bpf: Retry opening map after initial error

Current code used sync.Once and only attempted opening the map once using
sync.Once. Subsequent tried to open the map would always fail without
ever retrying, i.e. the initial failure always became a permanent failure.

Fixes: #2965

Reported-by: @mrdima
Signed-off-by: Thomas Graf <thomas@cilium.io>

pkg/bpf/map.go

@@ -1,4 +1,4 @@
-// Copyright 2016-2017 Authors of Cilium
+// Copyright 2016-2018 Authors of Cilium
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -120,6 +120,9 @@ type Map struct {
 	once sync.Once
 	lock lock.RWMutex
 
+	// openLock serializes calls to Map.Open()
+	openLock lock.Mutex
+
 	// NonPersistent is true if the map does not contain persistent data
 	// and should be removed on startup.
 	NonPersistent bool
@@ -353,25 +356,24 @@ reopen:
 }
 
 func (m *Map) Open() error {
-	var err error
-	m.once.Do(func() {
-		if m.fd != 0 {
-			err = nil
-			return
-		}
+	m.openLock.Lock()
+	defer m.openLock.Unlock()
 
-		if err = m.setPathIfUnset(); err != nil {
-			return
-		}
-		var fd int
-		fd, err = ObjGet(m.path)
-		if err != nil {
-			return
-		}
+	if m.fd != 0 {
+		return nil
+	}
 
-		m.fd = fd
-	})
-	return err
+	if err := m.setPathIfUnset(); err != nil {
+		return err
+	}
+
+	fd, err := ObjGet(m.path)
+	if err != nil {
+		return err
+	}
+
+	m.fd = fd
+	return nil
 }
 
 func (m *Map) Close() error {