/channels/nemesis/src/ch3_rma_shm.c

@@ -68,16 +68,19 @@ int MPIDI_CH3_SHM_Win_free(MPID_Win **win_ptr)
     if ((*win_ptr)->shm_allocated) {
         /* free shm_base_addrs that's only used for shared memory windows */
         MPIU_Free((*win_ptr)->shm_base_addrs);
-        /* detach from shared memory segment */
-        mpi_errno = MPIU_SHMW_Seg_detach((*win_ptr)->shm_segment_handle, (char **)&(*win_ptr)->shm_base_addr,
+
+        if ((*win_ptr)->shm_segment_len > 0) {
+            /* detach from shared memory segment */
+            mpi_errno = MPIU_SHMW_Seg_detach((*win_ptr)->shm_segment_handle, (char **)&(*win_ptr)->shm_base_addr,
                                          (*win_ptr)->shm_segment_len);
-        if (mpi_errno) MPIU_ERR_POP(mpi_errno);
+            if (mpi_errno) MPIU_ERR_POP(mpi_errno);
 
-        MPIU_SHMW_Hnd_finalize(&(*win_ptr)->shm_segment_handle);
+            MPIU_SHMW_Hnd_finalize(&(*win_ptr)->shm_segment_handle);
+        }
     }
 
     /* Free shared process mutex memory region */
-    if ((*win_ptr)->shm_mutex) {
+    if ((*win_ptr)->shm_mutex && (*win_ptr)->shm_segment_len > 0) {
 
         if ((*win_ptr)->comm_ptr->rank == 0) {
             MPIDI_CH3I_SHM_MUTEX_DESTROY(*win_ptr);

src/mpid/ch3/channels/nemesis/src/ch3_win_fns.c

@@ -133,6 +133,11 @@ static int MPIDI_CH3I_Win_allocate_shared(MPI_Aint size, int disp_unit, MPID_Inf
             (*win_ptr)->shm_segment_len += (*win_ptr)->sizes[i];
     }
 
+    if ((*win_ptr)->shm_segment_len == 0) {
+        (*win_ptr)->base = NULL;
+    }
+
+    else {
     mpi_errno = MPIU_SHMW_Hnd_init(&(*win_ptr)->shm_segment_handle);
     if (mpi_errno) MPIU_ERR_POP(mpi_errno);
 
@@ -252,6 +257,7 @@ static int MPIDI_CH3I_Win_allocate_shared(MPI_Aint size, int disp_unit, MPID_Inf
     }
 
     (*win_ptr)->base = (*win_ptr)->shm_base_addrs[rank];
+    }
 
     /* get the base addresses of the windows.  Note we reuse tmp_buf from above
        since it's at least as large as we need it for this allgather. */