etcdserver: metrics and monitor number of file descriptor

It exposes the metrics of file descriptor limit and file descriptor used.
Moreover, it prints out warning when more than 80% of fd limit has been used.

```
2015/04/08 01:26:19 etcdserver: 80% of the file descriptor limit is open
[open = 969, limit = 1024]
```

etcdserver/metrics.go

@@ -14,7 +14,13 @@
 
 package etcdserver
 
-import "github.com/coreos/etcd/Godeps/_workspace/src/github.com/prometheus/client_golang/prometheus"
+import (
+	"log"
+	"time"
+
+	"github.com/coreos/etcd/Godeps/_workspace/src/github.com/prometheus/client_golang/prometheus"
+	"github.com/coreos/etcd/pkg/runtime"
+)
 
 var (
 	// TODO: with label in v3?
@@ -32,10 +38,42 @@ var (
 		Name: "etcdserver_proposal_failed_total",
 		Help: "The total number of failed proposals.",
 	})
+
+	fileDescriptorUsed = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "file_descriptors_used",
+		Help: "The number of file descriptors used",
+	})
 )
 
 func init() {
 	prometheus.MustRegister(proposeDurations)
 	prometheus.MustRegister(proposePending)
 	prometheus.MustRegister(proposeFailed)
+	prometheus.MustRegister(fileDescriptorUsed)
+}
+
+func monitorFileDescriptor(done <-chan struct{}) {
+	ticker := time.NewTicker(5 * time.Second)
+	defer ticker.Stop()
+	for {
+		used, err := runtime.FDUsage()
+		if err != nil {
+			log.Printf("etcdserver: cannot monitor file descriptor usage (%v)", err)
+			return
+		}
+		fileDescriptorUsed.Set(float64(used))
+		limit, err := runtime.FDLimit()
+		if err != nil {
+			log.Printf("etcdserver: cannot monitor file descriptor usage (%v)", err)
+			return
+		}
+		if used >= limit/5*4 {
+			log.Printf("etcdserver: 80%% of the file descriptor limit is used [used = %d, limit = %d]", used, limit)
+		}
+		select {
+		case <-ticker.C:
+		case <-done:
+			return
+		}
+	}
 }

etcdserver/server.go

@@ -16,6 +16,7 @@ package etcdserver
 
 import (
 	"encoding/json"
+	"expvar"
 	"fmt"
 	"log"
 	"math/rand"
@@ -33,6 +34,7 @@ import (
 	"github.com/coreos/etcd/pkg/fileutil"
 	"github.com/coreos/etcd/pkg/idutil"
 	"github.com/coreos/etcd/pkg/pbutil"
+	"github.com/coreos/etcd/pkg/runtime"
 	"github.com/coreos/etcd/pkg/timeutil"
 	"github.com/coreos/etcd/pkg/types"
 	"github.com/coreos/etcd/pkg/wait"
@@ -69,6 +71,16 @@ var (
 
 func init() {
 	rand.Seed(time.Now().UnixNano())
+
+	expvar.Publish(
+		"file_descriptor_limit",
+		expvar.Func(
+			func() interface{} {
+				n, _ := runtime.FDLimit()
+				return n
+			},
+		),
+	)
 }
 
 type Response struct {
@@ -271,6 +283,7 @@ func (s *EtcdServer) Start() {
 	s.start()
 	go s.publish(defaultPublishRetryInterval)
 	go s.purgeFile()
+	go monitorFileDescriptor(s.done)
 }
 
 // start prepares and starts server in a new goroutine. It is no longer safe to

pkg/runtime/fds_linux.go

@@ -0,0 +1,36 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package runtime
+
+import (
+	"io/ioutil"
+	"syscall"
+)
+
+func FDLimit() (uint64, error) {
+	var rlimit syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rlimit); err != nil {
+		return 0, err
+	}
+	return rlimit.Cur, nil
+}
+
+func FDUsage() (uint64, error) {
+	fds, err := ioutil.ReadDir("/proc/self/fd")
+	if err != nil {
+		return 0, err
+	}
+	return uint64(len(fds)), nil
+}

pkg/runtime/fds_other.go

@@ -0,0 +1,30 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !linux
+
+package runtime
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func FDLimit() (uint64, error) {
+	return 0, fmt.Errorf("cannot get FDLimit on %s", runtime.GOOS)
+}
+
+func FDUsage() (uint64, error) {
+	return 0, fmt.Errorf("cannot get FDUsage on %s", runtime.GOOS)
+}