handles: use real pointers to keep track of handles

With the change to 1.6 rules, we couldn't use the Go pointers, so we
went with casting the list indices into pointers.

The runtime does not like this, however. It will sometimes detect that
we have a pointer with a very small value and consider it an invalid
pointer, bringing down the application with it.

Work around that by asking libc for the smallest amount of memory it'll
give us so we have an actual allocated pointer to use. We then use this
pointer value as the key in our map to find the Go object we're
tracking.

handles.go

@@ -1,5 +1,9 @@
 package git
 
+/*
+#include <stdlib.h>
+*/
+import "C"
 import (
 	"fmt"
 	"sync"
@@ -9,75 +13,45 @@ import (
 type HandleList struct {
 	sync.RWMutex
 	// stores the Go pointers
-	handles []interface{}
-	// Indicates which indices are in use.
-	set map[int]bool
+	handles map[unsafe.Pointer]interface{}
 }
 
 func NewHandleList() *HandleList {
 	return &HandleList{
-		handles: make([]interface{}, 5),
-		set:     make(map[int]bool),
+		handles: make(map[unsafe.Pointer]interface{}),
 	}
 }
 
-// findUnusedSlot finds the smallest-index empty space in our
-// list. You must only run this function while holding a write lock.
-func (v *HandleList) findUnusedSlot() int {
-	for i := 1; i < len(v.handles); i++ {
-		if !v.set[i] {
-			return i
-		}
-	}
-
-	// reaching here means we've run out of entries so append and
-	// return the new index, which is equal to the old length.
-	slot := len(v.handles)
-	v.handles = append(v.handles, nil)
-
-	return slot
-}
-
 // Track adds the given pointer to the list of pointers to track and
 // returns a pointer value which can be passed to C as an opaque
 // pointer.
 func (v *HandleList) Track(pointer interface{}) unsafe.Pointer {
+	handle := C.malloc(1)
+
 	v.Lock()
-
-	slot := v.findUnusedSlot()
-	v.handles[slot] = pointer
-	v.set[slot] = true
-
+	v.handles[handle] = pointer
 	v.Unlock()
 
-	return unsafe.Pointer(uintptr(slot))
+	return handle
 }
 
 // Untrack stops tracking the pointer given by the handle
 func (v *HandleList) Untrack(handle unsafe.Pointer) {
-	slot := int(uintptr(handle))
-
 	v.Lock()
-
-	v.handles[slot] = nil
-	delete(v.set, slot)
-
+	delete(v.handles, handle)
+	C.free(handle)
 	v.Unlock()
 }
 
 // Get retrieves the pointer from the given handle
 func (v *HandleList) Get(handle unsafe.Pointer) interface{} {
-	slot := int(uintptr(handle))
-
 	v.RLock()
+	defer v.RUnlock()
 
-	if !v.set[slot] {
+	ptr, ok := v.handles[handle]
+	if !ok {
 		panic(fmt.Sprintf("invalid pointer handle: %p", handle))
 	}
 
-	ptr := v.handles[slot]
-
-	v.RUnlock()
-
 	return ptr
 }