types/result, util/lineiter: add package for a result type, use it

This adds a new generic result type (motivated by golang/go#70084) to
try it out, and uses it in the new lineutil package (replacing the old
lineread package), changing that package to return iterators:
sometimes over []byte (when the input is all in memory), but sometimes
iterators over results of []byte, if errors might happen at runtime.

Updates #12912
Updates golang/go#70084

Change-Id: Iacdc1070e661b5fb163907b1e8b07ac7d51d3f83
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netmon/interfaces_android.go

@@ -5,7 +5,6 @@ package netmon
 
 import (
 	"bytes"
-	"errors"
 	"log"
 	"net/netip"
 	"os/exec"
@@ -15,7 +14,7 @@ import (
 	"golang.org/x/sys/unix"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/syncs"
-	"tailscale.com/util/lineread"
+	"tailscale.com/util/lineiter"
 )
 
 var (
@@ -34,11 +33,6 @@ func init() {
 
 var procNetRouteErr atomic.Bool
 
-// errStopReading is a sentinel error value used internally by
-// lineread.File callers to stop reading. It doesn't escape to
-// callers/users.
-var errStopReading = errors.New("stop reading")
-
 /*
 Parse 10.0.0.1 out of:
 
@@ -54,44 +48,42 @@ func likelyHomeRouterIPAndroid() (ret netip.Addr, myIP netip.Addr, ok bool) {
 	}
 	lineNum := 0
 	var f []mem.RO
-	err := lineread.File(procNetRoutePath, func(line []byte) error {
+	for lr := range lineiter.File(procNetRoutePath) {
+		line, err := lr.Value()
+		if err != nil {
+			procNetRouteErr.Store(true)
+			return likelyHomeRouterIP()
+		}
+
 		lineNum++
 		if lineNum == 1 {
 			// Skip header line.
-			return nil
+			continue
 		}
 		if lineNum > maxProcNetRouteRead {
-			return errStopReading
+			break
 		}
 		f = mem.AppendFields(f[:0], mem.B(line))
 		if len(f) < 4 {
-			return nil
+			continue
 		}
 		gwHex, flagsHex := f[2], f[3]
 		flags, err := mem.ParseUint(flagsHex, 16, 16)
 		if err != nil {
-			return nil // ignore error, skip line and keep going
+			continue // ignore error, skip line and keep going
 		}
 		if flags&(unix.RTF_UP|unix.RTF_GATEWAY) != unix.RTF_UP|unix.RTF_GATEWAY {
-			return nil
+			continue
 		}
 		ipu32, err := mem.ParseUint(gwHex, 16, 32)
 		if err != nil {
-			return nil // ignore error, skip line and keep going
+			continue // ignore error, skip line and keep going
 		}
 		ip := netaddr.IPv4(byte(ipu32), byte(ipu32>>8), byte(ipu32>>16), byte(ipu32>>24))
 		if ip.IsPrivate() {
 			ret = ip
-			return errStopReading
+			break
 		}
-		return nil
-	})
-	if errors.Is(err, errStopReading) {
-		err = nil
-	}
-	if err != nil {
-		procNetRouteErr.Store(true)
-		return likelyHomeRouterIP()
 	}
 	if ret.IsValid() {
 		// Try to get the local IP of the interface associated with
@@ -144,23 +136,26 @@ func likelyHomeRouterIPHelper() (ret netip.Addr, _ netip.Addr, ok bool) {
 		return
 	}
 	// Search for line like "default via 10.0.2.2 dev radio0 table 1016 proto static mtu 1500 "
-	lineread.Reader(out, func(line []byte) error {
+	for lr := range lineiter.Reader(out) {
+		line, err := lr.Value()
+		if err != nil {
+			break
+		}
 		const pfx = "default via "
 		if !mem.HasPrefix(mem.B(line), mem.S(pfx)) {
-			return nil
+			continue
 		}
 		line = line[len(pfx):]
 		sp := bytes.IndexByte(line, ' ')
 		if sp == -1 {
-			return nil
+			continue
 		}
 		ipb := line[:sp]
 		if ip, err := netip.ParseAddr(string(ipb)); err == nil && ip.Is4() {
 			ret = ip
 			log.Printf("interfaces: found Android default route %v", ip)
 		}
-		return nil
-	})
+	}
 	cmd.Process.Kill()
 	cmd.Wait()
 	return ret, netip.Addr{}, ret.IsValid()