common/fdlimit: Move fdlimit files to separate package (#15850)

* common/fdlimit: Move fdlimit files to separate package

When go-ethereum is used as a library the calling program need to set
the FD limit.

This commit extract fdlimit files to a separate package so it can be
used outside of go-ethereum.

* common/fdlimit: Remove FdLimit from functions signature

* common/fdlimit: Rename fdlimit functions

common/fdlimit/fdlimit_unix.go

@@ -0,0 +1,60 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of go-ethereum.
+//
+// go-ethereum is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// go-ethereum is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with go-ethereum. If not, see <http://www.gnu.org/licenses/>.
+
+// +build linux darwin netbsd openbsd solaris
+
+package fdlimit
+
+import "syscall"
+
+// Raise tries to maximize the file descriptor allowance of this process
+// to the maximum hard-limit allowed by the OS.
+func Raise(max uint64) error {
+	// Get the current limit
+	var limit syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &limit); err != nil {
+		return err
+	}
+	// Try to update the limit to the max allowance
+	limit.Cur = limit.Max
+	if limit.Cur > max {
+		limit.Cur = max
+	}
+	if err := syscall.Setrlimit(syscall.RLIMIT_NOFILE, &limit); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Current retrieves the number of file descriptors allowed to be opened by this
+// process.
+func Current() (int, error) {
+	var limit syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &limit); err != nil {
+		return 0, err
+	}
+	return int(limit.Cur), nil
+}
+
+// Maximum retrieves the maximum number of file descriptors this process is
+// allowed to request for itself.
+func Maximum() (int, error) {
+	var limit syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &limit); err != nil {
+		return 0, err
+	}
+	return int(limit.Max), nil
+}