readers: Initial Readers enumerator for detecting YubiKeys

Adds a `yubikey_piv::Readers` type which opens a PC/SC context and can
enumerate detected PC/SC readers with a slightly more ergonomic API than
what's provided in the upstream crate.

Does not support actually instantiating a `YubiKey` from a `Reader<'_>`
yet, but ideally all connections to YubiKeys should go through this API.

src/readers.rs

@@ -0,0 +1,83 @@
+//! Support for enumerating available readers
+
+use crate::error::Error;
+use std::{
+    borrow::Cow,
+    ffi::CStr,
+    sync::{Arc, Mutex},
+};
+
+/// Iterator over connected readers
+pub type Iter<'ctx> = std::vec::IntoIter<Reader<'ctx>>;
+
+/// Enumeration support for available readers
+pub struct Readers {
+    /// PC/SC context
+    ctx: Arc<Mutex<pcsc::Context>>,
+
+    /// Buffer for storing reader names
+    reader_names: Vec<u8>,
+}
+
+impl Readers {
+    /// Open a PC/SC context
+    pub fn open() -> Result<Self, Error> {
+        let ctx = pcsc::Context::establish(pcsc::Scope::System)?;
+        let reader_names = vec![0u8; ctx.list_readers_len()?];
+        Ok(Self {
+            ctx: Arc::new(Mutex::new(ctx)),
+            reader_names,
+        })
+    }
+
+    /// Iterate over the available readers
+    pub fn iter(&mut self) -> Result<Iter<'_>, Error> {
+        let Self { ctx, reader_names } = self;
+
+        let reader_cstrs: Vec<_> = {
+            let c = ctx.lock().unwrap();
+
+            // ensure PC/SC context is valid
+            c.is_valid()?;
+
+            c.list_readers(reader_names)?.collect()
+        };
+
+        let readers: Vec<_> = reader_cstrs
+            .iter()
+            .map(|name| Reader::new(name, Arc::clone(ctx)))
+            .collect();
+
+        Ok(readers.into_iter())
+    }
+}
+
+/// An individual connected reader
+pub struct Reader<'ctx> {
+    /// Name of this reader
+    name: &'ctx CStr,
+
+    /// PC/SC context
+    ctx: Arc<Mutex<pcsc::Context>>,
+}
+
+impl<'ctx> Reader<'ctx> {
+    /// Create a new reader from its name and context
+    fn new(name: &'ctx CStr, ctx: Arc<Mutex<pcsc::Context>>) -> Self {
+        // TODO(tarcieri): open devices, determine they're YubiKeys, get serial?
+        Self { name, ctx }
+    }
+
+    /// Get this reader's name
+    pub fn name(&self) -> Cow<'_, str> {
+        // TODO(tarcieri): is lossy ok here? try to avoid lossiness?
+        self.name.to_string_lossy()
+    }
+
+    /// Open the given reader
+    // TODO(tarcieri): return a `YubiKey` here rather than a `pcsc::Card`
+    pub fn open(&self) -> Result<pcsc::Card, Error> {
+        let ctx = self.ctx.lock().unwrap();
+        Ok(ctx.connect(self.name, pcsc::ShareMode::Shared, pcsc::Protocols::T1)?)
+    }
+}