fix: use EscapeCommFunction for USB reset on Windows

The go.bug.st/serial library uses SetCommState to toggle RTS/DTR,
which does not reliably send USB control messages on Windows for
USB CDC devices (e.g. ESP32-S3 USB-JTAG/Serial).

Add a Windows-specific hardResetUSB that closes the library port,
opens a raw handle via CreateFile, and uses EscapeCommFunction to
directly toggle the reset signals.

Non-Windows platforms delegate to the existing hardReset via the
serial library, which works correctly on Linux/macOS.

Author: deadprogram

pkg/espflasher/flasher.go

@@ -175,6 +175,9 @@ func New(portName string, opts *FlasherOptions) (*Flasher, error) {
 
 // Close releases the serial port and associated resources.
 func (f *Flasher) Close() error {
+	if f.port == nil {
+		return nil
+	}
 	return f.port.Close()
 }
 
@@ -844,7 +847,13 @@ func (f *Flasher) Reset() {
 	// For ROM bootloaders, skip flash_begin/flash_end — sending
 	// CMD_FLASH_BEGIN after a compressed download may interfere with
 	// the flash controller state at offset 0.
-	hardReset(f.port, false)
+	if f.usesUSB {
+		if hardResetUSB(f.portStr, f.port) {
+			f.port = nil
+		}
+	} else {
+		hardReset(f.port, false)
+	}
 	f.logf("Device reset.")
 }
 

pkg/espflasher/reset_other.go

@@ -0,0 +1,14 @@
+//go:build !windows
+
+package espflasher
+
+import "go.bug.st/serial"
+
+// hardResetUSB performs a hardware reset for USB-JTAG/Serial devices.
+// On non-Windows platforms, this delegates to the standard hardReset
+// which uses SetRTS/SetDTR through the serial library.
+// Returns false because the port remains open.
+func hardResetUSB(portName string, port serial.Port) bool {
+	hardReset(port, true)
+	return false
+}

pkg/espflasher/reset_windows.go

@@ -0,0 +1,55 @@
+//go:build windows
+
+package espflasher
+
+import (
+	"syscall"
+	"time"
+
+	"go.bug.st/serial"
+)
+
+var procEscapeCommFunction = syscall.NewLazyDLL("kernel32.dll").NewProc("EscapeCommFunction")
+
+const (
+	escSETRTS = 3
+	escCLRRTS = 4
+	escSETDTR = 5
+	escCLRDTR = 6
+)
+
+// hardResetUSB performs a hardware reset for USB-JTAG/Serial devices on Windows.
+//
+// The go.bug.st/serial library uses SetCommState to toggle RTS/DTR, but this
+// does not reliably send USB control messages on Windows for USB CDC devices.
+// This function closes the library port, opens the COM port with a raw Windows
+// handle, and uses EscapeCommFunction to directly toggle the signals.
+//
+// Returns true because the port is consumed (closed) and must not be reused.
+func hardResetUSB(portName string, port serial.Port) bool {
+	port.Close() //nolint:errcheck
+
+	name, err := syscall.UTF16PtrFromString(`\\.\` + portName)
+	if err != nil {
+		return true
+	}
+
+	handle, err := syscall.CreateFile(name,
+		syscall.GENERIC_READ|syscall.GENERIC_WRITE,
+		0, nil, syscall.OPEN_EXISTING, 0, 0)
+	if err != nil {
+		return true
+	}
+	defer syscall.CloseHandle(handle) //nolint:errcheck
+
+	// DTR=false (GPIO0 HIGH → normal boot mode, not bootloader)
+	procEscapeCommFunction.Call(uintptr(handle), escCLRDTR) //nolint:errcheck
+	// RTS=true (EN LOW → hold chip in reset)
+	procEscapeCommFunction.Call(uintptr(handle), escSETRTS) //nolint:errcheck
+	time.Sleep(200 * time.Millisecond)
+	// RTS=false (EN HIGH → chip boots)
+	procEscapeCommFunction.Call(uintptr(handle), escCLRRTS) //nolint:errcheck
+	time.Sleep(200 * time.Millisecond)
+
+	return true
+}