[libvirt] [PATCH] command: avoid potential deadlock on handshake

[Eric Blake]

Similar to commit 858c247, the parent process should close it's
copy of the end of the pipe given to the child; otherwise, if there
is an extreme bug where the parent thinks the child reported an
error and is waiting for the message to go along with it, but the
child thinks it reported success and is waiting for the parent
to acknowledge the success, we would get into deadlock.

Thankfully, I don't think this deadlock is possible without at
least one other bug in the code, but I did see exactly that sort
of situation prior to commit da831af - if a double close bug in
the parent causes the parent to read the wrong fd, it might assume
the child failed, even though the child really sent success if
only the parent had read from the correct location.

* src/util/command.c (virCommandHandshakeWait): Close unused fds
sooner.
---

 src/util/command.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/util/command.c b/src/util/command.c
index 62ea50b..62848cd 100644
--- a/src/util/command.c
+++ b/src/util/command.c
@@ -2507,6 +2507,11 @@ int virCommandHandshakeWait(virCommandPtr cmd)
             VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
             return -1;
         }
+        /* Close the handshakeNotify fd before trying to read anything
+         * further on the handshakeWait pipe; so that a child waiting
+         * on our acknowledgment will die rather than deadlock.  */
+        VIR_FORCE_CLOSE(cmd->handshakeNotify[1]);
+
         if ((len = saferead(cmd->handshakeWait[0], msg, 1024)) < 0) {
             VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
             VIR_FREE(msg);
-- 
1.7.10.2