Update comm_create to use sparse ctx id allocation

Update MPI_Comm_create to use sparse, rather than dense, context_id
allocation. This fixes incorrect context ID exhaustion errors caused by
including processes that are not in the group of the new communicator in
the allocation operation. This bug is exercised by
test/mpi/cerrors/comm/too_many_comms3.c.
Signed-off-by: Ken Raffenetti <raffenet@mcs.anl.gov>

src/mpi/comm/comm_create.c

@@ -233,8 +233,8 @@ int MPIR_Comm_create_intra(MPID_Comm *comm_ptr, MPID_Group *group_ptr,
        member of the group */
     /* In the multi-threaded case, MPIR_Get_contextid assumes that the
        calling routine already holds the single criticial section */
-    /* TODO should be converted to use MPIR_Get_contextid_sparse instead */
-    mpi_errno = MPIR_Get_contextid( comm_ptr, &new_context_id );
+    mpi_errno = MPIR_Get_contextid_sparse( comm_ptr, &new_context_id,
+                                           group_ptr->rank == MPI_UNDEFINED );
     if (mpi_errno) MPIU_ERR_POP(mpi_errno);
     MPIU_Assert(new_context_id != 0);
 
@@ -278,7 +278,6 @@ int MPIR_Comm_create_intra(MPID_Comm *comm_ptr, MPID_Group *group_ptr,
     }
     else {
         /* This process is not in the group */
-        MPIR_Free_contextid( new_context_id );
         new_context_id = 0;
     }
 
@@ -294,8 +293,9 @@ fn_fail:
         MPIR_Comm_release(*newcomm_ptr, 0/*isDisconnect*/);
         new_context_id = 0; /* MPIR_Comm_release frees the new ctx id */
     }
-    if (new_context_id != 0)
+    if (new_context_id != 0 && group_ptr->rank != MPI_UNDEFINED) {
         MPIR_Free_contextid(new_context_id);
+    }
     /* --END ERROR HANDLING-- */
     goto fn_exit;
 }