hid: fix and test HID enumeration on multiple threads

.travis.yml

@@ -22,3 +22,4 @@ matrix:
 
 script:
   - go install ./...
+  - go test -v ./...

appveyor.yml

@@ -29,3 +29,4 @@ install:
 
 build_script:
   - go install ./...
+  - go test -v ./...

hid_enabled.go

@@ -48,6 +48,15 @@ import (
 	"unsafe"
 )
 
+// enumerateLock is a mutex serializing access to USB device enumeration needed
+// by the macOS USB HID system calls, which require 2 consecutive method calls
+// for enumeration, causing crashes if called concurrently.
+//
+// For more details, see:
+//   https://developer.apple.com/documentation/iokit/1438371-iohidmanagersetdevicematching
+//   > "subsequent calls will cause the hid manager to release previously enumerated devices"
+var enumerateLock sync.Mutex
+
 func init() {
 	// Initialize the HIDAPI library
 	C.hid_init()
@@ -66,6 +75,9 @@ func Supported() bool {
 //  - If the product id is set to 0 then any product matches.
 //  - If the vendor and product id are both 0, all HID devices are returned.
 func Enumerate(vendorID uint16, productID uint16) []DeviceInfo {
+	enumerateLock.Lock()
+	defer enumerateLock.Unlock()
+
 	// Gather all device infos and ensure they are freed before returning
 	head := C.hid_enumerate(C.ushort(vendorID), C.ushort(productID))
 	if head == nil {

hid_test.go

@@ -0,0 +1,28 @@
+// hid - Gopher Interface Devices (USB HID)
+// Copyright (c) 2017 Péter Szilágyi. All rights reserved.
+//
+// This file is released under the 3-clause BSD license. Note however that Linux
+// support depends on libusb, released under GNU LGPL 2.1 or later.
+
+package hid
+
+import (
+	"sync"
+	"testing"
+)
+
+// Tests that device enumeration can be called concurrently from multiple threads.
+func TestThreadedEnumerate(t *testing.T) {
+	var pend sync.WaitGroup
+	for i := 0; i < 8; i++ {
+		pend.Add(1)
+
+		go func(index int) {
+			defer pend.Done()
+			for j := 0; j < 512; j++ {
+				Enumerate(uint16(index), 0)
+			}
+		}(i)
+	}
+	pend.Wait()
+}