wal: hold file lock while renaming WAL directory on non-Windows

Windows requires this lock to be released before the directory is
renamed. But on unix-like operating systems, releasing the lock and
trying to reacquire it immediately can be flaky if a process is forked
around the same time. The file descriptors are marked as close-on-exec
by the Go runtime, but there is a window between the fork and exec where
another process will be holding the lock.

wal/wal.go

@@ -131,22 +131,7 @@ func Create(dirpath string, metadata []byte) (*WAL, error) {
 		return nil, err
 	}
 
-	// rename of directory with locked files doesn't work on windows; close
-	// the WAL to release the locks so the directory can be renamed
-	w.Close()
-	if err := os.Rename(tmpdirpath, dirpath); err != nil {
-		return nil, err
-	}
-	// reopen and relock
-	newWAL, oerr := Open(dirpath, walpb.Snapshot{})
-	if oerr != nil {
-		return nil, oerr
-	}
-	if _, _, _, err := newWAL.ReadAll(); err != nil {
-		newWAL.Close()
-		return nil, err
-	}
-	return newWAL, nil
+	return w.renameWal(tmpdirpath)
 }
 
 // Open opens the WAL at the given snap.

wal/wal_test.go

@@ -666,7 +666,7 @@ func TestOpenOnTornWrite(t *testing.T) {
 		}
 	}
 
-	fn := w.tail().Name()
+	fn := path.Join(p, path.Base(w.tail().Name()))
 	w.Close()
 
 	// clobber some entry with 0's to simulate a torn write

wal/wal_unix.go

@@ -0,0 +1,38 @@
+// Copyright 2016 The etcd Authors
+//
+// 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 !windows
+
+package wal
+
+import "os"
+
+func (w *WAL) renameWal(tmpdirpath string) (*WAL, error) {
+	// On non-Windows platforms, hold the lock while renaming. Releasing
+	// the lock and trying to reacquire it quickly can be flaky because
+	// it's possible the process will fork to spawn a process while this is
+	// happening. The fds are set up as close-on-exec by the Go runtime,
+	// but there is a window between the fork and the exec where another
+	// process holds the lock.
+
+	if err := os.RemoveAll(w.dir); err != nil {
+		return nil, err
+	}
+	if err := os.Rename(tmpdirpath, w.dir); err != nil {
+		return nil, err
+	}
+
+	w.fp = newFilePipeline(w.dir, SegmentSizeBytes)
+	return w, nil
+}

wal/wal_windows.go

@@ -0,0 +1,41 @@
+// Copyright 2016 The etcd Authors
+//
+// 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 wal
+
+import (
+	"os"
+
+	"github.com/coreos/etcd/wal/walpb"
+)
+
+func (w *WAL) renameWal(tmpdirpath string) (*WAL, error) {
+	// rename of directory with locked files doesn't work on
+	// windows; close the WAL to release the locks so the directory
+	// can be renamed
+	w.Close()
+	if err := os.Rename(tmpdirpath, w.dir); err != nil {
+		return nil, err
+	}
+	// reopen and relock
+	newWAL, oerr := Open(w.dir, walpb.Snapshot{})
+	if oerr != nil {
+		return nil, oerr
+	}
+	if _, _, _, err := newWAL.ReadAll(); err != nil {
+		newWAL.Close()
+		return nil, err
+	}
+	return newWAL, nil
+}