Windows: Factor out CloseExecFrom

Signed-off-by: John Howard <jhoward@microsoft.com>

libcontainer/utils/utils_unix.go

@@ -0,0 +1,33 @@
+// +build !windows
+
+package utils
+
+import (
+	"io/ioutil"
+	"strconv"
+	"syscall"
+)
+
+func CloseExecFrom(minFd int) error {
+	fdList, err := ioutil.ReadDir("/proc/self/fd")
+	if err != nil {
+		return err
+	}
+	for _, fi := range fdList {
+		fd, err := strconv.Atoi(fi.Name())
+		if err != nil {
+			// ignore non-numeric file names
+			continue
+		}
+
+		if fd < minFd {
+			// ignore descriptors lower than our specified minimum
+			continue
+		}
+
+		// intentionally ignore errors from syscall.CloseOnExec
+		syscall.CloseOnExec(fd)
+		// the cases where this might fail are basically file descriptors that have already been closed (including and especially the one that was created when ioutil.ReadDir did the "opendir" syscall)
+	}
+	return nil
+}