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lock: reopen lockfile after changing mode to make lock effective

lock is acquired on open/create when file has DMEXCL set in mode,
so we need to reopen the file after setting the bit with wstat.

--- a/sys/src/cmd/lock.c

+++ b/sys/src/cmd/lock.c

@@ -41,14 +41,12 @@

 static int

 openlock(char *lock)

 {

-	int lckfd;

+	int lckfd, didwstat = 0;

 	Dir *dir;

-	if (lockwait)

-		while ((lckfd = open(lock, ORDWR)) < 0)

-			sleep(1000);

-	else

-		lckfd = open(lock, ORDWR);

+Reopen:

+	while ((lckfd = open(lock, ORDWR)) < 0 && lockwait)

+		sleep(1000);

 	if (lckfd < 0)

 		sysfatal("can't open %s read/write: %r", lock);

 	dir = dirfstat(lckfd);

@@ -55,10 +53,16 @@

 	if (dir == nil)

 		sysfatal("can't fstat %s: %r", lock);

 	if (!(dir->mode & DMEXCL)) {

+		if(didwstat++)

+			sysfatal("exclusive bit does not stick for %s", lock);

 		dir->mode |= DMEXCL;

 		dir->qid.type |= QTEXCL;

 		if (dirfwstat(lckfd, dir) < 0)

 			sysfatal("can't make %s exclusive access: %r", lock);

+		/* reopen for lock to be effective */

+		free(dir);

+		close(lckfd);

+		goto Reopen;

 	}

 	free(dir);

 	return lckfd;