Pure Rust reader for BSD 4.4 dbopen(3) btree (db 1.85) files

db185 reads the on-disk btree format used by the historical 4.4BSD dbopen(3) interface, as preserved in NetBSD libc and the pkgsrc libnbcompat sources. It exists because pkgsrc-rs will need to read pkgsrc's pkgdb.byfile.db, and the only pre-existing options for doing that are NetBSD libc (NetBSD-only) or linking against the libnbcompat C sources (everywhere else).

The crate targets only the subset of dbopen that pkgdb.byfile.db actually uses: btree, no duplicates, default memcmp ordering, no user-supplied compare or prefix callbacks. Hash and recno access methods are not supported and won't be. Write support will follow.

use db185::Db;

let db = Db::open("/var/db/pkg/pkgdb.byfile.db")?;

if let Some(value) = db.get(b"/opt/pkg/bin/foo\0")? {
    println!("{}", String::from_utf8_lossy(value.as_ref()));
}

for entry in &db {
    let entry = entry?;
    println!(
        "{} -> {}",
        String::from_utf8_lossy(entry.key()),
        String::from_utf8_lossy(entry.value()),
    );
}

The bundled dump example produces output byte-identical to pkg_admin dump when run against a pkgdb.byfile.db, which is how correctness is checked during development. On a real 199,996-entry pkgdb it runs in around 15 ms versus 35 ms for pkg_admin.