mesh-cli: carry what a pipe or a file gives standard input with the line, at most 64 KiB

So a secret reaches the controller's terminal without standing on a command line:
mesh-cli secret accept <node> <module> <name> --from - < file. A terminal is never read, more than the
bound is refused before anything is asked, and nothing carried is printed. mesh-sdk moves to f047d0a.

modules/mesh-cli/cmd/mesh-cli/main.go

@@ -23,12 +23,42 @@ import (
 // socket is where the node-engine answers; a variable so a test can name its own.
 var socket = cli.Socket
 
+// standardInput is mesh-cli's own standard input, and whether it is a terminal (or a device such as /dev/null),
+// which is never read; a variable so a test can say.
+var standardInput = func() (io.Reader, bool) {
+	info, err := os.Stdin.Stat()
+	if err != nil {
+		return os.Stdin, true
+	}
+	return os.Stdin, info.Mode()&os.ModeCharDevice != 0
+}
+
+// carried is what mesh-cli carries of its standard input for the line: what a pipe or a file held, at most
+// cli.MaxStdin, for the controller's terminal alone (a secret given with `secret accept … --from -`). A terminal is
+// never read: the line is asked without. More than the bound is refused, and nothing is asked.
+func carried() ([]byte, error) {
+	in, terminal := standardInput()
+	if terminal {
+		return nil, nil
+	}
+	body, err := io.ReadAll(io.LimitReader(in, cli.MaxStdin+1))
+	if err != nil {
+		return nil, fmt.Errorf("standard input could not be read (%v); nothing was asked", err)
+	}
+	if len(body) > cli.MaxStdin {
+		return nil, fmt.Errorf("standard input is more than %d bytes, which mesh-cli does not carry; nothing was asked", cli.MaxStdin)
+	}
+	return body, nil
+}
+
 const usage = `mesh-cli — the controller's command lines, from this node (novox/hq ADR 0272)
 
   mesh-cli <command line>     e.g. mesh-cli status, mesh-cli settings show zsh --node <node>
   nox <command line>          the same, in interactive zsh
 
 Run it as yourself, never under sudo: the node-engine tells the controller which account asks.
+What a pipe or a file gives its standard input (at most 64 KiB) goes with the line to the controller's
+terminal alone, e.g. a secret: mesh-cli secret accept <node> <module> <name> --from - < file
 On the control-node, from its operator's account, the line runs as the controller's terminal.
 Anywhere else it is an ordinary call, and a terminal-only change is refused with the reason.
 'mesh-cli help' asks the controller for its own list of command lines.
@@ -52,7 +82,12 @@ func run(ctx context.Context, args []string, stdout, stderr io.Writer) int {
 			"Nothing was asked\n", verb)
 		return 1
 	}
-	a, err := cli.Ask(ctx, socket, args)
+	in, err := carried()
+	if err != nil {
+		fmt.Fprintf(stderr, "mesh-cli: %v\n", err)
+		return 1
+	}
+	a, err := cli.AskWith(ctx, socket, cli.Request{Line: args, Stdin: in})
 	if err != nil {
 		if errors.Is(ctx.Err(), context.Canceled) {
 			fmt.Fprintln(stderr, "mesh-cli: stopped waiting; the line may still be running on the controller")