[client] Deduplicate SSH client handshake and bound it with a deadline

Extract the dial-then-handshake sequence into nbssh.Handshake, which
applies the context deadline to the socket for the duration of the
handshake. Previously only the Android client did this; the CLI, wasm
and SSH proxy paths could block forever on a peer that accepts the TCP
connection and then goes silent, since ClientConfig.Timeout is not used
by NewClientConn.

client/ssh/client/client.go

@@ -313,21 +313,23 @@ func Dial(ctx context.Context, addr, user string, opts DialOptions) (*Client, er
 
 // dialSSH establishes an SSH connection without JWT authentication
 func dialSSH(ctx context.Context, network, addr string, config *ssh.ClientConfig) (*Client, error) {
+	if config.Timeout > 0 {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithTimeout(ctx, config.Timeout)
+		defer cancel()
+	}
+
 	dialer := &net.Dialer{}
 	conn, err := dialer.DialContext(ctx, network, addr)
 	if err != nil {
 		return nil, fmt.Errorf("dial %s: %w", addr, err)
 	}
 
-	clientConn, chans, reqs, err := ssh.NewClientConn(conn, addr, config)
+	client, err := nbssh.Handshake(ctx, conn, addr, config)
 	if err != nil {
-		if closeErr := conn.Close(); closeErr != nil {
-			log.Debugf("connection close after handshake failure: %v", closeErr)
-		}
-		return nil, fmt.Errorf("ssh handshake: %w", err)
+		return nil, err
 	}
 
-	client := ssh.NewClient(clientConn, chans, reqs)
 	return &Client{
 		client: client,
 	}, nil

client/ssh/handshake.go

@@ -0,0 +1,45 @@
+package ssh
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/crypto/ssh"
+)
+
+// Handshake runs the SSH client handshake on an already dialed conn and
+// returns the resulting client. Dialing bounds only the TCP establishment;
+// without a deadline on the socket a peer that accepts and then goes silent
+// blocks the handshake forever, so the context deadline is applied to conn
+// for the duration of the handshake. conn is closed on any error.
+func Handshake(ctx context.Context, conn net.Conn, addr string, config *ssh.ClientConfig) (*ssh.Client, error) {
+	if deadline, ok := ctx.Deadline(); ok {
+		if err := conn.SetDeadline(deadline); err != nil {
+			closeHandshake(conn, "conn after deadline error")
+			return nil, fmt.Errorf("set handshake deadline: %w", err)
+		}
+	}
+
+	sshConn, chans, reqs, err := ssh.NewClientConn(conn, addr, config)
+	if err != nil {
+		closeHandshake(conn, "conn after handshake error")
+		return nil, fmt.Errorf("ssh handshake: %w", err)
+	}
+
+	if err := conn.SetDeadline(time.Time{}); err != nil {
+		closeHandshake(sshConn, "ssh conn after deadline clear error")
+		return nil, fmt.Errorf("clear handshake deadline: %w", err)
+	}
+
+	return ssh.NewClient(sshConn, chans, reqs), nil
+}
+
+func closeHandshake(c io.Closer, label string) {
+	if err := c.Close(); err != nil {
+		log.Debugf("ssh: close %s: %v", label, err)
+	}
+}

client/ssh/proxy/proxy.go

@@ -610,13 +610,10 @@ func (p *SSHProxy) dialBackend(ctx context.Context, addr, user, jwtToken string)
 		return nil, fmt.Errorf("connect to server: %w", err)
 	}
 
-	clientConn, chans, reqs, err := cryptossh.NewClientConn(conn, addr, config)
-	if err != nil {
-		_ = conn.Close()
-		return nil, fmt.Errorf("SSH handshake: %w", err)
-	}
+	handshakeCtx, cancel := context.WithTimeout(ctx, sshHandshakeTimeout)
+	defer cancel()
 
-	return cryptossh.NewClient(clientConn, chans, reqs), nil
+	return nbssh.Handshake(handshakeCtx, conn, addr, config)
 }
 
 func (p *SSHProxy) verifyHostKey(hostname string, remote net.Addr, key cryptossh.PublicKey) error {