Warn if ulimit is too low when serving production sites

main.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"net"
 	"os"
+	"os/exec"
 	"path"
 	"runtime"
 	"strconv"
@@ -86,17 +87,32 @@ func main() {
 		}(s)
 
 		if !quiet {
-			for _, config := range configs {
-				fmt.Println(config.Address())
-			}
-
+			var checkedFdLimit bool
 			for addr, configs := range addresses {
 				for _, conf := range configs {
+					// Print address of site
+					fmt.Println(conf.Address())
+
+					// Note if non-localhost site resolves to loopback interface
 					if addr.IP.IsLoopback() && !isLocalhost(conf.Host) {
 						fmt.Printf("Notice: %s is only accessible on this machine (%s)\n",
 							conf.Host, addr.IP.String())
 					}
 				}
+
+				// Warn if ulimit is too low for production sites
+				if (runtime.GOOS == "linux" || runtime.GOOS == "darwin") &&
+					addr.IP.IsLoopback() && !checkedFdLimit {
+					out, err := exec.Command("ulimit", "-n").Output()
+					if err == nil {
+						// Note that an error here need not be reported
+						lim, err := strconv.Atoi(string(bytes.TrimSpace(out)))
+						if err == nil && lim < 4096 {
+							fmt.Printf("Warning: File descriptor limit is too low (%d) for production sites\n", lim)
+						}
+						checkedFdLimit = true
+					}
+				}
 			}
 		}
 	}