Add context.Context parameter to NewDevice

NewDevice now takes a context as its first parameter, consistent
with Command, Query, and the rest of the API. Update all call
sites in examples and example tests. Add pkgsite docs recipe to
Justfile.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: matthewrankin

Justfile

@@ -12,6 +12,11 @@ coverage_file := "coverage.out"
 loc:
   scc --remap-unknown "-*- Justfile -*-":"justfile"
 
+# View documentation in web browser using pkgsite.
+[group('general')]
+docs:
+  pkgsite -open .
+
 # Format and vet Go code. Runs before tests.
 [group('test')]
 check:

asrl.go

@@ -62,9 +62,14 @@ func WithReadTimeout(t time.Duration) DeviceOption {
 }
 
 // NewDevice opens a serial Device using the given VISA address resource string.
-// Optional DeviceOption values can be provided to override the default settings
-// for EndMark, HWHandshaking, DelayTime, and ReadTimeout.
-func NewDevice(address string, opts ...DeviceOption) (*Device, error) {
+// The context is checked before opening the serial port. Optional DeviceOption
+// values can be provided to override the default settings for EndMark,
+// HWHandshaking, DelayTime, and ReadTimeout.
+func NewDevice(ctx context.Context, address string, opts ...DeviceOption) (*Device, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
 	v, err := NewVisaResource(address)
 	if err != nil {
 		return nil, err

example_test.go

@@ -14,15 +14,15 @@ import (
 )
 
 func Example() {
+	ctx := context.Background()
+
 	// Open a serial device using a VISA resource string.
-	dev, err := asrl.NewDevice("ASRL::/dev/tty.usbserial-PX484GRU::9600::8N2::INSTR")
+	dev, err := asrl.NewDevice(ctx, "ASRL::/dev/tty.usbserial-PX484GRU::9600::8N2::INSTR")
 	if err != nil {
 		log.Fatal(err)
 	}
 	defer func() { _ = dev.Close() }()
 
-	ctx := context.Background()
-
 	// Query the instrument identification.
 	idn, err := dev.Query(ctx, "*IDN?")
 	if err != nil {
@@ -37,8 +37,10 @@ func Example() {
 }
 
 func Example_withOptions() {
+	ctx := context.Background()
+
 	// Open a device with functional options.
-	dev, err := asrl.NewDevice(
+	dev, err := asrl.NewDevice(ctx,
 		"ASRL::/dev/tty.usbserial-PX8X3YR6::9600::8N2::INSTR",
 		asrl.WithHWHandshaking(true),
 	)
@@ -47,8 +49,6 @@ func Example_withOptions() {
 	}
 	defer func() { _ = dev.Close() }()
 
-	ctx := context.Background()
-
 	// With hardware handshaking enabled, Command polls DSR before writing.
 	if err := dev.Command(ctx, "SYST:REM"); err != nil {
 		log.Fatal(err)

examples/keysight/e3631a/main.go

@@ -53,7 +53,8 @@ func main() {
 	// Open virtual comm port.
 	address := fmt.Sprintf("ASRL::%s::9600::8N2::INSTR", serialPort)
 	log.Printf("VISA Address = %s", address)
-	dev, err := asrl.NewDevice(address, asrl.WithHWHandshaking(true))
+	ctx := context.Background()
+	dev, err := asrl.NewDevice(ctx, address, asrl.WithHWHandshaking(true))
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -62,7 +63,6 @@ func main() {
 			log.Printf("error closing device: %v", err)
 		}
 	}()
-	ctx := context.Background()
 
 	// Query the identification of the function generator.
 	idn, err := dev.Query(ctx, "*IDN?")

examples/srs/ds345/main.go

@@ -42,7 +42,8 @@ func main() {
 	// Open virtual comm port.
 	address := fmt.Sprintf("ASRL::%s::%d::8N2::INSTR", serialPort, baudRate)
 	log.Printf("VISA Address = %s", address)
-	dev, err := asrl.NewDevice(address)
+	ctx := context.Background()
+	dev, err := asrl.NewDevice(ctx, address)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -52,8 +53,6 @@ func main() {
 		}
 	}()
 
-	ctx := context.Background()
-
 	// Query the identification of the function generator.
 	idn, err := dev.Query(ctx, "*idn?")
 	if err != nil && !errors.Is(err, io.EOF) {