fix(hub): don't read the SSH client after it is closed (#2277)

createSessionWithTimeout checked sys.client for nil and then dereferenced
it again inside the goroutine that calls NewSession. update() runs the
SMART fetch in its own goroutine, so closeSSHConnection can clear the
field between those two reads and the goroutine dereferences a nil
client, panicking the whole hub process.

Make client an atomic.Pointer, load it once before starting the
goroutine, and clear it with Swap so a concurrent close cannot be
observed mid-session-creation. NewSession on an already-closed client
returns an error, which the existing retry path already handles.

Closes #2157

internal/hub/systems/ssh_timeout_test.go

@@ -4,11 +4,13 @@ package systems
 
 import (
 	"errors"
+	"sync"
 	"testing"
 	"testing/synctest"
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"golang.org/x/crypto/ssh"
 )
 
 // TestRunWithTimeout covers the guard added for issue #2041: the per-system SSH
@@ -54,3 +56,38 @@ func TestRunWithTimeout(t *testing.T) {
 		})
 	})
 }
+
+// closedConn stands in for a connection whose peer has gone away: opening a
+// channel fails rather than succeeding, which is what NewSession does on a
+// client that closeSSHConnection has already closed.
+type closedConn struct{ ssh.Conn }
+
+func (closedConn) OpenChannel(string, []byte) (ssh.Channel, <-chan *ssh.Request, error) {
+	return nil, nil, errors.New("use of closed network connection")
+}
+
+func (closedConn) Close() error { return nil }
+
+// TestCreateSessionDuringClose covers issue #2157: the background SMART fetch
+// creates a session while the updater can be tearing the same connection down,
+// so session creation must not read the client field after it is cleared.
+func TestCreateSessionDuringClose(t *testing.T) {
+	for range 500 {
+		sys := &System{ctx: t.Context()}
+		sys.client.Store(&ssh.Client{Conn: closedConn{}})
+
+		var wg sync.WaitGroup
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			session, err := sys.createSessionWithTimeout(time.Second)
+			assert.Nil(t, session)
+			assert.Error(t, err, "a closed connection must surface an error, not a session")
+		}()
+		go func() {
+			defer wg.Done()
+			sys.closeSSHConnection()
+		}()
+		wg.Wait()
+	}
+}