rustls: keep WRITABLE and drain buffered ciphertext when the socket backs up, and bound the plaintext buffer at 256KiB — rustls accepts all plaintext and buffers ciphertext internally on WouldBlock (unlike openssl, which surfaces backpressure through write); expose wants_write()/flush() so the reactor drains it and the sendq caps govern a slow reader. no-op for the openssl and plaintext paths

src/socketengine.rs

@@ -204,6 +204,21 @@ impl Sock {
             Sock::Tls(t) => t.write(buf),
         }
     }
+    /// Whether a TLS session still has outbound bytes buffered internally (rustls
+    /// after a WouldBlock); a plaintext socket never buffers in-process.
+    fn wants_write(&self) -> bool {
+        match self {
+            Sock::Plain(_) => false,
+            Sock::Tls(t) => t.wants_write(),
+        }
+    }
+    /// Push a TLS session's buffered ciphertext to the socket; no-op for plaintext.
+    fn flush(&mut self) -> io::Result<()> {
+        match self {
+            Sock::Plain(_) => Ok(()),
+            Sock::Tls(t) => t.flush(),
+        }
+    }
     /// Best-effort graceful close. TLS sends a close_notify alert; plaintext relies on
     /// the socket's own FIN when the stream drops.
     fn shutdown(&mut self) {
@@ -245,8 +260,9 @@ impl Conn {
 fn set_interest(poll: &mut Poll, c: &mut Conn, t: usize) {
     let want_read = !c.paused;
     // a TLS handshake may need to write (its flight) as well as read, so keep both
-    // until it completes; after that, write only when there's a backlog to drain.
-    let want_write = c.handshaking || !c.wbuf.is_empty() || c.paused;
+    // until it completes; after that, write when there's a backlog to drain — either
+    // our own queued plaintext, or ciphertext still buffered inside a TLS session.
+    let want_write = c.handshaking || !c.wbuf.is_empty() || c.paused || c.sock.wants_write();
     if want_read == c.want_read && want_write == c.want_write {
         return;
     }
@@ -979,12 +995,20 @@ fn flush_conn(poll: &mut Poll, conns: &mut HashMap<usize, Conn>, t: usize, core:
             c.wbuf.clear();
             c.wpos = 0;
         }
+        // push any ciphertext a TLS session still holds buffered (rustls keeps it when
+        // the socket filled mid-write); our plaintext queue draining doesn't mean the
+        // socket has it all. WouldBlock leaves the rest for the next writable event.
+        match c.sock.flush() {
+            Ok(()) => {}
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {}
+            Err(_) => close = true,
+        }
         if c.paused && c.pending() <= c.softsendq {
             c.paused = false;
             unpaused = true;
         }
         set_interest(poll, c, t);
-        if c.closing && c.wbuf.is_empty() {
+        if c.closing && c.wbuf.is_empty() && !c.sock.wants_write() {
             close = true;
         }
     }

src/tls.rs

@@ -55,6 +55,17 @@ pub trait TlsSession: Send {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize>;
     /// Encrypt+queue application data; returns the plaintext bytes accepted.
     fn write(&mut self, buf: &[u8]) -> io::Result<usize>;
+    /// Whether the session still holds outbound TLS bytes not yet pushed to the
+    /// socket. rustls buffers ciphertext internally when the socket is full;
+    /// openssl surfaces backpressure through `write`, so it never buffers.
+    fn wants_write(&self) -> bool {
+        false
+    }
+    /// Push any buffered outbound TLS bytes to the socket. `WouldBlock` leaves the
+    /// remainder for the next writable event; a no-op when nothing is buffered.
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
     /// The underlying mio socket, for the reactor's poll (re)registration.
     fn source(&mut self) -> &mut MioStream;
     /// SHA-256 fingerprint of the peer certificate (CertFP / SASL EXTERNAL), if any.

src/tls_rustls.rs

@@ -219,7 +219,11 @@ impl TlsBackend for RustlsBackend {
     }
 
     fn start(&self, sock: MioStream) -> io::Result<Box<dyn TlsSession>> {
-        let conn = ServerConnection::new(self.cfg()).map_err(err)?;
+        let mut conn = ServerConnection::new(self.cfg()).map_err(err)?;
+        // bound the buffered plaintext so a slow-reading client makes writer().write()
+        // return short (backpressure) instead of growing without limit; the reactor's
+        // sendq caps then govern it, matching the openssl backend.
+        conn.set_buffer_limit(Some(256 * 1024));
         Ok(Box::new(RustlsSession { conn, sock }))
     }
 }
@@ -307,6 +311,14 @@ impl TlsSession for RustlsSession {
         self.pump_write()?;
         Ok(n)
     }
+    fn wants_write(&self) -> bool {
+        // rustls holds encrypted bytes when the socket filled mid-flush; the reactor
+        // must keep WRITABLE interest and drain them, or a burst strands here.
+        self.conn.wants_write()
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.pump_write()
+    }
     fn source(&mut self) -> &mut MioStream {
         &mut self.sock
     }