contrib/fs-idmap: Move logic to a new function

We can't call log.Fatalf() and defer functions, as the former doesn't
call any defers. Let's just move the code to a new function and call
os.Exit() only in main, when all defer executed.

Now that all the code is one function, we only print twice to stderr. It
is simpler to just print to stderr instead of logging and having also
the timestamp we don't really want.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

contrib/cmd/fs-idmap/fs-idmap.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"fmt"
-	"log"
 	"os"
 	"os/exec"
 	"syscall"
@@ -12,13 +11,20 @@ import (
 
 func main() {
 	if len(os.Args) != 2 {
-		log.Fatalf("usage: %s path_to_mount_set_attr", os.Args[0])
+		fmt.Fprintln(os.Stderr, "usage:", os.Args[0], "path_to_mount_set_attr")
+		os.Exit(1)
 	}
-
 	src := os.Args[1]
+	if err := supportsIDMap(src); err != nil {
+		fmt.Fprintln(os.Stderr, "fatal error:", err)
+		os.Exit(1)
+	}
+}
+
+func supportsIDMap(src string) error {
 	treeFD, err := unix.OpenTree(unix.AT_FDCWD, src, uint(unix.OPEN_TREE_CLONE|unix.OPEN_TREE_CLOEXEC|unix.AT_EMPTY_PATH))
 	if err != nil {
-		log.Fatalf("error calling open_tree %q: %v", src, err)
+		return fmt.Errorf("error calling open_tree %q: %w", src, err)
 	}
 	defer unix.Close(treeFD)
 
@@ -29,7 +35,7 @@ func main() {
 		GidMappings: []syscall.SysProcIDMap{{ContainerID: 0, HostID: 65536, Size: 65536}},
 	}
 	if err := cmd.Start(); err != nil {
-		log.Fatalf("failed to run the helper binary: %v", err)
+		return fmt.Errorf("failed to run the helper binary: %w", err)
 	}
 	defer func() {
 		_ = cmd.Process.Kill()
@@ -39,8 +45,7 @@ func main() {
 	path := fmt.Sprintf("/proc/%d/ns/user", cmd.Process.Pid)
 	var userNsFile *os.File
 	if userNsFile, err = os.Open(path); err != nil {
-		log.Fatalf("unable to get user ns file descriptor: %v", err)
-		return
+		return fmt.Errorf("unable to get user ns file descriptor: %w", err)
 	}
 	defer userNsFile.Close()
 
@@ -49,6 +54,8 @@ func main() {
 		Userns_fd: uint64(userNsFile.Fd()),
 	}
 	if err := unix.MountSetattr(treeFD, "", unix.AT_EMPTY_PATH, &attr); err != nil {
-		log.Fatalf("error calling mount_setattr: %v", err)
+		return fmt.Errorf("error calling mount_setattr: %w", err)
 	}
+
+	return nil
 }