When running
$ go test ./tests/matrix/
in isolation, it failed like this:
fd leak? before, after:
[0r=/dev/null 3w=/dev/null 5r=/proc/8078/fd (hidden:4)]
[0r=/dev/null 3w=/dev/null 5w=/tmp/go-build366655199/b001/testlog.txt 7r=/proc/8078/fd (hidden:4)]
Filter by prefix to get rid of this spurious test failure.