Introduce portable, endian-clean structures using shifts and masks instead of bitfields

[jmwilson]

Bitfields are nonportable and often have buggy implementations. Shifts and masks, while more verbose, have better defined semantics. This is a preview of using them for the SCSI communication structures. The end of this change will be to get rid of the need for the #if STENC_BIG_ENDIAN checks and the run-time endian check in main (yuck) and ensure that the code is endian-clean at compile time.

I went back to the references of SCP-5 and SSP-5 to get the layout of the SCSI command structures and reimplemented them side-by-side with the old structures for comparison. Following NL.8 and NL.9, I went with a snake case naming convention for these structures. I also followed a style based on ARM's register naming convention:

struct foo {
  uint8_t flags1;
static constexpr auto flags1_field1_pos {0u};
static constexpr uint8_t flags1_field1_mask {1u << flags1_field1_pos};
static constexpr auto flags1_field2_pos {1u};
static constexpr uint8_t flags1_field1_mask {3u << flags1_field2_pos};
  ...
};

vs

struct foo {
  uint8_t field1 : 1;
  uint8_t field2 : 2;
  ...
};

Endian conversion is handled by using htons/ntohs functions - these are the most portable option since /usr/include/arpa/inet.h should be on every UNIX system. There are some better options in endian.h but this file is not fully standard across all UNIX variants.

[jonasstein]

Would Scott M. suggest auto instead of uint8_t here? Just curious...
The enum is so much cleaner already. Nice.

[jmwilson]

By default enums are sized as int but the underlying field in the SCSI page is a single byte, so it is necessary to require a smaller size.

[jonasstein]

Is this well tested already? What is left to do before merging?

[jmwilson]

Will need to rewrite much of scsiencrypt.cpp to use these structures before it makes sense to merge. The new tests should ensure strong coverage.

[lgtm-com]

This pull request introduces 2 alerts when merging 40e97bb into 1508f43 - view on LGTM.com

new alerts:

• 2 for Catching by value

[jmwilson]

Since this is a large PR, I have broken it into 5 commits:

1. introduces the new structures in the scsi namespace and adds some unit tests
2. introduces a modernized version of SCSIExecute using RAII and exceptions that will replace the old versions. Since the device code can't be unit tested, this was tested on both Linux and FreeBSD
3. Switches the SP-IN code (everything that is used when running stenc --detail) over to use the new structures.
4. Switches the SP-OUT (changes to the encryption setting) over to use the new structures
5. Deletes the old code

At each commit, the tests pass showing the SCSI data written and the interpretation of data received remains stable regardless of the representation used. This was accompanied by live testing on Linux and FreeBSD to verify normal and error behavior.

[jmwilson]

This will also close #63 since the new scsi::get_nbes will not move the tape.