Terminal: show the shell's error output, so a missing shell is explained

The shell's stderr was discarded, so a shell that could not start looked like an
immediate disconnect. It now goes to the terminal like other output, followed by
the exit code. An end-to-end test covers a machine with no shell. Builds against
Bamboo v0.1.10, which says why when there is no shell to run.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

e2e/terminal_relay_test.go

@@ -137,6 +137,33 @@ func TestWebTerminalWithoutAnyOpenPort(t *testing.T) {
 		t.Errorf("the test opened a port after all: %+v", maps.Mappings)
 	}
 
+	// a machine with no shell (the published container image has none) says why, instead of the
+	// terminal just disconnecting: the shell's error output reaches the browser, then the exit code
+	if sshd.Supported {
+		t.Setenv("SHELL", "/nonexistent/shell")
+		ws := dial()
+		defer ws.Close()
+		var out bytes.Buffer
+		var exit termMsg
+		for exit.Type != "exit" {
+			ws.SetReadDeadline(time.Now().Add(15 * time.Second))
+			var f []byte
+			if err := websocket.Message.Receive(ws, &f); err != nil {
+				t.Fatalf("waiting for the shell to end (got %q): %v", out.String(), err)
+			}
+			if len(f) > 0 && f[0] == '{' {
+				if m := parseTermMsg(f); m.Type == "exit" {
+					exit = m
+				}
+				continue
+			}
+			out.Write(f)
+		}
+		if exit.Code != 127 || !strings.Contains(out.String(), "no shell to run") {
+			t.Errorf("a missing shell: exit %d, output %q", exit.Code, out.String())
+		}
+	}
+
 	// the SSH server still decides who gets in
 	other := newSSHSigner(t)
 	if err := os.WriteFile(keys, ssh.MarshalAuthorizedKey(other.PublicKey()), 0o600); err != nil {

go.mod

@@ -3,7 +3,7 @@ module git.veillax.com/Veillax/hammerhead
 go 1.27.3
 
 require (
-	git.veillax.com/Veillax/bamboo v0.1.9
+	git.veillax.com/Veillax/bamboo v0.1.10
 	git.veillax.com/Veillax/blacktip v0.1.10
 	git.veillax.com/Veillax/reef v0.1.12
 	github.com/emersion/go-msgauth v0.7.0

go.sum

@@ -1,5 +1,5 @@
-git.veillax.com/Veillax/bamboo v0.1.9 h1:D+DWIFRxy2MZ9Oa7fRrdeM7xNUBZPwjfO5CRMgc9+q4=
+git.veillax.com/Veillax/bamboo v0.1.10 h1:R21+L54cS87VXmOS4eAsfRwp6vL+ORLqJUBaRHBIlJE=
-git.veillax.com/Veillax/bamboo v0.1.9/go.mod h1:Z3O7xoa52mFaADs7RxaBQl0a0K+5HlHPB1eOYQjM15k=
+git.veillax.com/Veillax/bamboo v0.1.10/go.mod h1:Z3O7xoa52mFaADs7RxaBQl0a0K+5HlHPB1eOYQjM15k=
 git.veillax.com/Veillax/blacktip v0.1.10 h1:2e1wUbMCp4z/DvQjAJCRmLyqu4PX+qV3mo5WBliqGiE=
 git.veillax.com/Veillax/blacktip v0.1.10/go.mod h1:tarAhdKU9hx5pdufe9wG3dRWYkWgPYZvawKM5DWgt6o=
 git.veillax.com/Veillax/reef v0.1.12 h1:Yizgcbh/TUz/dkO/4F+MOZkXlN7FU3/et/5uq1dH72s=

terminal.go

@@ -360,6 +360,9 @@ func (s *Server) runTerminal(ws *websocket.Conn, c *orgCtx, siteID string, cols,
 		problem(err.Error())
 		return
 	}
+	// What the shell writes to its error output (including why it could not start) goes to the
+	// terminal like everything else, instead of being thrown away.
+	sess.Stderr = termWriter(func(p []byte) error { return send(termFrame{data: append([]byte(nil), p...)}) })
 	if err := sess.Shell(); err != nil {
 		problem(fmt.Sprintf("Couldn't start a shell: %v", err))
 		return
@@ -566,3 +569,13 @@ func clipText(s string, n int) string {
 	}
 	return s
 }
+
+// termWriter sends what is written to it to the browser as terminal output.
+type termWriter func(p []byte) error
+
+func (w termWriter) Write(p []byte) (int, error) {
+	if err := w(p); err != nil {
+		return 0, err
+	}
+	return len(p), nil
+}