Revise custom group id test (#139)

* Add comments explaining the purpose of the test

* Revise test for the custom group

This solves https://github.com/mozilla/cubeb-coreaudio-rs/issues/138.
When the Blackhole device is installed, both its input and output data
source are the same. As a result, the hash-map whose key is data source
value has collision when the hash-map is used to store both input and
output device's group ids.

To avoid the collision, we should use different hash-map for input
device and output device to store the group id values.

src/backend/tests/api.rs

@@ -1238,50 +1238,55 @@ fn test_get_device_group_id() {
 fn test_get_same_group_id_for_builtin_device_pairs() {
     use std::collections::HashMap;
 
+    // These device sources have custom group id. See `get_custom_group_id`.
     const IMIC: u32 = 0x696D_6963; // "imic"
     const ISPK: u32 = 0x6973_706B; // "ispk"
     const EMIC: u32 = 0x656D_6963; // "emic"
     const HDPN: u32 = 0x6864_706E; // "hdpn"
     let pairs = [(IMIC, ISPK), (EMIC, HDPN)];
 
-    // TODO: group_ids will have collision if one input device and one output device
-    //       has same source value.
-    let mut group_ids = HashMap::<u32, String>::new();
+    let mut input_group_ids = HashMap::<u32, String>::new();
     let input_devices = test_get_devices_in_scope(Scope::Input);
     for device in input_devices.iter() {
         match get_device_source(*device, DeviceType::INPUT) {
             Ok(source) => match get_device_group_id(*device, DeviceType::INPUT) {
-                Ok(id) => assert!(group_ids
+                Ok(id) => assert!(input_group_ids
                     .insert(source, id.into_string().unwrap())
                     .is_none()),
-                Err(e) => assert!(group_ids.insert(source, format!("Error {}", e)).is_none()),
+                Err(e) => assert!(input_group_ids
+                    .insert(source, format!("Error {}", e))
+                    .is_none()),
             },
             _ => {} // do nothing when failing to get source.
         }
     }
+
+    let mut output_group_ids = HashMap::<u32, String>::new();
     let output_devices = test_get_devices_in_scope(Scope::Output);
     for device in output_devices.iter() {
         match get_device_source(*device, DeviceType::OUTPUT) {
             Ok(source) => match get_device_group_id(*device, DeviceType::OUTPUT) {
-                Ok(id) => assert!(group_ids
+                Ok(id) => assert!(output_group_ids
                     .insert(source, id.into_string().unwrap())
                     .is_none()),
-                Err(e) => assert!(group_ids.insert(source, format!("Error {}", e)).is_none()),
+                Err(e) => assert!(output_group_ids
+                    .insert(source, format!("Error {}", e))
+                    .is_none()),
             },
             _ => {} // do nothing when failing to get source.
         }
     }
 
-    for (dev1, dev2) in pairs.iter() {
-        let id1 = group_ids.get(dev1);
-        let id2 = group_ids.get(dev2);
+    for (input, output) in pairs.iter() {
+        let input_group_id = input_group_ids.get(input);
+        let output_group_id = output_group_ids.get(output);
 
-        if id1.is_some() && id2.is_some() {
-            assert_eq!(id1, id2);
+        if input_group_id.is_some() && output_group_id.is_some() {
+            assert_eq!(input_group_id, output_group_id);
         }
 
-        group_ids.remove(dev1);
-        group_ids.remove(dev2);
+        input_group_ids.remove(input);
+        output_group_ids.remove(output);
     }
 }