[PATCH ucspi-vsock] vsockclient: allow null application

[Alyssa Ross <hi@alyssa.is>]

I found myself running

    vsockclient 2 1024 true

in a situation where it just mattered that a connection was made, and
no data needed to be exchanged of it.  That I found myself doing this
is quite a strong indicator that the program to exec into should be
optional.

UCSPI-1996[1] doesn't specify what should happen if no application is
provided, so I think this is a reasonable interpretation.  I'm not
sure if I feel the same for vsockserver(d), so I haven't made the
corresponding change there.  I'll wait to see if there's ever a need
for it.

[1]: https://cr.yp.to/proto/ucspi.txt
---
 vsockclient.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/vsockclient.c b/vsockclient.c
index 934cfa0..456810d 100644
--- a/vsockclient.c
+++ b/vsockclient.c
@@ -41,11 +41,9 @@ int main(int argc, char *argv[])
 		}
 	}
 
-	// Check there are enough positional arguments (two for the
-	// address and at least one to exec into).
-	if (optind > argc - 3)
+	// Find the address (cid and port) to connect to.
+	if (optind > argc - 2)
 		ex_usage();
-
 	if (getu32(argv[optind++], 0, UINT32_MAX, &rcid))
 		ex_usage();
 	if (getu32(argv[optind++], 0, UINT32_MAX, &rport))
@@ -69,6 +67,11 @@ int main(int argc, char *argv[])
 		     "connect to %" PRIu32 ":%" PRIu32 " failed", rcid, rport);
 	ilog("connected to %" PRIu32 ":%" PRIu32 "!", rcid, rport);
 
+	// If we have nothing to exec into, we're done.  (Sometimes
+	// just connecting on its own is enough.)
+	if (optind == argc)
+		return 0;
+
 	// Set up the file descriptors to the well-known UCSPI numbers.
 	if (dup2(fd, 6) == -1)
 		diee(EX_OSERR, "dup2");
-- 
2.30.0