Support malformed interface descriptors (#79)

* Add an invalid interface descriptor to one of the test devices.
* Add a test for getting that interface description.
* Handle the malformed interface/alternate setting descriptors, where
interface/alt numbers don't follow the spec (not 0-based, not contiguous
etc.)

Addresses issue #65.

config.go

@@ -50,14 +50,26 @@ func (c ConfigDesc) String() string {
 }
 
 func (c ConfigDesc) intfDesc(num, alt int) (*InterfaceSetting, error) {
-	if num < 0 || num >= len(c.Interfaces) {
-		return nil, fmt.Errorf("interface %d not found, available interfaces 0..%d", num, len(c.Interfaces)-1)
+	// 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
 		}
-	ifInfo := c.Interfaces[num]
-	if alt < 0 || alt >= len(ifInfo.AltSettings) {
-		return nil, fmt.Errorf("alternate setting %d not found for %s, available alt settings 0..%d", alt, ifInfo, len(ifInfo.AltSettings)-1)
+		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 &ifInfo.AltSettings[alt], nil
+		}
+		return nil, fmt.Errorf("alternate setting %d not found for %s, available alt settings: %v", alt, c.Interfaces[i], alts)
+	}
+	return nil, fmt.Errorf("interface %d not found, available interface numbers: %v", num, ifs)
 }
 
 // Config represents a USB device set to use a particular configuration.

device_test.go

@@ -81,6 +81,7 @@ func TestClaimAndRelease(t *testing.T) {
 		{1, 0, "Fast streaming"},
 		{1, 1, "Slower streaming"},
 		{1, 2, ""},
+		{3, 2, "Interface for https://github.com/google/gousb/issues/65"},
 	} {
 		if got, err := dev.InterfaceDescription(1, tc.intf, tc.alt); err != nil {
 			t.Errorf("%s.InterfaceDescription(1, %d, %d): %v", dev, tc.intf, tc.alt, err)

fakelibusb_devices.go

@@ -158,6 +158,18 @@ var fakeDevices = []fakeDevice{
 							},
 						},
 					}},
+				}, {
+					// Malformed descriptior, non contiguous interface
+					// number (previous interface is #1), alt settings
+					// not starting from 0.
+					// See https://github.com/google/gousb/issues/65
+					Number: 3,
+					AltSettings: []InterfaceSetting{{
+						Number:     3,
+						Alternate:  2,
+						Class:      ClassVendorSpec,
+						iInterface: 9,
+					}},
 				}},
 			}},
 			iManufacturer: 1,
@@ -172,6 +184,7 @@ var fakeDevices = []fakeDevice{
 			6: "Boring setting",
 			7: "Fast streaming",
 			8: "Slower streaming",
+			9: "Interface for https://github.com/google/gousb/issues/65",
 		},
 	},
 	// Bus 001 Device 003: ID 9999:0002

interface.go

@@ -23,8 +23,7 @@ import (
 // InterfaceDesc contains information about a USB interface, extracted from
 // the descriptor.
 type InterfaceDesc struct {
-	// Number is the number of this interface, a zero-based index in the array
-	// of interfaces supported by the device configuration.
+	// Number is the number of this interface.
 	Number int
 	// AltSettings is a list of alternate settings supported by the interface.
 	AltSettings []InterfaceSetting

libusb.go

@@ -271,7 +271,9 @@ func (libusbImpl) getDeviceDesc(d *libusbDevice) (*DeviceDesc, error) {
 			Cap:  int(cfg.bNumInterfaces),
 		}
 		c.Interfaces = make([]InterfaceDesc, 0, len(ifaces))
-		for ifNum, iface := range ifaces {
+		// a map of interface numbers to a set of alternate settings numbers
+		hasIntf := make(map[int]map[int]bool)
+		for _, iface := range ifaces {
 			if iface.num_altsetting == 0 {
 				continue
 			}
@@ -283,7 +285,7 @@ func (libusbImpl) getDeviceDesc(d *libusbDevice) (*DeviceDesc, error) {
 				Cap:  int(iface.num_altsetting),
 			}
 			descs := make([]InterfaceSetting, 0, len(alts))
-			for altNum, alt := range alts {
+			for _, alt := range alts {
 				i := InterfaceSetting{
 					Number:     int(alt.bInterfaceNumber),
 					Alternate:  int(alt.bAlternateSetting),
@@ -292,12 +294,16 @@ func (libusbImpl) getDeviceDesc(d *libusbDevice) (*DeviceDesc, error) {
 					Protocol:   Protocol(alt.bInterfaceProtocol),
 					iInterface: int(alt.iInterface),
 				}
-				if ifNum != i.Number {
-					return nil, fmt.Errorf("config %d interface at index %d has number %d, USB standard states they should be identical", c.Number, ifNum, i.Number)
+
+				if hasIntf[i.Number][i.Alternate] {
+					log.Printf("Device on bus %d address %d offered a descriptor for config %d with two different entries with the same interface number (%d) and the same alternate setting number (%d). gousb will use only the first one.", dev.Bus, dev.Address, c.Number, i.Number, i.Alternate)
+					continue
 				}
-				if altNum != i.Alternate {
-					return nil, fmt.Errorf("config %d interface %d alternate settings at index %d has number %d, USB standard states they should be identical", c.Number, i.Number, altNum, i.Alternate)
+				if hasIntf[i.Number] == nil {
+					hasIntf[i.Number] = make(map[int]bool)
 				}
+				hasIntf[i.Number][i.Alternate] = true
+
 				var ends []C.struct_libusb_endpoint_descriptor
 				*(*reflect.SliceHeader)(unsafe.Pointer(&ends)) = reflect.SliceHeader{
 					Data: uintptr(unsafe.Pointer(alt.endpoint)),