fix: BIO_set_retry_write when BIO_CTRL_FLUSH to allow writer returns WouldBlock on flush

boring/src/ssl/bio.rs

@@ -166,9 +166,13 @@ unsafe extern "C" fn ctrl<S: Write>(
     let state = state::<S>(bio);
 
     if cmd == BIO_CTRL_FLUSH {
+        BIO_clear_retry_flags(bio);
         match catch_unwind(AssertUnwindSafe(|| state.stream.flush())) {
             Ok(Ok(())) => 1,
             Ok(Err(err)) => {
+                if retriable_error(&err) {
+                    BIO_set_retry_write(bio);
+                }
                 state.error = Some(err);
                 0
             }

boring/src/ssl/test/mod.rs

@@ -563,6 +563,68 @@ fn test_select_cert_alpn_extension() {
     );
 }
 
+#[test]
+fn test_io_retry() {
+    #[derive(Debug)]
+    struct RetryStream {
+        inner: TcpStream,
+        first_read: bool,
+        first_write: bool,
+        first_flush: bool,
+    }
+
+    impl Read for RetryStream {
+        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+            if mem::replace(&mut self.first_read, false) {
+                Err(io::Error::new(io::ErrorKind::WouldBlock, "first read"))
+            } else {
+                self.inner.read(buf)
+            }
+        }
+    }
+
+    impl Write for RetryStream {
+        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+            if mem::replace(&mut self.first_write, false) {
+                Err(io::Error::new(io::ErrorKind::WouldBlock, "first write"))
+            } else {
+                self.inner.write(buf)
+            }
+        }
+
+        fn flush(&mut self) -> io::Result<()> {
+            if mem::replace(&mut self.first_flush, false) {
+                Err(io::Error::new(io::ErrorKind::WouldBlock, "first flush"))
+            } else {
+                self.inner.flush()
+            }
+        }
+    }
+
+    let server = Server::builder().build();
+
+    let stream = RetryStream {
+        inner: server.connect_tcp(),
+        first_read: true,
+        first_write: true,
+        first_flush: true,
+    };
+
+    let ctx = SslContext::builder(SslMethod::tls()).unwrap();
+    let mut s = match Ssl::new(&ctx.build()).unwrap().connect(stream) {
+        Ok(mut s) => return s.read_exact(&mut [0]).unwrap(),
+        Err(HandshakeError::WouldBlock(s)) => s,
+        Err(_) => panic!("should not fail on setup"),
+    };
+    loop {
+        match s.handshake() {
+            Ok(mut s) => return s.read_exact(&mut [0]).unwrap(),
+            Err(HandshakeError::WouldBlock(mid_s)) => s = mid_s,
+            Err(_) => panic!("should not fail on handshake"),
+        }
+    }
+}
+
 #[test]
 #[should_panic(expected = "blammo")]
 fn write_panic() {