Add connection prehook for scripted node traversal

Implemented by spawning a user-defined process communicating with the
remote station over stdio.

Issue #114

conn_prehook.go

@@ -0,0 +1,111 @@
+package main
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"os/exec"
+	"time"
+
+	"golang.org/x/sync/errgroup"
+)
+
+type prehookConn struct {
+	net.Conn
+	br *bufio.Reader
+
+	executable string
+	args       []string
+}
+
+func NewPrehookConn(conn net.Conn, executable string, args ...string) prehookConn {
+	return prehookConn{
+		Conn:       conn,
+		br:         bufio.NewReader(conn),
+		executable: executable,
+		args:       args,
+	}
+}
+
+func (p prehookConn) Read(b []byte) (int, error) { return p.br.Read(b) }
+
+// Wait waits for the prehook process to exit, returning nil if the process
+// terminated successfully (exit code 0).
+func (p prehookConn) Wait(ctx context.Context) error {
+	cmd := exec.CommandContext(ctx, p.executable, p.args...)
+	cmd.WaitDelay = time.Second
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = p.Conn
+	cmdStdin, err := cmd.StdinPipe()
+	if err != nil {
+		return err
+	}
+
+	// Copy environment to the child process. Also include additional
+	// relevant variables: REMOTE_ADDR, LOCAL_ADDR and the output of the
+	// env command.
+	cmd.Env = append(append(os.Environ(),
+		"PAT_REMOTE_ADDR="+p.RemoteAddr().String(),
+		"PAT_LOCAL_ADDR="+p.LocalAddr().String(),
+	), envAll()...)
+
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+
+	g, ctx := errgroup.WithContext(ctx)
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	g.Go(func() error { return p.forwardLines(ctx, cmdStdin) })
+	g.Go(func() error { defer cancel(); return cmd.Wait() })
+	return g.Wait()
+}
+
+// forwardLines forwards data from to the spawned process line by line.
+//
+// The line delimiter is CR or LF, but to facilitate scripting we append LF if
+// it's missing.
+//
+// Wait one second after each line, to give the process time to terminate
+// before delivering the next line.
+func (p prehookConn) forwardLines(ctx context.Context, w io.Writer) error {
+	// Copy the lines to stdout so the user can see what's going on.
+	stdinBuffered := bufio.NewWriter(io.MultiWriter(w, os.Stdout))
+	defer stdinBuffered.Flush()
+
+	var isPrefix bool // true if we're in the middle of a line
+	for {
+		if !isPrefix {
+			// A line was just terminated (or no data has been read yet).
+			// Flush and wait one second to check if the process
+			// exited. If not we assume it expects an upcoming line.
+			if err := stdinBuffered.Flush(); err != nil {
+				return fmt.Errorf("child process exited prematurely: %w", err)
+			}
+			select {
+			case <-ctx.Done():
+				return nil
+			case <-time.After(time.Second):
+			}
+		}
+
+		b, err := p.br.ReadByte()
+		if err != nil {
+			return err
+		}
+		stdinBuffered.WriteByte(b)
+		isPrefix = !(b == '\n' || b == '\r')
+
+		// Make sure CR is always followed by LF. It's easier to deal with in scripts.
+		if b == '\r' {
+			stdinBuffered.WriteByte('\n')
+			// Peek to check if the next byte is the LF we just wrote, in which case discard it.
+			if peek, _ := p.br.Peek(1); len(peek) > 0 && peek[0] == '\n' {
+				p.br.Discard(1)
+			}
+		}
+	}
+}

connect.go

@@ -202,6 +202,18 @@ func Connect(connectStr string) (success bool) {
 		return
 	}
 
+	if exec := url.Params.Get("prehook"); exec != "" {
+		log.Println("Running prehook...")
+		prehookConn := NewPrehookConn(conn, exec, url.Params["prehook-param"]...)
+		if err := prehookConn.Wait(ctx); err != nil {
+			conn.Close()
+			log.Printf("Prehook script failed: %s", err)
+			return
+		}
+		log.Println("Prehook succeeded")
+		conn = prehookConn
+	}
+
 	err = exchange(conn, url.Target, false)
 	if err != nil {
 		log.Printf("Exchange failed: %s", err)

go.mod

@@ -20,6 +20,7 @@ require (
 	github.com/pd0mz/go-maidenhead v1.0.0
 	github.com/peterh/liner v1.2.1
 	github.com/spf13/pflag v1.0.5
+	golang.org/x/sync v0.5.0
 )
 
 require (

go.sum

@@ -82,6 +82,8 @@ golang.org/x/crypto v0.0.0-20210813211128-0a44fdfbc16e/go.mod h1:GvvjBRRGRdwPK5y
 golang.org/x/net v0.0.0-20210614182718-04defd469f4e/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.0.0-20210813160813-60bc85c4be6d h1:LO7XpTYMwTqxjLcGWPijK3vRXg1aWdlNOVOHRq45d7c=
 golang.org/x/net v0.0.0-20210813160813-60bc85c4be6d/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
+golang.org/x/sync v0.5.0 h1:60k92dhOjHxJkrqnwsfl8KuaHbn/5dl0lUPUklKo3qE=
+golang.org/x/sync v0.5.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
 golang.org/x/sys v0.0.0-20191005200804-aed5e4c7ecf9/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210223212115-eede4237b368/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=