internal/divert: WinDivert handle wrapper

Thin Go layer over imgk/divert-go. Exposes Open/Close/Recv/Send and
maps the most relevant Windows errors to sentinels (ErrAccessDenied,
ErrDriverFailedPriorUnload, ErrInvalidHandle, ErrShutdown) so the
engine's recovery classifier can reason about them without importing
golang.org/x/sys/windows.

Verified imgk/divert-go@v0.1.0 API matches plan; only deviation is
Recv/Send returning uint (cast to int at our boundary).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/divert/divert.go

@@ -0,0 +1,130 @@
+//go:build windows
+
+package divert
+
+import (
+	"errors"
+	"fmt"
+
+	idivert "github.com/imgk/divert-go"
+)
+
+// Handle wraps a WinDivert handle.
+type Handle struct {
+	h *idivert.Handle
+}
+
+// Open opens a WinDivert handle at NETWORK layer for outbound capture.
+// The filter expression is the standard WinDivert syntax (see
+// internal/divert/filter.go for our builder).
+//
+// Returns ErrAccessDenied when the calling process is not elevated.
+// Returns ErrDriverFailedPriorUnload when an outdated WinDivert
+// (e.g. v1.x from zapret) is already loaded.
+func Open(filter string) (*Handle, error) {
+	h, err := idivert.Open(filter, idivert.LayerNetwork, 0, 0)
+	if err != nil {
+		return nil, mapWinDivertErr(err)
+	}
+	return &Handle{h: h}, nil
+}
+
+// Close closes the handle. Safe to call multiple times.
+func (h *Handle) Close() error {
+	if h == nil || h.h == nil {
+		return nil
+	}
+	err := h.h.Close()
+	h.h = nil
+	return err
+}
+
+// Recv blocks until a packet arrives that matches the filter, or until
+// the handle is closed (Close from another goroutine returns
+// ErrShutdown to the recv'er). buf must be sized for a full Ethernet
+// MTU (~1600 bytes is fine).
+//
+// Returns the captured packet length, the WinDivertAddress (containing
+// direction, interface index, etc), and any error.
+func (h *Handle) Recv(buf []byte) (int, *idivert.Address, error) {
+	if h == nil || h.h == nil {
+		return 0, nil, errors.New("handle closed")
+	}
+	addr := new(idivert.Address)
+	n, err := h.h.Recv(buf, addr)
+	if err != nil {
+		return 0, nil, mapWinDivertErr(err)
+	}
+	return int(n), addr, nil
+}
+
+// Send reinjects a packet. The address typically comes from a previous
+// Recv call (so the kernel knows whether it's outbound or inbound, which
+// interface, etc).
+func (h *Handle) Send(buf []byte, addr *idivert.Address) (int, error) {
+	if h == nil || h.h == nil {
+		return 0, errors.New("handle closed")
+	}
+	n, err := h.h.Send(buf, addr)
+	if err != nil {
+		return 0, mapWinDivertErr(err)
+	}
+	return int(n), nil
+}
+
+// Sentinel errors mapped from raw Windows errors so the engine layer
+// can pattern-match without importing windows package.
+var (
+	ErrAccessDenied            = errors.New("WinDivert: access denied (need admin)")
+	ErrDriverFailedPriorUnload = errors.New("WinDivert: outdated driver from another tool is loaded; reboot or stop the other tool first")
+	ErrInvalidHandle           = errors.New("WinDivert: handle invalidated (driver crashed?)")
+	ErrShutdown                = errors.New("WinDivert: shutdown")
+)
+
+func mapWinDivertErr(err error) error {
+	if err == nil {
+		return nil
+	}
+	msg := err.Error()
+	switch {
+	case contains(msg, "access is denied"), contains(msg, "ACCESS_DENIED"):
+		return ErrAccessDenied
+	case contains(msg, "FAILED_PRIOR_UNLOAD"), contains(msg, "prior unload"):
+		return ErrDriverFailedPriorUnload
+	case contains(msg, "INVALID_HANDLE"):
+		return ErrInvalidHandle
+	case contains(msg, "SHUTDOWN"):
+		return ErrShutdown
+	}
+	return fmt.Errorf("WinDivert: %w", err)
+}
+
+func contains(s, sub string) bool {
+	// case-insensitive
+	if len(sub) == 0 {
+		return true
+	}
+	if len(s) < len(sub) {
+		return false
+	}
+	for i := 0; i+len(sub) <= len(s); i++ {
+		match := true
+		for j := 0; j < len(sub); j++ {
+			a, b := s[i+j], sub[j]
+			if a >= 'A' && a <= 'Z' {
+				a += 32
+			}
+			if b >= 'A' && b <= 'Z' {
+				b += 32
+			}
+			if a != b {
+				match = false
+				break
+			}
+		}
+		if match {
+			return true
+		}
+	}
+	return false
+}