chirp: add a 10s timeout when communicating with BIRD (#5444)

Prior to this change, if BIRD stops responding wgengine.watchdogEngine
will crash tailscaled.

This happens because in wgengine.userspaceEngine, we end up blocking
forever trying to write a request to or read a response from BIRD with
wgLock held, and then future watchdog'd calls will block on acquiring
that mutex until the watchdog kills the process. With the timeout, we at
least get the chance to print an error message and decide whether we
want to crash or not.

Updates tailscale/coral#72

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

chirp/chirp.go

@@ -11,15 +11,31 @@ import (
 	"fmt"
 	"net"
 	"strings"
+	"time"
+)
+
+const (
+	// Maximum amount of time we should wait when reading a response from BIRD.
+	responseTimeout = 10 * time.Second
 )
 
 // New creates a BIRDClient.
 func New(socket string) (*BIRDClient, error) {
+	return newWithTimeout(socket, responseTimeout)
+}
+
+func newWithTimeout(socket string, timeout time.Duration) (*BIRDClient, error) {
 	conn, err := net.Dial("unix", socket)
 	if err != nil {
 		return nil, fmt.Errorf("failed to connect to BIRD: %w", err)
 	}
-	b := &BIRDClient{socket: socket, conn: conn, scanner: bufio.NewScanner(conn)}
+	b := &BIRDClient{
+		socket:  socket,
+		conn:    conn,
+		scanner: bufio.NewScanner(conn),
+		timeNow: time.Now,
+		timeout: timeout,
+	}
 	// Read and discard the first line as that is the welcome message.
 	if _, err := b.readResponse(); err != nil {
 		return nil, err
@@ -32,6 +48,8 @@ type BIRDClient struct {
 	socket  string
 	conn    net.Conn
 	scanner *bufio.Scanner
+	timeNow func() time.Time
+	timeout time.Duration
 }
 
 // Close closes the underlying connection to BIRD.
@@ -81,10 +99,15 @@ func (b *BIRDClient) EnableProtocol(protocol string) error {
 // 1 means ‘table entry’, 8 ‘runtime error’ and 9 ‘syntax error’.
 
 func (b *BIRDClient) exec(cmd string, args ...any) (string, error) {
+	if err := b.conn.SetWriteDeadline(b.timeNow().Add(b.timeout)); err != nil {
+		return "", err
+	}
 	if _, err := fmt.Fprintf(b.conn, cmd, args...); err != nil {
 		return "", err
 	}
-	fmt.Fprintln(b.conn)
+	if _, err := fmt.Fprintln(b.conn); err != nil {
+		return "", err
+	}
 	return b.readResponse()
 }
 
@@ -105,14 +128,20 @@ func hasResponseCode(s []byte) bool {
 }
 
 func (b *BIRDClient) readResponse() (string, error) {
+	// Set the read timeout before we start reading anything.
+	if err := b.conn.SetReadDeadline(b.timeNow().Add(b.timeout)); err != nil {
+		return "", err
+	}
+
 	var resp strings.Builder
 	var done bool
 	for !done {
 		if !b.scanner.Scan() {
-			return "", fmt.Errorf("reading response from bird failed: %q", resp.String())
-		}
-		if err := b.scanner.Err(); err != nil {
-			return "", err
+			if err := b.scanner.Err(); err != nil {
+				return "", err
+			}
+
+			return "", fmt.Errorf("reading response from bird failed (EOF): %q", resp.String())
 		}
 		out := b.scanner.Bytes()
 		if _, err := resp.Write(out); err != nil {