cmd/osutil: Increase maxfiles on macOS properly (Fixes syncthing#6206)

go.mod

@@ -26,6 +26,8 @@ require (
 	github.com/maruel/panicparse v1.3.0
 	github.com/mattn/go-isatty v0.0.10
 	github.com/minio/sha256-simd v0.1.1
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.1 // indirect
 	github.com/onsi/ginkgo v1.9.0 // indirect
 	github.com/onsi/gomega v1.6.0 // indirect
 	github.com/oschwald/geoip2-golang v1.3.0

lib/osutil/rlimit_unix.go

@@ -8,7 +8,14 @@
 
 package osutil
 
-import "syscall"
+import (
+	"runtime"
+	"syscall"
+)
+
+const (
+	darwinOpenMax = 10240
+)
 
 // MaximizeOpenFileLimit tries to set the resource limit RLIMIT_NOFILE (number
 // of open file descriptors) to the max (hard limit), if the current (soft
@@ -26,6 +33,12 @@ func MaximizeOpenFileLimit() (int, error) {
 		return int(lim.Cur), nil
 	}
 
+	// macOS doesn't like a soft limit greater then OPEN_MAX
+	// See also: man setrlimit
+	if runtime.GOOS == "darwin" && lim.Max > darwinOpenMax {
+		lim.Cur = darwinOpenMax
+	}
+
 	// Try to increase the limit to the max.
 	oldLimit := lim.Cur
 	lim.Cur = lim.Max