Make mmutex actually acquire and release file locks

mmutex/lib/blocker.go

Index: mmutex/lib/blocker.go
diff --git a/mmutex/lib/blocker.go b/mmutex/lib/blocker.go
new file mode 100644
index 0000000000000000000000000000000000000000..f69b17b92f750432b7c62e0babd02b7133c23933
--- /dev/null
+++ b/mmutex/lib/blocker.go
@@ -0,0 +1,43 @@
+// Copyright 2017 The LUCI 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 lib
+
+import (
+	"time"
+
+	"github.com/danjacques/gofslock/fslock"
+)
+
+func minDuration(d1 time.Duration, d2 time.Duration) time.Duration {
+	if d1.Nanoseconds() <= d2.Nanoseconds() {
+		return d1
+	} else {
+		return d2
+	}
+}
+
+func CreateBlockerUntil(t time.Time, pollingInterval time.Duration) fslock.Blocker {
+	return func() error {
+		timeRemaining := t.Sub(time.Now())
+		if timeRemaining <= 0 {
+			return fslock.ErrLockHeld
+		}
+
+		time.Sleep(minDuration(pollingInterval, timeRemaining))
+
+		// Returning nil signals that the lock should be retried.
+		return nil
+	}
+}