libpod: add execSessionNoCopy

To avoid creating an expensive deep copy, create an internal function to
access the exec session.

[NO TESTS NEEDED]

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

libpod/container.go

@@ -774,9 +774,9 @@ func (c *Container) ExecSessions() ([]string, error) {
 	return ids, nil
 }
 
-// ExecSession retrieves detailed information on a single active exec session in
-// a container
-func (c *Container) ExecSession(id string) (*ExecSession, error) {
+// execSessionNoCopy returns the associated exec session to id.
+// Note that the session is not a deep copy.
+func (c *Container) execSessionNoCopy(id string) (*ExecSession, error) {
 	if !c.batched {
 		c.lock.Lock()
 		defer c.lock.Unlock()
@@ -791,6 +791,17 @@ func (c *Container) ExecSession(id string) (*ExecSession, error) {
 		return nil, errors.Wrapf(define.ErrNoSuchExecSession, "no exec session with ID %s found in container %s", id, c.ID())
 	}
 
+	return session, nil
+}
+
+// ExecSession retrieves detailed information on a single active exec session in
+// a container
+func (c *Container) ExecSession(id string) (*ExecSession, error) {
+	session, err := c.execSessionNoCopy(id)
+	if err != nil {
+		return nil, err
+	}
+
 	returnSession := new(ExecSession)
 	if err := JSONDeepCopy(session, returnSession); err != nil {
 		return nil, errors.Wrapf(err, "error copying contents of container %s exec session %s", c.ID(), session.ID())

libpod/container_exec.go

@@ -747,7 +747,7 @@ func (c *Container) Exec(config *ExecConfig, streams *define.AttachStreams, resi
 		return -1, err
 	}
 
-	session, err := c.ExecSession(sessionID)
+	session, err := c.execSessionNoCopy(sessionID)
 	if err != nil {
 		if errors.Cause(err) == define.ErrNoSuchExecSession {
 			// TODO: If a proper Context is ever plumbed in here, we