Remove runtime lockDir and add in-memory lock manager

Remove runtime's lockDir as it is no longer needed after the lock
rework.

Add a trivial in-memory lock manager for unit testing

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

libpod/lock/in_memory_locks.go

@@ -0,0 +1,91 @@
+package lock
+
+import (
+	"sync"
+
+	"github.com/pkg/errors"
+)
+
+// Mutex holds a single mutex and whether it has been allocated.
+type Mutex struct {
+	id        uint32
+	lock      sync.Mutex
+	allocated bool
+}
+
+// ID retrieves the ID of the mutex
+func (m *Mutex) ID() uint32 {
+	return m.id
+}
+
+// Lock locks the mutex
+func (m *Mutex) Lock() {
+	m.lock.Lock()
+}
+
+// Unlock unlocks the mutex
+func (m *Mutex) Unlock() {
+	m.lock.Unlock()
+}
+
+// Free deallocates the mutex to allow its reuse
+func (m *Mutex) Free() error {
+	m.allocated = false
+
+	return nil
+}
+
+// InMemoryManager is a lock manager that allocates and retrieves local-only
+// locks - that is, they are not multiprocess. This lock manager is intended
+// purely for unit and integration testing and should not be used in production
+// deployments.
+type InMemoryManager struct {
+	locks     []*Mutex
+	numLocks  uint32
+	localLock sync.Mutex
+}
+
+// NewInMemoryManager creates a new in-memory lock manager with the given number
+// of locks.
+func NewInMemoryManager(numLocks uint32) (Manager, error) {
+	if numLocks == 0 {
+		return nil, errors.Errorf("must provide a non-zero number of locks!")
+	}
+
+	manager := new(InMemoryManager)
+	manager.numLocks = numLocks
+	manager.locks = make([]*Mutex, numLocks)
+
+	var i uint32
+	for i = 0; i < numLocks; i++ {
+		lock := new(Mutex)
+		lock.id = i
+		manager.locks[i] = lock
+	}
+
+	return manager, nil
+}
+
+// AllocateLock allocates a lock from the manager.
+func (m *InMemoryManager) AllocateLock() (Locker, error) {
+	m.localLock.Lock()
+	defer m.localLock.Unlock()
+
+	for _, lock := range m.locks {
+		if !lock.allocated {
+			lock.allocated = true
+			return lock, nil
+		}
+	}
+
+	return nil, errors.Errorf("all locks have been allocated")
+}
+
+// RetrieveLock retrieves a lock from the manager.
+func (m *InMemoryManager) RetrieveLock(id uint32) (Locker, error) {
+	if id >= m.numLocks {
+		return nil, errors.Errorf("given lock ID %d is too large - this manager only supports lock indexes up to %d", id, m.numLocks - 1)
+	}
+
+	return m.locks[id], nil
+}