Add secondary group impersonation w/ !cgo support

pkg/util/syscall_credentials.go

@@ -0,0 +1,44 @@
+package util
+
+import (
+	"strconv"
+	"syscall"
+
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+)
+
+func SyscallCredentials(userStr string) (*syscall.Credential, error) {
+	uid, gid, err := GetUIDAndGIDFromString(userStr, true)
+	if err != nil {
+		return nil, errors.Wrap(err, "get uid/gid")
+	}
+
+	u, err := Lookup(userStr)
+	if err != nil {
+		return nil, errors.Wrap(err, "lookup")
+	}
+	logrus.Infof("util.Lookup returned: %+v", u)
+
+	var groups []uint32
+
+	gidStr, err := groupIDs(u)
+	if err != nil {
+		return nil, errors.Wrap(err, "group ids for user")
+	}
+
+	for _, g := range gidStr {
+		i, err := strconv.ParseUint(g, 10, 32)
+		if err != nil {
+			return nil, errors.Wrap(err, "parseuint")
+		}
+
+		groups = append(groups, uint32(i))
+	}
+
+	return &syscall.Credential{
+		Uid:    uid,
+		Gid:    gid,
+		Groups: groups,
+	}, nil
+}