rootless: Merge rootless.go back into rootless_linux.go

The files were split apart by b96be3af (changes to allow for darwin
compilation, 2018-06-20, #1015), but the C import and two functions
left in rootless.go are all Linux-specific as well.  This commit moves
all of the pre-b96be3af rootless.go into rootless_linux.go, just
adding the '// +build linux' header (b96be3af also scrambled the + in
that header) and keeping the new GetRootlessUID from a1545fe6
(rootless: add function to retrieve the original UID, 2018-07-05, #1048).

Signed-off-by: W. Trevor King <wking@tremily.us>

Closes: #1034
Approved by: baude

pkg/rootless/rootless_linux.go

@@ -1,4 +1,4 @@
-// build +linux
+// +build linux
 
 package rootless
 
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	gosignal "os/signal"
 	"runtime"
 	"strconv"
@@ -22,6 +23,11 @@ extern int reexec_in_user_namespace_wait(int pid);
 */
 import "C"
 
+func runInUser() error {
+	os.Setenv("_LIBPOD_USERNS_CONFIGURED", "done")
+	return nil
+}
+
 // IsRootless tells us if we are running in rootless mode
 func IsRootless() bool {
 	return os.Getuid() != 0 || os.Getenv("_LIBPOD_USERNS_CONFIGURED") != ""
@@ -37,6 +43,30 @@ func GetRootlessUID() int {
 	return os.Getuid()
 }
 
+func tryMappingTool(tool string, pid int, hostID int, mappings []idtools.IDMap) error {
+	path, err := exec.LookPath(tool)
+	if err != nil {
+		return err
+	}
+
+	appendTriplet := func(l []string, a, b, c int) []string {
+		return append(l, fmt.Sprintf("%d", a), fmt.Sprintf("%d", b), fmt.Sprintf("%d", c))
+	}
+
+	args := []string{path, fmt.Sprintf("%d", pid)}
+	args = appendTriplet(args, 0, hostID, 1)
+	if mappings != nil {
+		for _, i := range mappings {
+			args = appendTriplet(args, i.ContainerID+1, i.HostID, i.Size)
+		}
+	}
+	cmd := exec.Cmd{
+		Path: path,
+		Args: args,
+	}
+	return cmd.Run()
+}
+
 // BecomeRootInUserNS re-exec podman in a new userNS
 func BecomeRootInUserNS() (bool, error) {