refactor: optimize file cleanups, propagate exit codes, and fix Makefile

- Unlink the temporary bootstrap launcher binary immediately after opening a read-only descriptor to it, then execute via `/proc/self/fd/<fd>` to ensure zero-disk footprint on execution.
- Unlink temporary `/tmp/resolvconf*` files immediately after successful bind-mounting over `/etc/resolv.conf`.
- Prune parent ephemeral profile directories when unpinning a namespace, leaving zero directories behind once empty.
- Propagate the exact exit status of the wrapped command to the host process using `errors.As` and `*exec.ExitError` instead of defaulting to exit code 1.
- Added E2E automated test `TestExitCodePropagation` to verify exit status delivery.
- Added the `$(BINARY)` target to `.PHONY` in the Makefile to delegate dependency tracking to Go's compiler cache, ensuring modified Go files are rebuilt during `make test`.

1 files changed, 21 insertions, 1 deletions

diff --git a/internal/namespace/namespace.go b/internal/namespace/namespace.go
index a1e7ad9..e2ef2f1 100644
--- a/internal/namespace/namespace.go
+++ b/internal/namespace/namespace.go
@@ -100,16 +100,36 @@ func Bootstrap() error {
 	// 2. Write the embedded launcher binary to the temp file.
 	if _, err := tmpFile.Write(launcherBytes); err != nil {
 		_ = tmpFile.Close()
+		_ = os.Remove(launcherPath)
 		return fmt.Errorf("failed to write launcher binary: %w", err)
 	}
 
 	// Ensure the binary is executable (0700)
 	if err := tmpFile.Chmod(0700); err != nil {
 		_ = tmpFile.Close()
+		_ = os.Remove(launcherPath)
 		return fmt.Errorf("failed to set launcher permissions: %w", err)
 	}
+
+	// 2b. Open a read-only fd of the launcher to exec
+	execFd, err := syscall.Open(launcherPath, syscall.O_RDONLY, 0)
+	if err != nil {
+		_ = tmpFile.Close()
+		_ = os.Remove(launcherPath)
+		return fmt.Errorf("failed to open launcher for exec: %w", err)
+	}
+
+	// Close the write file descriptor (to avoid ETXTBSY)
 	_ = tmpFile.Close()
 
+	// Unlink the file from disk (makes it invisible and ensures it is deleted on exit)
+	_ = os.Remove(launcherPath)
+
+	// Clear close-on-exec so it remains open across syscall.Exec
+	if flags, err := unix.FcntlInt(uintptr(execFd), unix.F_GETFD, 0); err == nil {
+		_, _ = unix.FcntlInt(uintptr(execFd), unix.F_SETFD, flags&^unix.FD_CLOEXEC)
+	}
+
 	// 3. Prepare arguments for the launcher.
 	// The launcher expects: launcher <command_to_run> [args...]
 	args := []string{self}
@@ -153,7 +173,7 @@ func Bootstrap() error {
 		}
 	}
 
-	err = syscall.Exec(launcherPath, args, env)
+	err = syscall.Exec(fmt.Sprintf("/proc/self/fd/%d", execFd), args, env)
 	if err != nil {
 		return fmt.Errorf("launcher exec failed: %w", err)
 	}