etcd: ensure data dir is writable

etcd checks that the data dir is writable by writing and removing an
empty file to the data dir during startup and exits non-zero if that
fails.

fixes #876

main.go

@@ -27,6 +27,7 @@ import (
 	"github.com/coreos/etcd/etcdserver"
 	"github.com/coreos/etcd/etcdserver/etcdhttp"
 	"github.com/coreos/etcd/pkg/cors"
+	"github.com/coreos/etcd/pkg/fileutil"
 	"github.com/coreos/etcd/pkg/flags"
 	"github.com/coreos/etcd/pkg/transport"
 	"github.com/coreos/etcd/pkg/types"
@@ -152,6 +153,9 @@ func startEtcd() {
 	if err := os.MkdirAll(*dir, privateDirMode); err != nil {
 		log.Fatalf("etcd: cannot create data directory: %v", err)
 	}
+	if err := fileutil.IsDirWriteable(*dir); err != nil {
+		log.Fatalf("etcd: cannot write to data directory: %v", err)
+	}
 
 	pt, err := transport.NewTransport(peerTLSInfo)
 	if err != nil {

pkg/fileutil/fileutil.go

@@ -0,0 +1,37 @@
+/*
+   Copyright 2014 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 fileutil
+
+import (
+	"io/ioutil"
+	"os"
+	"path"
+)
+
+const (
+	privateFileMode = 0600
+)
+
+// IsDirWriteable checks if dir is writable by writing and removing a file
+// to dir. It returns nil if dir is writable.
+func IsDirWriteable(dir string) error {
+	f := path.Join(dir, ".touch")
+	if err := ioutil.WriteFile(f, []byte(""), privateFileMode); err != nil {
+		return err
+	}
+	return os.Remove(f)
+}

pkg/fileutil/fileutil_test.go

@@ -0,0 +1,39 @@
+/*
+   Copyright 2014 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 fileutil
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestIsDirWriteable(t *testing.T) {
+	tmpdir, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatalf("unexpected ioutil.TempDir error: %v", err)
+	}
+	if err := IsDirWriteable(tmpdir); err != nil {
+		t.Fatalf("unexpected IsDirWriteable error: %v", err)
+	}
+	if err := os.Chmod(tmpdir, 0444); err != nil {
+		t.Fatalf("unexpected os.Chmod error: %v", err)
+	}
+	if err := IsDirWriteable(tmpdir); err == nil {
+		t.Fatalf("expected IsDirWriteable to error")
+	}
+}