Add a test checking number of ranks is correct (#907)

* Add a test checking number of ranks is correct

* Update test_gather.jl

Author: giordano

test/runtests.jl

@@ -78,7 +78,11 @@ istest(f) = endswith(f, ".jl") && startswith(f, "test_") && !in(f, excludefiles)
 testfiles = sort(filter(istest, readdir(testdir)))
 
 @testset "$f" for f in testfiles
-    cmd(n=nprocs) = `$(mpiexec()) -n $n $(Base.julia_cmd()) --startup-file=no $(joinpath(testdir, f))`
+    cmd(n=nprocs) =
+        addenv(`$(mpiexec()) -n $n $(Base.julia_cmd()) --startup-file=no $(joinpath(testdir, f))`,
+               # `JULIA_MPI_TEST_NUM_PROCESSES` is used in `test_gather.jl` to
+               # test number of processes.
+               "JULIA_MPI_TEST_NUM_PROCESSES"=>string(n))
     if f == "test_spawn.jl"
         # Some command as the others, but always use a single process
         run(cmd(1))

test/test_gather.jl

@@ -97,5 +97,23 @@ else
     @test C === nothing
 end
 
+# Test that the number of ranks is as expected:
+# <https://github.com/JuliaParallel/MPI.jl/issues/905>.
+num_processes = get(ENV, "JULIA_MPI_TEST_NUM_PROCESSES", nothing)
+if isnothing(num_processes)
+    @test_broken false
+else
+    ranks = MPI.Gather(rank, comm)
+    comm_sizes = MPI.Gather(sz, comm)
+
+    if isroot
+        n = parse(Int, num_processes)
+        # Note: there's a similar test above, but not comparing with the
+        # externally set `JULIA_MPI_TEST_NUM_PROCESSES`.
+        @test sort(ranks) == 0:(n - 1)
+        @test all(==(n), comm_sizes)
+    end
+end
+
 MPI.Finalize()
 @test MPI.Finalized()