Do not assume interface numbers follow the slice indices.

config.go

@@ -49,25 +49,17 @@ func (c ConfigDesc) String() string {
 	return fmt.Sprintf("Configuration %d", c.Number)
 }
 
-func (c ConfigDesc) intfDesc(num, alt int) (*InterfaceSetting, error) {
+func (c ConfigDesc) intfDesc(num int) (*InterfaceDesc, error) {
 	// In an ideal world, interfaces in the descriptor would be numbered
 	// contiguously starting from 0, as required by the specification. In the
 	// real world however the specification is sometimes ignored:
 	// https://github.com/google/gousb/issues/65
 	ifs := make([]int, len(c.Interfaces))
-	for i := range c.Interfaces {
-		ifs[i] = c.Interfaces[i].Number
-		if ifs[i] != num {
-			continue
+	for i, desc := range c.Interfaces {
+		if desc.Number == num {
+			return &desc, nil
 		}
-		alts := make([]int, len(c.Interfaces[i].AltSettings))
-		for a := range c.Interfaces[i].AltSettings {
-			alts[a] = c.Interfaces[i].AltSettings[a].Alternate
-			if alts[a] == alt {
-				return &c.Interfaces[i].AltSettings[a], nil
-			}
-		}
-		return nil, fmt.Errorf("alternate setting %d not found for %s, available alt settings: %v", alt, c.Interfaces[i], alts)
+		ifs[i] = desc.Number
 	}
 	return nil, fmt.Errorf("interface %d not found, available interface numbers: %v", num, ifs)
 }
@@ -120,9 +112,13 @@ func (c *Config) Interface(num, alt int) (*Interface, error) {
 		return nil, fmt.Errorf("Interface(%d, %d) called on %s after Close", num, alt, c)
 	}
 
-	altInfo, err := c.Desc.intfDesc(num, alt)
+	intf, err := c.Desc.intfDesc(num)
+	if err != nil {
+		return nil, fmt.Errorf("descriptor of interface %d in %s: %v", num, c, err)
+	}
+	altInfo, err := intf.altSetting(alt)
 	if err != nil {
-		return nil, fmt.Errorf("descriptor of interface (%d, %d) in %s: %v", num, alt, c, err)
+		return nil, fmt.Errorf("descriptor of alternate setting %d fo interface %d in %s: %v", alt, num, c, err)
 	}
 
 	c.mu.Lock()
@@ -137,7 +133,7 @@ func (c *Config) Interface(num, alt int) (*Interface, error) {
 	}
 
 	// Select an alternate setting if needed (device has multiple alternate settings).
-	if len(c.Desc.Interfaces[num].AltSettings) > 1 {
+	if len(intf.AltSettings) > 1 {
 		if err := c.dev.ctx.libusb.setAlt(c.dev.handle, uint8(num), uint8(alt)); err != nil {
 			c.dev.ctx.libusb.release(c.dev.handle, uint8(num))
 			return nil, fmt.Errorf("failed to set alternate config %d on interface %d of %s: %v", alt, num, c, err)

device.go

@@ -276,9 +276,13 @@ func (d *Device) InterfaceDescription(cfgNum, intfNum, altNum int) (string, erro
 	if err != nil {
 		return "", fmt.Errorf("%s: %v", d, err)
 	}
-	alt, err := cfg.intfDesc(intfNum, altNum)
+	intf, err := cfg.intfDesc(intfNum)
 	if err != nil {
-		return "", fmt.Errorf("%s, configuration %d: %v", d, cfgNum, err)
+		return "", fmt.Errorf("%s, configuration %d interface %d: %v", d, cfgNum, intfNum, err)
+	}
+	alt, err := intf.altSetting(altNum)
+	if err != nil {
+		return "", fmt.Errorf("%s, configuration %d interface %d alternate setting %d: %v", d, cfgNum, intfNum, altNum, err)
 	}
 	return d.GetStringDescriptor(alt.iInterface)
 }

device_test.go

@@ -117,26 +117,26 @@ func TestClaimAndRelease(t *testing.T) {
 		t.Errorf("%s.InEndpoint(%d): got %+v, want %+v", intf, ep1Addr, got, want)
 	}
 
-	if _, err := cfg.Interface(1, 0); err == nil {
+	if _, err := cfg.Interface(if1Num, 0); err == nil {
 		t.Fatalf("%s.Interface(1, 0): got nil, want non nil, because Interface 1 is already claimed.", cfg)
 	}
 
 	// intf2 is interface #0, not claimed yet.
-	intf2, err := cfg.Interface(0, 0)
+	intf2, err := cfg.Interface(if2Num, alt2Num)
 	if err != nil {
 		t.Fatalf("%s.Interface(0, 0): got %v, want nil", cfg, err)
 	}
 
 	if err := cfg.Close(); err == nil {
-		t.Fatalf("%s.Close(): got nil, want non nil, because the Interface was not released.", cfg)
+		t.Fatalf("%s.Close(): got nil, want non nil, because the Interfaces #1/2 was not released.", cfg)
 	}
 	if err := dev.Close(); err == nil {
 		t.Fatalf("%s.Close(): got nil, want non nil, because the Config was not released.", cfg)
 	}
 
 	intf.Close()
 	if err := cfg.Close(); err == nil {
-		t.Fatalf("%s.Close(): got nil, want non nil, because the Interface was not released.", cfg)
+		t.Fatalf("%s.Close(): got nil, want non nil, because the Interface #2 was not released.", cfg)
 	}
 	if err := dev.Close(); err == nil {
 		t.Fatalf("%s.Close(): got nil, want non nil, because the Config was not released.", dev)
@@ -244,3 +244,61 @@ func TestAutoDetachFailure(t *testing.T) {
 		t.Fatalf("%s.Config(1) got nil, but want no nil because interface fails to detach", dev)
 	}
 }
+
+func TestInterface(t *testing.T) {
+	t.Parallel()
+	c := newContextWithImpl(newFakeLibusb())
+	defer func() {
+		if err := c.Close(); err != nil {
+			t.Errorf("Context.Close: %v", err)
+		}
+	}()
+
+	for _, tc := range []struct {
+		name                  string
+		vid, pid              ID
+		cfgNum, ifNum, altNum int
+	}{{
+		name:   "simple",
+		vid:    0x8888,
+		pid:    0x0002,
+		cfgNum: 1,
+		ifNum:  0,
+		altNum: 0,
+	}, {
+		name:   "alt_setting",
+		vid:    0x8888,
+		pid:    0x0002,
+		cfgNum: 1,
+		ifNum:  1,
+		altNum: 1,
+	}, {
+		name:   "noncontiguous_interfaces",
+		vid:    0x8888,
+		pid:    0x0002,
+		cfgNum: 1,
+		ifNum:  3,
+		altNum: 2,
+	}} {
+		t.Run(tc.name, func(t *testing.T) {
+			dev, err := c.OpenDeviceWithVIDPID(0x8888, 0x0002)
+			if err != nil {
+				t.Fatalf("OpenDeviceWithVIDPID(0x8888, 0x0002): %v", err)
+			}
+			if dev == nil {
+				t.Fatal("OpenDeviceWithVIDPID(0x8888, 0x0002): got nil device, need non-nil")
+			}
+			defer dev.Close()
+			cfg, err := dev.Config(tc.cfgNum)
+			if err != nil {
+				t.Fatalf("%s.Config(%d): %v", dev, tc.cfgNum, err)
+			}
+			defer cfg.Close()
+			intf, err := cfg.Interface(tc.ifNum, tc.altNum)
+			if err != nil {
+				t.Fatalf("%s.Interface(%d, %d): %v", cfg, tc.ifNum, tc.altNum, err)
+			}
+			intf.Close()
+		})
+	}
+}

interface.go

@@ -29,6 +29,17 @@ type InterfaceDesc struct {
 	AltSettings []InterfaceSetting
 }
 
+func (i *InterfaceDesc) altSetting(alt int) (*InterfaceSetting, error) {
+	alts := make([]int, len(i.AltSettings))
+	for a, s := range i.AltSettings {
+		if s.Alternate == alt {
+			return &s, nil
+		}
+		alts[a] = s.Alternate
+	}
+	return nil, fmt.Errorf("alternate setting %d not found for %s, available alt settings: %v", alt, i, alts)
+}
+
 // String returns a human-readable description of the interface descriptor and
 // its alternate settings.
 func (i InterfaceDesc) String() string {