Running the LilyPond development version inside a container

README.md

Containerized lilypond-git

I’ve lately taken up LilyPond transcription as a hobby, and recently I encountered a score that needed a feature from the LilyPond development version. I wanted to keep the stable version of LilyPond installed on my main system, so I created a container (machinectl clone Arch-Base Arch-lilypond-git) and built the lilypond-git package inside it. In order to build the score in that container, I then use the below Makefile and small wrapper program: lilypond-git invokes lilypond inside the Arch-lilypond-git container, with the current working directory bind-mounted into it, and afterwards adjusts the ownership of the generated PDF file (because lilypond was running as root inside the container). This would be easier to write as a shell script, of course, but Linux ignores the setuid bit on shell scripts, and I need this to be a setuid binary if I want to invoke it from Emacs’ compilation mode without entering my password for sudo all the time (which is extremely cumbersome anyways, since you have to use C-u M-x compile instead of just M-x compile, which doesn’t match the F12 keybinding).

lilypond-git.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(int argc, char *argv[]) {
  if (argc != 3) {
    fprintf(stderr, "Usage: %s FILE.ly FILE.pdf\n", argv[0]);
    return EXIT_FAILURE;
  }

  char *cwd = get_current_dir_name();
  int child = fork();
  if (child < 0) {
    perror("fork");
    return EXIT_FAILURE;
  }
  if (child == 0) {
    return execlp("systemd-nspawn",
                  "systemd-nspawn",
                  "--quiet",
                  "-M",
                  "Arch-lilypond-git",
                  "--ephemeral",
                  "--read-only",
                  "--private-network",
                  "--bind",
                  cwd,
                  "--chdir",
                  cwd,
                  "lilypond",
                  argv[1],
                  (char *)NULL);
  } else {
    free(cwd);
    int wstatus, exitstatus = -1;
    do {
      pid_t w = waitpid(child, &wstatus, 0);
      if (w == -1) {
        perror("waitpid");
        exit(EXIT_FAILURE);
      }
      if (WIFEXITED(wstatus)) {
        exitstatus = WEXITSTATUS(wstatus);
      }
    } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
    (void)chown(argv[2], getuid(), getgid());
    return exitstatus >= 0 ? exitstatus : EXIT_FAILURE;
  }
}

Makefile

.PHONY: all

CFLAGS := -Wall -Wextra -Wpedantic -Werror

all: the-score.pdf

the-score.pdf: the-score.ly lilypond-git
	./lilypond-git $< $@

# based on Make’s implicit rule, search for %: in make -p
lilypond-git: lilypond-git.c
	$(LINK.c) $^ $(LOADLIBES) $(LDLIBS) -o $@
	sudo sh -c 'chown root $@ && chmod u+s $@'