Fix EndpointDescriptor

Fix creation of EndpointDescriptor and add unit tests for both possible
lengths.

usb_protocol/types/descriptors/standard.py

@@ -128,22 +128,23 @@ class DeviceCapabilityTypes(IntEnum):
 )
 
 
-EndpointDescriptor = DescriptorFormat(
+EndpointDescriptorLength = construct.Rebuild(construct.Int8ul, 7 if (this.bRefresh is None) and (this.bSynchAddress is None) else 9)
 
+EndpointDescriptor = DescriptorFormat(
     # [USB2.0: 9.6; USB Audio Device Class Definition 1.0: 4.6.1.1, 4.6.2.1]
     # Interfaces of the Audio 1.0 class extend their subordinate endpoint descriptors with
     # 2 additional bytes (extending it from 7 to 9 bytes). Thankfully, this is the only extension that
     # changes the length of a standard descriptor type, but we do have to handle this case in Construct.
-    "bLength"             / construct.Default(construct.OneOf(construct.Int8ul, [7, 9]), 7),
+    "bLength"             / EndpointDescriptorLength,
     "bDescriptorType"     / DescriptorNumber(StandardDescriptorNumbers.ENDPOINT),
     "bEndpointAddress"    / DescriptorField("Endpoint Address"),
     "bmAttributes"        / DescriptorField("Attributes", default=2),
     "wMaxPacketSize"      / DescriptorField("Maximum Packet Size", default=64),
     "bInterval"           / DescriptorField("Polling interval", default=255),
 
     # 2 bytes that are only present on endpoint descriptors for Audio 1.0 class interfaces.
-    ("bRefresh"           / construct.Optional(construct.Int8ul)) * "Refresh Rate",
-    ("bSynchAddress"      / construct.Optional(construct.Int8ul)) * "Synch Endpoint Address",
+    ("bRefresh"           / construct.If(this.bLength == 9, construct.Int8ul)) * "Refresh Rate",
+    ("bSynchAddress"      / construct.If(this.bLength == 9, construct.Int8ul)) * "Synch Endpoint Address",
 )
 
 
@@ -198,7 +199,6 @@ class DeviceCapabilityTypes(IntEnum):
 )
 
 
-
 class DescriptorParserCases(unittest.TestCase):
 
     STRING_DESCRIPTOR = bytes([
@@ -225,7 +225,6 @@ class DescriptorParserCases(unittest.TestCase):
         ord('s'), 0x00,
     ])
 
-
     def test_string_descriptor_parse(self):
 
         # Parse the relevant string...
@@ -236,23 +235,20 @@ def test_string_descriptor_parse(self):
         self.assertEqual(parsed.bDescriptorType,             3)
         self.assertEqual(parsed.bString, "Great Scott Gadgets")
 
-
     def test_string_descriptor_build(self):
         data = StringDescriptor.build({
             'bString': "Great Scott Gadgets"
         })
 
         self.assertEqual(data, self.STRING_DESCRIPTOR)
 
-
     def test_string_language_descriptor_build(self):
         data = StringLanguageDescriptor.build({
             'wLANGID': (LanguageIDs.ENGLISH_US,)
         })
 
         self.assertEqual(data, b"\x04\x03\x09\x04")
 
-
     def test_device_descriptor(self):
 
         device_descriptor = [
@@ -291,7 +287,6 @@ def test_device_descriptor(self):
         self.assertEqual(parsed.iSerialNumber,        3)
         self.assertEqual(parsed.bNumConfigurations,   1)
 
-
     def test_bcd_constructor(self):
 
         emitter = BCDFieldAdapter(construct.Int16ul)
@@ -300,5 +295,71 @@ def test_bcd_constructor(self):
         self.assertEqual(result, b"\x40\x01")
 
 
+    def test_parse_endpoint_descriptor(self):
+        # Parse the relevant descriptor ...
+        parsed = EndpointDescriptor.parse([
+                0x07,        # Length
+                0x05,        # Type
+                0x81,        # Endpoint address
+                0x02,        # Attributes
+                0x40, 0x00,  # Maximum packet size
+                0xFF,        # Interval
+            ])
+
+        # ... and check the descriptor's fields.
+        self.assertEqual(parsed.bLength, 7)
+        self.assertEqual(parsed.bDescriptorType, StandardDescriptorNumbers.ENDPOINT)
+        self.assertEqual(parsed.bEndpointAddress, 0x81)
+        self.assertEqual(parsed.bmAttributes, 2)
+        self.assertEqual(parsed.wMaxPacketSize, 64)
+        self.assertEqual(parsed.bInterval, 255)
+
+    def test_parse_endpoint_descriptor_audio(self):
+        # Parse the relevant descriptor ...
+        parsed = EndpointDescriptor.parse([
+                0x09,        # Length
+                0x05,        # Type
+                0x81,        # Endpoint address
+                0x02,        # Attributes
+                0x40, 0x00,  # Maximum packet size
+                0xFF,        # Interval
+                0x20,        # Refresh rate
+                0x05,        # Synch endpoint address
+            ])
+
+        # ... and check the descriptor's fields.
+        self.assertEqual(parsed.bLength, 9)
+        self.assertEqual(parsed.bDescriptorType, StandardDescriptorNumbers.ENDPOINT)
+        self.assertEqual(parsed.bEndpointAddress, 0x81)
+        self.assertEqual(parsed.bmAttributes, 2)
+        self.assertEqual(parsed.wMaxPacketSize, 64)
+        self.assertEqual(parsed.bInterval, 255)
+        self.assertEqual(parsed.bRefresh, 32)
+        self.assertEqual(parsed.bSynchAddress, 0x05)
+
+    def test_build_endpoint_descriptor_audio(self):
+        # Build the relevant descriptor
+        data = EndpointDescriptor.build({
+            'bEndpointAddress': 0x81,
+            'bmAttributes': 2,
+            'wMaxPacketSize': 64,
+            'bInterval': 255,
+            'bRefresh': 32,
+            'bSynchAddress': 0x05,
+        })
+
+        # ... and check the binary output
+        self.assertEqual(data, bytes([
+                0x09,        # Length
+                0x05,        # Type
+                0x81,        # Endpoint address
+                0x02,        # Attributes
+                0x40, 0x00,  # Maximum packet size
+                0xFF,        # Interval
+                0x20,        # Refresh rate
+                0x05,        # Synch endpoint address
+            ]))
+
+
 if __name__ == "__main__":
     unittest.main()