Raise RLIMIT_NPROC only if maxnprocs higher than current limits

Otherwise maxnprocs may actually lower the limit. Especially when using the
default limit of 512, this quickly causes quark's fork() to fail when started
with a non-exclusive user.

To mitigate this, we respect system defaults and only raise the limit if it is
an actual raise.

.drone.yml

@@ -7,18 +7,31 @@ steps:
   image: gcc
   commands:
     - make
+    - make minibomb
 
 - name: run
   image: debian
   commands:
     - cp quark /usr/local/bin
-    - useradd web && su web && cd
+    - useradd web
     - mkdir -p web && cd web && echo "hello from quark" > index.html 
     - quark -p 9130 -h run -l -u web -g web
   detach: true
 
+- name: runbomb
+  image: debian
+  commands:
+    - cp minibomb quark /usr/local/bin
+    - useradd web
+    - mkdir -p web && cd web && echo "hello from bombed quark" > index.html 
+    - su web -c minibomb
+    - quark -p 9131 -h runbomb -l -u web -g web
+  detach: true
+
 - name: test
   image: curlimages/curl
   commands:
-    - sleep 15
+    - sleep 20
     - curl http://run:9130
+    - curl http://runbomb:9131
+

Makefile

@@ -42,3 +42,9 @@ install: all
 uninstall:
 	rm -f "$(DESTDIR)$(PREFIX)/bin/quark"
 	rm -f "$(DESTDIR)$(MANPREFIX)/man1/quark.1"
+
+minibomb: minibomb.c
+	$(CC) -pthread -o $@ $(CPPFLAGS) $(CFLAGS) minibomb.c $(LDFLAGS)
+
+clean-minibomb:
+	rm -f minibomb

main.c

@@ -285,7 +285,12 @@ main(int argc, char *argv[])
 	}
 
 	/* raise the process limit */
-	rlim.rlim_cur = rlim.rlim_max = maxnprocs;
+  if (getrlimit(RLIMIT_NPROC, &rlim) < 0) {
+    die("getrlimit RLIMIT_NPROC:");
+  }
+
+  rlim.rlim_cur = MAX(rlim.rlim_cur, maxnprocs);
+  rlim.rlim_max = MAX(rlim.rlim_max, maxnprocs);
   if (setrlimit(RLIMIT_NPROC, &rlim) < 0) {
     die("setrlimit RLIMIT_NPROC:");
   }

minibomb.c

@@ -0,0 +1,22 @@
+#include <stdio.h>
+#include <unistd.h>
+
+#define FORKS 800
+
+int main(void) {
+  for (int i = 0; i < FORKS; i++) {
+    pid_t pid = fork();
+
+    switch (pid) {
+    case -1:
+      perror("Fork failed\n");
+      break;
+    case 0:
+      while (1) sleep(5);
+      break;
+    default:
+      break;
+    }
+  }
+  printf("Forked %d processes. Letting someone else clean up. Bye.\n", FORKS);
+}