>Number: 170775
>Category: bin
>Synopsis: lockf(1) should support a flag to not create a lock file if
>one does not exist
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: update
>Submitter-Id: current-users
>Arrival-Date: Mon Aug 20 00:00:23 UTC 2012
>Closed-Date:
>Last-Modified:
>Originator: Matthew Story
>Release: 10.0-CURRENT
>Organization:
>Environment:
>Description:
I sometimes want to scan a directory and dispatch a job per file in the
directory. In such cases I typically run a bunch of scanning/executing jobs on
the directory and use lock(1) to control concurrent and potentially competing
attempts to scan, think something like:
jot - 1 10 | while read line; do find work/ -depth 1 -print0 | xargs -0 -n1 -I
{} lockf -k -t0 {} sh -c '
# do something
mv "$1" done/' worker {} &
done
In cases where I want to lock on a file that I know exists, do some work and
move on, there is a small race-condition were a secondary job to attempt to
lock on a file that had been moved.
In other cases, it would be nice to use mv to short-circuit a coordinated lockf
(with -k) effort:
lockf -k foo/bar sh -c 'something || rm foo/bar' &
lockf -k foo/bar sh -c 'second thing || rm foo/bar' &
To this end, I propose the attached `-n' flag, which augments the behavior of
lockf(1) by removing the O_CREAT flag at open(2) time: making the above
jot - 1 10 | while read line; do find work/ -depth 1 -print0 | xargs -0 -n1 -I
{} lockf -kn -t0 {} sh -c '
# do something
mv "$1" done/' worker {} &
done
and
lockf -k foo/bar sh -c 'something || rm foo/bar' &
lockf -kn foo/bar sh -c 'second thing || rm foo/bar'
The patch causes exit due to nonexistence to exit with EX_UNAVAILABLE if -n is
specified and the lock file does not exist.
The patch does not distinguish between failure due to missing intermediate
directory and missing lock-file, although lockf could probably be cleaned up to
use open + fopenat to cut down on stats for retried locks, which would also
allow -n to distinguish between the two failure cases trivially.
The patch does not cause failure due to missing lock file to have it's
diagnostic information suppressed by the -s (silent) flag, although perhaps it
should (I was unsure about which way to go on this one).
>How-To-Repeat:
>Fix:
patch attached, also available via HTTP:
http://axe0.blackskyresearch.net/patches/matt/freebsd.nocreate_lockf.patch.txt
Patch attached with submission follows:
Index: usr.bin/lockf/lockf.c
===================================================================
--- usr.bin/lockf/lockf.c (revision 239407)
+++ usr.bin/lockf/lockf.c (working copy)
@@ -56,12 +56,13 @@
int
main(int argc, char **argv)
{
- int ch, silent, status, waitsec;
+ int ch, flags, silent, status, waitsec;
pid_t child;
silent = keep = 0;
+ flags = O_CREAT;
waitsec = -1; /* Infinite. */
- while ((ch = getopt(argc, argv, "skt:")) != -1) {
+ while ((ch = getopt(argc, argv, "sknt:")) != -1) {
switch (ch) {
case 'k':
keep = 1;
@@ -69,6 +70,9 @@
case 's':
silent = 1;
break;
+ case 'n':
+ flags &= ~O_CREAT;
+ break;
case 't':
{
char *endptr;
@@ -118,13 +122,13 @@
* avoiding the separate step of waiting for the lock. This
* yields fairness and improved performance.
*/
- lockfd = acquire_lock(lockname, O_NONBLOCK);
+ lockfd = acquire_lock(lockname, flags|O_NONBLOCK);
while (lockfd == -1 && !timed_out && waitsec != 0) {
- if (keep)
- lockfd = acquire_lock(lockname, 0);
+ if (keep)
+ lockfd = acquire_lock(lockname, flags);
else {
wait_for_lock(lockname);
- lockfd = acquire_lock(lockname, O_NONBLOCK);
+ lockfd = acquire_lock(lockname, flags|O_NONBLOCK);
}
}
if (waitsec > 0)
@@ -165,9 +169,12 @@
{
int fd;
- if ((fd = open(name, O_RDONLY|O_CREAT|O_EXLOCK|flags, 0666)) == -1) {
+ if ((fd = open(name, O_RDONLY|O_EXLOCK|flags, 0666)) == -1) {
if (errno == EAGAIN || errno == EINTR)
return (-1);
+ else if (errno == ENOENT && (flags&O_CREAT) == 0)
+ err(EX_UNAVAILABLE, "%s", name);
+
err(EX_CANTCREAT, "cannot open %s", name);
}
return (fd);
@@ -215,7 +222,7 @@
{
fprintf(stderr,
- "usage: lockf [-ks] [-t seconds] file command [arguments]\n");
+ "usage: lockf [-ksn] [-t seconds] file command [arguments]\n");
exit(EX_USAGE);
}
Index: usr.bin/lockf/lockf.1
===================================================================
--- usr.bin/lockf/lockf.1 (revision 239407)
+++ usr.bin/lockf/lockf.1 (working copy)
@@ -90,6 +90,18 @@
.Nm
to operate silently.
Failure to acquire the lock is indicated only in the exit status.
+.It Fl n
+Causes
+.Nm
+to fail if the specified lock
+.Ar file
+does not exist. If
+.Fl n
+is not specified,
+.Nm
+will create
+.Ar file
+if necessary.
.It Fl t Ar seconds
Specifies a timeout for waiting for the lock.
By default,
@@ -130,6 +142,10 @@
utility
was unable to create the lock file, e.g., because of insufficient access
privileges.
+.It Dv EX_UNAVAILABLE
+The
+.Fl n
+option is specified and the specified lock file does not exist.
.It Dv EX_USAGE
There was an error on the
.Nm
>Release-Note:
>Audit-Trail:
>Unformatted:
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