compctl: Fail more gracefully if ran on a non-compctld enabled machine

cgroup.c

@@ -55,6 +55,14 @@ cgroup_create(const char *chier, const char *cgroup)
 	return 1;
 }
 
+bool
+cgroup_available(const char *chier, const char *cgroup)
+{
+	char cgroupdir[PATH_MAX];
+	snprintf(cgroupdir, sizeof(cgroupdir), "%s%s/%s", cgroupfs, chier, cgroup);
+	return (access(cgroupdir, F_OK) >= 0);
+}
+
 
 int
 cgroup_add_task(const char *chier, const char *cgroup, pid_t pid)

cgroup.h

@@ -18,6 +18,8 @@ int cgroup_setup(const char *chier, const char *controllers);
 /* Setup a given control group. Return 0 if cgroups already set up,
  * 1 on success, -1 on failure. */
 int cgroup_create(const char *chier, const char *cgroup);
+/* Return true if cgroup control is set up, false otherwise. */
+bool cgroup_available(const char *chier, const char *cgroup);
 
 /* Add a task to a given cgroup. Return 1 on success, -1 on failure. */
 int cgroup_add_task(const char *chier, const char *cgroup, pid_t pid);

compctl.c

@@ -20,6 +20,7 @@ daemon_chat(char *cmd)
 	struct sockaddr_un sun = { .sun_family = AF_UNIX, .sun_path = SOCKFILE };
 	if (connect(s, (struct sockaddr *) &sun, sizeof(sun.sun_family) + strlen(sun.sun_path) + 1) < 0) {
 		perror(SOCKFILE);
+		fputs("Plese contact " ADMINCONTACT " about this error.\n", stderr);
 		exit(EXIT_FAILURE);
 	}
 
@@ -202,6 +203,14 @@ main(int argc, char *argv[])
 		return EXIT_FAILURE;
 	}
 
+	if (!cgroup_available(chier, cgroup)) {
+		fputs("CGroup-based computation control is not available on this host.\n", stderr);
+		fputs("Most likely, this computer is not meant for computations,\n", stderr);
+		fputs("please consider switching to a different host.\n", stderr);
+		fputs("Plese contact " ADMINCONTACT " if you believe this is an error.\n", stderr);
+		return EXIT_FAILURE;
+	}
+
 	while (argc > optind) {
 		char *cmd = argv[optind++];
 		if (!strcmp(cmd, "--run")) {