Check for correctness of feature argument number when parsing feature arguments

[waltdisgrace]

Preliminary PR for #485

[jbaublitz]

@waltdisgrace Ping me here once you've figured out the testing issues, happy to help if you get stuck.

[waltdisgrace]

Rebased.

[mulkieran]

@waltdisgrace Please post the error that you obtain from your newly added test with the unmodified code, i.e., the code w/out your additional changes in the source.

[waltdisgrace]

@mulkieran:

Here is the relevant snippet of the make test output:

failures:

---- lineardev::tests::test_flakey_incorrect_feature_args_input stdout ----
thread 'lineardev::tests::test_flakey_incorrect_feature_args_input' panicked at 'index 9 out of range for slice of length 8', src/lineardev.rs:303:18
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::io::impls::<impl std::io::Write for alloc::boxed::Box<W>>::write_fmt
   4: std::panicking::default_hook::{{closure}}
   5: std::panicking::default_hook
   6: std::panicking::rust_panic_with_hook
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::slice::slice_index_len_fail
  10: <core::ops::range::Range<usize> as core::slice::SliceIndex<[T]>>::index
             at /builddir/build/BUILD/rustc-1.44.0-src/src/libcore/slice/mod.rs:2919
  11: core::slice::<impl core::ops::index::Index<I> for [T]>::index
             at /builddir/build/BUILD/rustc-1.44.0-src/src/libcore/slice/mod.rs:2732
  12: <alloc::vec::Vec<T> as core::ops::index::Index<I>>::index
             at /builddir/build/BUILD/rustc-1.44.0-src/src/liballoc/vec.rs:1945
  13: <devicemapper::lineardev::FlakeyTargetParams as core::str::FromStr>::from_str
             at src/lineardev.rs:303
  14: core::str::<impl str>::parse
             at /builddir/build/BUILD/rustc-1.44.0-src/src/libcore/str/mod.rs:4280
  15: devicemapper::lineardev::tests::test_flakey_incorrect_feature_args_input
             at src/lineardev.rs:923
  16: devicemapper::lineardev::tests::test_flakey_incorrect_feature_args_input::{{closure}}
             at src/lineardev.rs:922
  17: core::ops::function::FnOnce::call_once
             at /builddir/build/BUILD/rustc-1.44.0-src/src/libcore/ops/function.rs:232
  18: test::run_test::run_test_inner::{{closure}}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[mulkieran]

@waltdisgrace: Ok! That failure looks right to me.

[waltdisgrace]

Rebased

[mulkieran]

The implementation for Flakey looks reasonable to me. Linear won't have any feature args, because it is so simple, but I think all the other targets that are supported have feature args and will therefore each implementation will contain the same bug.

Please go ahead and pick another, e.g., ThinPoolDev and apply the same fix.

[waltdisgrace]

@mulkieran Should the fixes for the other targets be done in separate PRs or the same PR as Flakey?

[mulkieran]

Unlikely to manage to get to this before next release.