Always allow extra file descriptors when setting the connection maximum

When setting the maximum number of connections allowed by the OS,
always allow some extra file descriptors for other files.

Fixes bug 22797; bugfix on 0.2.0.10-alpha.

changes/bug22797

@@ -0,0 +1,4 @@
+  o Minor bugfixes (file limits):
+    - When setting the maximum number of connections allowed by the OS,
+      always allow some extra file descriptors for other files.
+      Fixes bug 22797; bugfix on 0.2.0.10-alpha.

src/common/compat.c

@@ -1568,19 +1568,24 @@ set_max_file_descriptors(rlim_t limit, int *max_out)
   if (setrlimit(RLIMIT_NOFILE, &rlim) != 0) {
     int bad = 1;
 #ifdef OPEN_MAX
-    if (errno == EINVAL && OPEN_MAX < rlim.rlim_cur) {
+    uint64_t try_limit = OPEN_MAX - ULIMIT_BUFFER;
+    if (errno == EINVAL && try_limit < rlim.rlim_cur) {
       /* On some platforms, OPEN_MAX is the real limit, and getrlimit() is
        * full of nasty lies.  I'm looking at you, OSX 10.5.... */
-      rlim.rlim_cur = OPEN_MAX;
+      rlim.rlim_cur = try_limit;
       if (setrlimit(RLIMIT_NOFILE, &rlim) == 0) {
         if (rlim.rlim_cur < (rlim_t)limit) {
           log_warn(LD_CONFIG, "We are limited to %lu file descriptors by "
-                 "OPEN_MAX, and ConnLimit is %lu.  Changing ConnLimit; sorry.",
-                   (unsigned long)OPEN_MAX, (unsigned long)limit);
+                   "OPEN_MAX (%lu), and ConnLimit is %lu.  Changing "
+                   "ConnLimit; sorry.",
+                   (unsigned long)try_limit, (unsigned long)OPEN_MAX,
+                   (unsigned long)limit);
         } else {
-          log_info(LD_CONFIG, "Dropped connection limit to OPEN_MAX (%lu); "
-                   "Apparently, %lu was too high and rlimit lied to us.",
-                   (unsigned long)OPEN_MAX, (unsigned long)rlim.rlim_max);
+          log_info(LD_CONFIG, "Dropped connection limit to %lu based on "
+                   "OPEN_MAX (%lu); Apparently, %lu was too high and rlimit "
+                   "lied to us.",
+                   (unsigned long)try_limit, (unsigned long)OPEN_MAX,
+                   (unsigned long)rlim.rlim_max);
         }
         bad = 0;
       }