[PATCH V2] xenconsole: add file lock to xenconsole

[Yufang Zhang <yufang521247@gmail.com>]

From: Yufang Zhang <yuzhang@redhat.com>

This patch add a file lock to xenconsole for each console id, so
that only one console could be attached to a guest at a time.
Otherwise, consoles would get stuck and print strange outputs.

The version2 patch fix bellow problems:
1) fix for indent level and coding style as the surrounding code
2) put the lock files in /var/run/
3) use snprintf instead of sprintf

Considering the lock files are just blank files and they can be
reused, leaving them behind when xenconsole crashes(or killed by
kill -9) is not so bad.

Signed-off-by: Yufang Zhang <yufang521247@gmail.com>
---
 tools/console/client/main.c |   31 +++++++++++++++++++++++++++++++
 1 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/tools/console/client/main.c b/tools/console/client/main.c
index df3636f..ff1c64e 100644
--- a/tools/console/client/main.c
+++ b/tools/console/client/main.c
@@ -46,6 +46,28 @@
 
 static volatile sig_atomic_t received_signal = 0;
 
+static int console_locked(const char *file)
+{
+	int fd;
+
+	fd = open(file, O_RDWR | O_CREAT, S_IRUSR|S_IWUSR);
+	if (fd == -1) {
+		fprintf(stderr,"can't open lock file: %s\n", file);
+		exit(1);
+	}
+
+	if (lockf(fd, F_TLOCK, 0) == -1) {
+		if ( errno == EACCES || errno == EAGAIN ) {
+			close(fd);
+			return (1);
+		}
+		fprintf(stderr,"can't lock file: %s\n", file);
+		exit(1);
+	}
+
+	return (0);
+}
+
 static void sighandler(int signum)
 {
 	received_signal = 1;
@@ -280,6 +302,7 @@ int main(int argc, char **argv)
 	int spty, xsfd;
 	struct xs_handle *xs;
 	char *end;
+	char buf[100];
 	console_type type = CONSOLE_INVAL;
 
 	while((ch = getopt_long(argc, argv, sopt, lopt, &opt_ind)) != -1) {
@@ -368,6 +391,13 @@ int main(int argc, char **argv)
 		exit(EINVAL);
 	}
 
+	/* Make sure only one console is attached to guest */
+	snprintf(buf, strlen("/var/run/xenconsole-%d-%d") + 1, "/var/run/xenconsole-%d-%d", domid, num);
+	if (console_locked(buf) == 1) {
+		fprintf(stderr, "Another console has already been attached to guest\n");
+		exit(EINVAL);
+	}
+
 	/* Set a watch on this domain's console pty */
 	if (!xs_watch(xs, path, ""))
 		err(errno, "Can't set watch for console pty");
@@ -387,6 +417,7 @@ int main(int argc, char **argv)
 	console_loop(spty, xs, path);
 	restore_term(STDIN_FILENO, &attr);
 
+	unlink(buf);
 	free(path);
 	free(dom_path);
 	return 0;
-- 
1.7.4.4