Implement MSG_COPY option for msgrcv(2).

Implement Queue.Copy and add more tests for it.

Updates #135

1 files changed, 23 insertions, 4 deletions

diff --git a/pkg/sentry/kernel/msgqueue/msgqueue.go b/pkg/sentry/kernel/msgqueue/msgqueue.go
index 28520b19a..c111297d7 100644
--- a/pkg/sentry/kernel/msgqueue/msgqueue.go
+++ b/pkg/sentry/kernel/msgqueue/msgqueue.go
@@ -386,13 +386,21 @@ func (q *Queue) pop(ctx context.Context, creds *auth.Credentials, mType int64, m
 	return msg, nil
 }
 
-// Copy copies a message from the queue without deleting it. See
-// msgrcv(MSG_COPY).
-func (q *Queue) Copy() (*Message, error) {
+// Copy copies a message from the queue without deleting it. If no message
+// exists, an error is returned. See msgrcv(MSG_COPY).
+func (q *Queue) Copy(mType int64) (*Message, error) {
 	q.mu.Lock()
 	defer q.mu.Unlock()
 
-	return nil, linuxerr.ENOSYS
+	if mType < 0 || q.messages.Empty() {
+		return nil, linuxerr.ENOMSG
+	}
+
+	msg := q.msgAtIndex(mType)
+	if msg == nil {
+		return nil, linuxerr.ENOMSG
+	}
+	return msg, nil
 }
 
 // msgOfType returns the first message with the specified type, nil if no
@@ -433,6 +441,17 @@ func (q *Queue) msgOfTypeLessThan(mType int64) (m *Message) {
 	return m
 }
 
+// msgAtIndex returns a pointer to a message at given index, nil if non exits.
+//
+// Precondition: caller must hold q.mu.
+func (q *Queue) msgAtIndex(mType int64) *Message {
+	msg := q.messages.Front()
+	for ; mType != 0 && msg != nil; mType-- {
+		msg = msg.Next()
+	}
+	return msg
+}
+
 // Lock implements ipc.Mechanism.Lock.
 func (q *Queue) Lock() {
 	q.mu.Lock()