hardware/serial: improve windows I/O handling

pkg/tools/hardware/serial.go

@@ -413,3 +413,37 @@ func serialContextErr(ctx context.Context) error {
 		return nil
 	}
 }
+
+func serialWriteAll(
+	ctx context.Context,
+	data []byte,
+	timeout time.Duration,
+	now func() time.Time,
+	write func([]byte) (int, error),
+) (int, error) {
+	if err := serialContextErr(ctx); err != nil {
+		return 0, err
+	}
+
+	total := 0
+	deadline := now().Add(timeout)
+	for total < len(data) {
+		if err := serialContextErr(ctx); err != nil {
+			return total, err
+		}
+		if deadline.Sub(now()) <= 0 {
+			return total, fmt.Errorf("timeout while writing serial data")
+		}
+
+		n, err := write(data[total:])
+		total += n
+		if err != nil {
+			return total, err
+		}
+		if n == 0 {
+			continue
+		}
+	}
+
+	return total, nil
+}

pkg/tools/hardware/serial_windows.go

@@ -25,6 +25,17 @@ var (
 const (
 	purgeTxClear = 0x0004
 	purgeRxClear = 0x0008
+
+	dcbFlagBinary           = 0x00000001
+	dcbFlagParity           = 0x00000002
+	dcbFlagOutxCtsFlow      = 0x00000004
+	dcbFlagOutxDsrFlow      = 0x00000008
+	dcbFlagDtrControlMask   = 0x00000030
+	dcbFlagDsrSensitivity   = 0x00000040
+	dcbFlagTXContinueOnXoff = 0x00000080
+	dcbFlagOutX             = 0x00000100
+	dcbFlagInX              = 0x00000200
+	dcbFlagRtsControlMask   = 0x00003000
 )
 
 type dcb struct {
@@ -113,6 +124,8 @@ func serialRead(ctx context.Context, cfg serialConfig, length int, timeout time.
 
 	buf := make([]byte, length)
 	var read uint32
+	// Synchronous serial I/O on Windows cannot be interrupted once the syscall starts.
+	// COMMTIMEOUTS bounds how long turn cancellation may take to surface.
 	if err := windows.ReadFile(handle, buf, &read, nil); err != nil {
 		return nil, err
 	}
@@ -134,11 +147,15 @@ func serialWrite(ctx context.Context, cfg serialConfig, data []byte, timeout tim
 		return 0, err
 	}
 
-	var written uint32
-	if err := windows.WriteFile(handle, data, &written, nil); err != nil {
-		return int(written), err
-	}
-	return int(written), nil
+	return serialWriteAll(ctx, data, timeout, time.Now, func(chunk []byte) (int, error) {
+		var written uint32
+		// Like ReadFile above, this synchronous WriteFile call relies on COMMTIMEOUTS
+		// rather than context preemption once the syscall is in flight.
+		if err := windows.WriteFile(handle, chunk, &written, nil); err != nil {
+			return int(written), err
+		}
+		return int(written), nil
+	})
 }
 
 func openAndConfigureWindowsSerial(cfg serialConfig, timeout time.Duration) (windows.Handle, error) {
@@ -171,18 +188,19 @@ func configureWindowsSerialPort(handle windows.Handle, cfg serialConfig, timeout
 
 	state.BaudRate = uint32(cfg.Baud)
 	state.ByteSize = byte(cfg.DataBits)
-	state.Flags |= 0x00000001 // fBinary
+	state.Flags = sanitizeWindowsSerialFlags(state.Flags)
+	state.Flags |= dcbFlagBinary
 
 	switch cfg.Parity {
 	case "even":
 		state.Parity = 2
-		state.Flags |= 0x00000002 // fParity
+		state.Flags |= dcbFlagParity
 	case "odd":
 		state.Parity = 1
-		state.Flags |= 0x00000002 // fParity
+		state.Flags |= dcbFlagParity
 	default:
 		state.Parity = 0
-		state.Flags &^= 0x00000002
+		state.Flags &^= dcbFlagParity
 	}
 
 	switch cfg.StopBits {
@@ -216,3 +234,15 @@ func configureWindowsSerialPort(handle windows.Handle, cfg serialConfig, timeout
 	procPurgeComm.Call(uintptr(handle), uintptr(purgeRxClear|purgeTxClear))
 	return nil
 }
+
+func sanitizeWindowsSerialFlags(flags uint32) uint32 {
+	flags &^= dcbFlagOutxCtsFlow |
+		dcbFlagOutxDsrFlow |
+		dcbFlagDtrControlMask |
+		dcbFlagDsrSensitivity |
+		dcbFlagTXContinueOnXoff |
+		dcbFlagOutX |
+		dcbFlagInX |
+		dcbFlagRtsControlMask
+	return flags
+}

pkg/tools/hardware/serial_windows_test.go

@@ -0,0 +1,39 @@
+//go:build windows
+
+package hardwaretools
+
+import "testing"
+
+func TestSanitizeWindowsSerialFlags(t *testing.T) {
+	flags := uint32(
+		dcbFlagBinary |
+			dcbFlagParity |
+			dcbFlagOutxCtsFlow |
+			dcbFlagOutxDsrFlow |
+			dcbFlagDtrControlMask |
+			dcbFlagDsrSensitivity |
+			dcbFlagTXContinueOnXoff |
+			dcbFlagOutX |
+			dcbFlagInX |
+			dcbFlagRtsControlMask,
+	)
+
+	got := sanitizeWindowsSerialFlags(flags)
+
+	if got&dcbFlagBinary == 0 {
+		t.Fatal("sanitizeWindowsSerialFlags() should preserve fBinary")
+	}
+	if got&dcbFlagParity == 0 {
+		t.Fatal("sanitizeWindowsSerialFlags() should preserve fParity")
+	}
+	if got&(dcbFlagOutxCtsFlow|
+		dcbFlagOutxDsrFlow|
+		dcbFlagDtrControlMask|
+		dcbFlagDsrSensitivity|
+		dcbFlagTXContinueOnXoff|
+		dcbFlagOutX|
+		dcbFlagInX|
+		dcbFlagRtsControlMask) != 0 {
+		t.Fatalf("sanitizeWindowsSerialFlags() = %#x, want flow-control bits cleared", got)
+	}
+}

pkg/tools/hardware/serial_write_common_test.go

@@ -0,0 +1,87 @@
+package hardwaretools
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestSerialWriteAllRetriesPartialWritesUntilComplete(t *testing.T) {
+	now := time.Unix(0, 0)
+	calls := 0
+
+	written, err := serialWriteAll(context.Background(), []byte("PING"), time.Second, func() time.Time {
+		return now
+	}, func(chunk []byte) (int, error) {
+		calls++
+		now = now.Add(100 * time.Millisecond)
+		switch calls {
+		case 1:
+			if string(chunk) != "PING" {
+				t.Fatalf("first chunk = %q, want %q", chunk, "PING")
+			}
+			return 2, nil
+		case 2:
+			if string(chunk) != "NG" {
+				t.Fatalf("second chunk = %q, want %q", chunk, "NG")
+			}
+			return 2, nil
+		default:
+			t.Fatalf("unexpected extra write call %d", calls)
+			return 0, nil
+		}
+	})
+	if err != nil {
+		t.Fatalf("serialWriteAll() error = %v", err)
+	}
+	if written != 4 {
+		t.Fatalf("serialWriteAll() wrote %d bytes, want 4", written)
+	}
+}
+
+func TestSerialWriteAllTimesOutAfterZeroByteWrites(t *testing.T) {
+	now := time.Unix(0, 0)
+	calls := 0
+
+	written, err := serialWriteAll(context.Background(), []byte("A"), 250*time.Millisecond, func() time.Time {
+		return now
+	}, func(chunk []byte) (int, error) {
+		calls++
+		now = now.Add(100 * time.Millisecond)
+		return 0, nil
+	})
+	if err == nil || err.Error() != "timeout while writing serial data" {
+		t.Fatalf("serialWriteAll() error = %v, want timeout", err)
+	}
+	if written != 0 {
+		t.Fatalf("serialWriteAll() wrote %d bytes, want 0", written)
+	}
+	if calls != 3 {
+		t.Fatalf("write calls = %d, want 3", calls)
+	}
+}
+
+func TestSerialWriteAllReturnsContextCancellationAfterRetryBoundary(t *testing.T) {
+	now := time.Unix(0, 0)
+	ctx, cancel := context.WithCancel(context.Background())
+	calls := 0
+
+	written, err := serialWriteAll(ctx, []byte("A"), time.Second, func() time.Time {
+		return now
+	}, func(chunk []byte) (int, error) {
+		calls++
+		now = now.Add(100 * time.Millisecond)
+		cancel()
+		return 0, nil
+	})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("serialWriteAll() error = %v, want context canceled", err)
+	}
+	if written != 0 {
+		t.Fatalf("serialWriteAll() wrote %d bytes, want 0", written)
+	}
+	if calls != 1 {
+		t.Fatalf("write calls = %d, want 1", calls)
+	}
+}