go/cli: carry mesh-cli's standard input with its line, bounded at 64 KiB, for the controller's terminal alone

A secret given at the terminal (secret accept … --from -) has no way through mesh-cli today: the controller
runs every line with no standard input. The request now carries it, refused over 64 KiB by mesh-cli and by
the reader alike, and MaxRequest grows by what base64 makes of that bound.

go/cli/cli.go

@@ -24,9 +24,13 @@ import (
 // address (ADR 0272 §3).
 const Socket = "/run/nox-mesh-host/cli.sock"
 
-// MaxRequest bounds what the node-engine reads of a request. A command line is a few hundred bytes; settings given
-// inline are a few kilobytes.
-const MaxRequest = 64 << 10
+// MaxStdin bounds the standard input mesh-cli carries for a line: a secret, a key, a settings file — kilobytes. More
+// is refused by mesh-cli before anything is asked, and by the node-engine and the controller again.
+const MaxStdin = 64 << 10
+
+// MaxRequest bounds what the node-engine reads of a request: a command line of a few hundred bytes, settings given
+// inline of a few kilobytes, and standard input of at most MaxStdin, which JSON writes as base64 (4 bytes for every 3).
+const MaxRequest = 64<<10 + MaxStdin*4/3 + 4
 
 // MaxAnswer bounds what mesh-cli reads of an answer. The controller cuts what it runs to fit one bus message, and
 // says so; this is the local bound above that.
@@ -36,9 +40,16 @@ const MaxAnswer = 16 << 20
 // and a minute for the link on either side.
 const AskWithin = 6 * time.Minute
 
-// Request is what mesh-cli asks: the controller's command line, as words.
+// Request is what mesh-cli asks: the controller's command line, as words, and what mesh-cli's own standard input
+// held when it was not a terminal.
+//
+// **Stdin is for the controller's terminal alone** (novox/hq ADR 0259 §10, ADR 0272): a secret given with
+// `secret accept … --from -`, so it never stands on a command line. The controller gives it to a line that runs as
+// its terminal and refuses any other line that carries it; nobody keeps it — not the node-engine, not the
+// controller's record of calls, not its journal.
 type Request struct {
-	Line []string `json:"line"`
+	Line  []string `json:"line"`
+	Stdin []byte   `json:"stdin,omitempty"`
 }
 
 // Answer is what comes back: what the command printed, how it exited, and whether it ran as the controller's
@@ -87,6 +98,9 @@ func ReadRequest(r io.Reader) (Request, error) {
 	if len(req.Line) == 0 {
 		return Request{}, errors.New("the request names no command")
 	}
+	if len(req.Stdin) > MaxStdin {
+		return Request{}, fmt.Errorf("%w: standard input of more than %d bytes", ErrTooLong, MaxStdin)
+	}
 	return req, nil
 }
 
@@ -102,9 +116,18 @@ func ReadAnswer(r io.Reader) (Answer, error) {
 
 // Ask is mesh-cli's side: connect to the socket, ask, and wait for the answer within AskWithin or ctx.
 func Ask(ctx context.Context, socket string, line []string) (Answer, error) {
+	return AskWith(ctx, socket, Request{Line: line})
+}
+
+// AskWith asks a whole request: a line, and the standard input it carries, at most MaxStdin.
+func AskWith(ctx context.Context, socket string, req Request) (Answer, error) {
+	line := req.Line
 	if len(line) == 0 {
 		return Answer{}, errors.New("no command given")
 	}
+	if len(req.Stdin) > MaxStdin {
+		return Answer{}, fmt.Errorf("standard input is more than %d bytes, which mesh-cli does not carry; nothing was asked", MaxStdin)
+	}
 	ctx, cancel := context.WithTimeout(ctx, AskWithin)
 	defer cancel()
 	var d net.Dialer
@@ -116,7 +139,7 @@ func Ask(ctx context.Context, socket string, line []string) (Answer, error) {
 	if deadline, ok := ctx.Deadline(); ok {
 		_ = conn.SetDeadline(deadline)
 	}
-	if err := WriteRequest(conn, Request{Line: line}); err != nil {
+	if err := WriteRequest(conn, req); err != nil {
 		return Answer{}, fmt.Errorf("asking the node-engine: %w", err)
 	}
 	a, err := ReadAnswer(conn)