maxima_fork.c

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <pwd.h>
#include <signal.h>

#include <sys/resource.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>

#include <fcntl.h>
#include <bsd/unistd.h>
#include <limits.h>
#include <grp.h>
#define N_SLOT 16
#define RNOFILE 256
#define FILEPATH_LEN (PATH_MAX + 1)
char filepath[FILEPATH_LEN];

// inits a maxima process for web service:
// changes gid/uid to maxima-{slot}
// redirects input/output, creates temporary subdirectories
char *fork_new_process() {
	fflush(stdout);

	// send an S for Synchronization, so that
	// the server process doesn't accidentally write into
	// sbcl's buffer
	// the server should not write anything before it has read this
	write(STDOUT_FILENO, "S", 1);

	// while the loop is running, the SIGCHLD handler
	// is deactivated so that children are automatically reaped
	// after that, it is again restored
	struct sigaction old, new;
	new.sa_handler = SIG_IGN;
	sigemptyset(&new.sa_mask);
	new.sa_flags = SA_NOCLDWAIT;
	char *ret = NULL;
	if (sigaction(SIGCHLD, &new, &old) == -1) {
		perror("Could not set signal error for children");
		return NULL;
	}

	// when sbcl spawns a child process through lisp, sbcl tries to close all
	// filedescriptors until RLIMIT_NOFILE
	// in docker containers, this is by standard quite high, so it takes long
	// which is remediated here by setting it lower manually
	struct rlimit nofile = { .rlim_cur = RNOFILE, .rlim_max = RNOFILE };
	if (setrlimit(RLIMIT_NOFILE, &nofile) == -1) {
		perror("Error setting rlimit_nofile");
		sigaction(SIGCHLD, &old, NULL);
		return NULL;
	}

	for (;;) {
		// can't flush enough
		fflush(stdout);
		int slot;
		
		// the slot number and temp directory is sent to the process
		// over stdin in the format "%d%s", where %s can contain anything
		// but newlines and musn't start with a number, which isn't a
		// problem for absolute paths
		if (scanf("%d", &slot) == EOF) {
			if (errno != 0) {
				perror("Error getting slot number from stdin");
				ret = NULL;