Refactor locks package to build on non-Linux

Move SHM specific code into a subpackage. Within the main locks
package, move the manager to be linux-only and add a non-Linux
unsupported build file.

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

libpod/lock/shm_lock_manager_linux.go

@@ -0,0 +1,112 @@
+// +build linux
+
+package lock
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+	"syscall"
+
+	"github.com/pkg/errors"
+	"github.com/projectatomic/libpod/libpod/lock/shm"
+)
+
+// SHMLockManager manages shared memory locks.
+type SHMLockManager struct {
+	locks *shm.SHMLocks
+}
+
+// NewSHMLockManager makes a new SHMLockManager with the given number of locks.
+func NewSHMLockManager(numLocks uint32) (Manager, error) {
+	locks, err := shm.CreateSHMLock(numLocks)
+	if err != nil {
+		return nil, err
+	}
+
+	manager := new(SHMLockManager)
+	manager.locks = locks
+
+	return manager, nil
+}
+
+// OpenSHMLockManager opens an existing SHMLockManager with the given number of
+// locks.
+func OpenSHMLockManager(numLocks uint32) (Manager, error) {
+	locks, err := shm.OpenSHMLock(numLocks)
+	if err != nil {
+		return nil, err
+	}
+
+	manager := new(SHMLockManager)
+	manager.locks = locks
+
+	return manager, nil
+}
+
+// AllocateLock allocates a new lock from the manager.
+func (m *SHMLockManager) AllocateLock() (Locker, error) {
+	semIndex, err := m.locks.AllocateSemaphore()
+	if err != nil {
+		return nil, err
+	}
+
+	lock := new(SHMLock)
+	lock.lockID = semIndex
+	lock.manager = m
+
+	return lock, nil
+}
+
+// RetrieveLock retrieves a lock from the manager given its ID.
+func (m *SHMLockManager) RetrieveLock(id string) (Locker, error) {
+	intID, err := strconv.ParseInt(id, 16, 64)
+	if err != nil {
+		return nil, errors.Wrapf(err, "given ID %q is not a valid SHMLockManager ID - cannot be parsed as int", id)
+	}
+
+	if intID < 0 {
+		return nil, errors.Wrapf(syscall.EINVAL, "given ID %q is not a valid SHMLockManager ID - must be positive", id)
+	}
+
+	if intID > math.MaxUint32 {
+		return nil, errors.Wrapf(syscall.EINVAL, "given ID %q is not a valid SHMLockManager ID - too large", id)
+	}
+
+	var u32ID uint32 = uint32(intID)
+	if u32ID >= m.locks.GetMaxLocks() {
+		return nil, errors.Wrapf(syscall.EINVAL, "given ID %q is not a valid SHMLockManager ID - too large to fit", id)
+	}
+
+	lock := new(SHMLock)
+	lock.lockID = u32ID
+	lock.manager = m
+
+	return lock, nil
+}
+
+// SHMLock is an individual shared memory lock.
+type SHMLock struct {
+	lockID  uint32
+	manager *SHMLockManager
+}
+
+// ID returns the ID of the lock.
+func (l *SHMLock) ID() string {
+	return fmt.Sprintf("%x", l.lockID)
+}
+
+// Lock acquires the lock.
+func (l *SHMLock) Lock() error {
+	return l.manager.locks.LockSemaphore(l.lockID)
+}
+
+// Unlock releases the lock.
+func (l *SHMLock) Unlock() error {
+	return l.manager.locks.UnlockSemaphore(l.lockID)
+}
+
+// Free releases the lock, allowing it to be reused.
+func (l *SHMLock) Free() error {
+	return l.manager.locks.DeallocateSemaphore(l.lockID)
+}