Make build work (#546)

The mbedtls example has caused problem in the main build a number of
times. By making it a standalone `cargo new --bin`, we can keep it in
the source tree as a good example but avoid having it break the main
build.

Also, fix some clippy lints.

examples/mbedtls/src/mbedtls_connector.rs

@@ -0,0 +1,145 @@
+use std::fmt;
+use std::io;
+use ureq::{Error, ReadWrite, TlsConnector};
+
+use std::net::TcpStream;
+use std::sync::{Arc, Mutex};
+
+use mbedtls::rng::CtrDrbg;
+use mbedtls::ssl::config::{Endpoint, Preset, Transport};
+use mbedtls::ssl::{Config, Context};
+
+fn entropy_new() -> mbedtls::rng::OsEntropy {
+    mbedtls::rng::OsEntropy::new()
+}
+
+pub struct MbedTlsConnector {
+    context: Arc<Mutex<Context>>,
+}
+
+#[derive(Debug)]
+struct MbedTlsError;
+impl fmt::Display for MbedTlsError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "MedTLS handshake failed")
+    }
+}
+
+impl std::error::Error for MbedTlsError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        None
+    }
+}
+
+#[allow(dead_code)]
+pub(crate) fn default_tls_config() -> std::sync::Arc<dyn TlsConnector> {
+    Arc::new(MbedTlsConnector::new(
+        mbedtls::ssl::config::AuthMode::Required,
+    ))
+}
+
+impl MbedTlsConnector {
+    pub fn new(mode: mbedtls::ssl::config::AuthMode) -> MbedTlsConnector {
+        let entropy = Arc::new(entropy_new());
+        let mut config = Config::new(Endpoint::Client, Transport::Stream, Preset::Default);
+        let rng = Arc::new(CtrDrbg::new(entropy, None).unwrap());
+        config.set_rng(rng);
+        config.set_authmode(mode);
+        let ctx = Context::new(Arc::new(config));
+        MbedTlsConnector {
+            context: Arc::new(Mutex::new(ctx)),
+        }
+    }
+}
+
+impl TlsConnector for MbedTlsConnector {
+    fn connect(
+        &self,
+        _dns_name: &str,
+        io: Box<dyn ReadWrite>,
+    ) -> Result<Box<dyn ReadWrite>, Error> {
+        let mut ctx = self.context.lock().unwrap();
+        let sync = SyncIo(Mutex::new(io));
+        match ctx.establish(sync, None) {
+            Err(_) => {
+                let io_err = io::Error::new(io::ErrorKind::InvalidData, MbedTlsError);
+                return Err(io_err.into());
+            }
+            Ok(()) => Ok(MbedTlsStream::new(self)),
+        }
+    }
+}
+
+struct SyncIo(Mutex<Box<dyn ReadWrite>>);
+
+impl io::Read for SyncIo {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let mut lock = self.0.lock().unwrap();
+        lock.read(buf)
+    }
+}
+
+impl io::Write for SyncIo {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        let mut lock = self.0.lock().unwrap();
+        lock.write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        let mut lock = self.0.lock().unwrap();
+        lock.flush()
+    }
+}
+
+struct MbedTlsStream {
+    context: Arc<Mutex<Context>>, //tcp_stream: TcpStream,
+}
+
+impl fmt::Debug for MbedTlsStream {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MbedTlsStream").finish()
+    }
+}
+
+impl MbedTlsStream {
+    pub fn new(mtc: &MbedTlsConnector) -> Box<MbedTlsStream> {
+        Box::new(MbedTlsStream {
+            context: mtc.context.clone(),
+        })
+    }
+}
+
+impl ReadWrite for MbedTlsStream {
+    // no obvious way to get socket back out of mbedtls context
+    // context.io() returns Any, which is hard to turn back into
+    // TcpStream reference, and what is lifetime of reference?
+    fn socket(&self) -> Option<&TcpStream> {
+        None
+    }
+}
+
+impl io::Read for MbedTlsStream {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let mut ctx = self.context.lock().unwrap();
+        ctx.read(buf)
+    }
+}
+
+impl io::Write for MbedTlsStream {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        let mut ctx = self.context.lock().unwrap();
+        ctx.write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        let mut ctx = self.context.lock().unwrap();
+        ctx.flush()
+    }
+}
+
+/*
+ * Local Variables:
+ * compile-command: "cd ../.. && cargo build --example mbedtls-req"
+ * mode: rust
+ * End:
+ */