inotify: Avoid most of the race conditions, get rid of Cint

There's still a condition that could be a race with fd-reuse, if one
goroutine is calling inotify.{AddWatch,RmWatch,Read}(); another
goroutine is calling inotify.Close(), and several things happen between
loadFd() running and the add_watch/rm_watch/read syscall launching:
 - syscall.Close() returns
 - syscall.Open() reuses the filedescriptor

           A                                  B
    syscall(loadFd())         inotify.Close(); syscall.Open()
    ----------------------------------------------------------
           loadFd()
                                     syscall.Close()
                                     syscall.Open()
           syscall()

Given that Read() can't be allowed to block Close() from running, I'm not
sure there's a way to fix this.

1 files changed, 14 insertions, 1 deletions

diff --git a/inotify/bits.go b/inotify/bits.go
index 3606b52..9162435 100644
--- a/inotify/bits.go
+++ b/inotify/bits.go
@@ -1,5 +1,7 @@
 package inotify
 
+import "sync/atomic"
+
 const (
 	// Flags for the parameter of InotifyInit1().
 	// These, oddly, appear to be 24-bit numbers.
@@ -7,8 +9,19 @@ const (
 	IN_NONBLOCK uint32 = 00004000
 )
 
-type Mask uint32
+// Logically, Fd and Wd should be 'int', not 'int64', to match the OS.
+// But, because there's no 'sync/atomic.SwapInt', we cast up to int64.
+type Wd int64
+type Fd int64
+
+func swapFd(addr *Fd, new Fd) (old Fd) {
+	return Fd(atomic.SwapInt64((*int64)(addr), int64(new)))
+}
+func loadFd(addr *Fd) Fd {
+	return Fd(atomic.LoadInt64((*int64)(addr)))
+}
 
+type Mask uint32
 const (
 	// Supported events suitable for the `mask` parameter of Inotify.AddWatch().
 	IN_ACCESS        Mask = (1<< 0) // File was accessed.