fdtrack: temporary hack for tracking file descriptor usage

Package fdtrack logs statistics about open file descriptors.
This should help identify the source of #1549.

fdtrack/fdusage_linux.go

@@ -0,0 +1,53 @@
+// Copyright 2015 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library 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 Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// +build linux
+
+package fdtrack
+
+import (
+	"io"
+	"os"
+	"syscall"
+)
+
+func fdlimit() int {
+	var nofile syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &nofile); err != nil {
+		return 0
+	}
+	return int(nofile.Cur)
+}
+
+func fdusage() (int, error) {
+	f, err := os.Open("/proc/self/fd")
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+	const batchSize = 100
+	n := 0
+	for {
+		list, err := f.Readdirnames(batchSize)
+		n += len(list)
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			return 0, err
+		}
+	}
+	return n, nil
+}