[libvirt] [PATCH 2/3] qemu: support passing pre-opened UNIX socket listen FD

[Daniel P. Berrangé]

There is a race condition when spawning QEMU where libvirt has spawned
QEMU but the monitor socket is not yet open. Libvirt has to repeatedly
try to connect() to QEMU's monitor until eventually it succeeds, or
times out. We use kill() to check if QEMU is still alive so we avoid
waiting a long time if QEMU exited, but having a timeout at all is still
unpleasant.

With QEMU 2.12 we can pass in a pre-opened FD for UNIX domain or TCP
sockets. If libvirt has called bind() and listen() on this FD, then we
have a guarantee that libvirt can immediately call connect() and
succeed without any race.

Although we only really care about this for the monitor socket and agent
socket, this patch does FD passing for all UNIX socket based character
devices since there appears to be no downside to it.

We don't do FD passing for TCP sockets, however, because it is only
possible to pass a single FD, while some hostnames may require listening
on multiple FDs to cover IPv4 and IPv6 concurrently.

Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
---
 src/qemu/qemu_command.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 56 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index fa0aa5d5c3..291aad13cf 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -5010,8 +5010,62 @@ qemuBuildChrChardevStr(virLogManagerPtr logManager,
         break;
 
     case VIR_DOMAIN_CHR_TYPE_UNIX:
-        virBufferAsprintf(&buf, "socket,id=%s,path=", charAlias);
-        virQEMUBuildBufferEscapeComma(&buf, dev->data.nix.path);
+#ifndef WIN32
+        if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_CHARDEV_FD_PASS)) {
+            struct sockaddr_un addr;
+            socklen_t addrlen = sizeof(addr);
+            int fd;
+
+            if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
+                virReportSystemError(errno, "%s",
+                                     _("Unable to create UNIX socket"));
+                goto cleanup;
+            }
+
+            memset(&addr, 0, sizeof(addr));
+            addr.sun_family = AF_UNIX;
+            if (virStrcpyStatic(addr.sun_path, dev->data.nix.path) == NULL) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("Monitor path %s too big for destination"),
+                               dev->data.nix.path);
+                VIR_FORCE_CLOSE(fd);
+                goto cleanup;
+            }
+
+            if (unlink(dev->data.nix.path) < 0 && errno != ENOENT) {
+                virReportSystemError(errno,
+                                     _("Unable to unlink %s"),
+                                     dev->data.nix.path);
+                VIR_FORCE_CLOSE(fd);
+                goto cleanup;
+            }
+
+            if (bind(fd, (struct sockaddr *)&addr, addrlen) < 0) {
+                virReportSystemError(errno,
+                                     _("Unable to bind to monitor %s"),
+                                     dev->data.nix.path);
+                VIR_FORCE_CLOSE(fd);
+                goto cleanup;
+            }
+
+            if (listen(fd, 1) < 0) {
+                virReportSystemError(errno,
+                                     _("Unable to listen to monitor %s"),
+                                     dev->data.nix.path);
+                VIR_FORCE_CLOSE(fd);
+                goto cleanup;
+            }
+
+            virBufferAsprintf(&buf, "socket,id=%s,fd=%d", charAlias, fd);
+
+            virCommandPassFD(cmd, fd, VIR_COMMAND_PASS_FD_CLOSE_PARENT);
+        } else {
+#endif /* WIN32 */
+            virBufferAsprintf(&buf, "socket,id=%s,path=", charAlias);
+            virQEMUBuildBufferEscapeComma(&buf, dev->data.nix.path);
+#ifndef WIN32
+        }
+#endif /* WIN32 */
         if (dev->data.nix.listen)
             virBufferAdd(&buf, nowait ? ",server,nowait" : ",server", -1);
 
-- 
2.14.3