Initial implementation of flock(1)

Very useful to prevent overlapping cron jobs amongst other things.

Makefile

@@ -97,6 +97,7 @@ BIN =\
 	expr\
 	false\
 	find\
+	flock\
 	fold\
 	getconf\
 	grep\

flock.1

@@ -0,0 +1,29 @@
+.Dd October 7, 2015
+.Dt FLOCK 1
+.Os sbase
+.Sh NAME
+.Nm flock
+.Nd tool to manage locks on files
+.Sh SYNOPSIS
+.Nm
+.Op Fl nsux
+.Ar file
+.Ar cmd Op arg ...
+.Sh DESCRIPTION
+.Nm
+is used to manage advisory locks on open files.  It is commonly used to prevent
+long running cron jobs from running in parallel.  If
+.Ar file
+does not exist, it will be created.
+.Sh OPTIONS
+.Bl -tag -width Ds
+.It Fl n
+Set non-blocking mode on the lock.  Fail immediately if the lock
+cannot be acquired.
+.It Fl s
+Acquire a shared lock.
+.It Fl u
+Release the lock.
+.It Fl x
+Acquire an exclusive lock.  This is the default.
+.El

flock.c

@@ -0,0 +1,75 @@
+/* See LICENSE file for copyright and license details. */
+#include <sys/file.h>
+#include <sys/wait.h>
+
+#include <errno.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: %s [-nsux] file cmd [arg ...]\n", argv0);
+}
+
+int
+main(int argc, char *argv[])
+{
+	int fd, status, savederrno, flags = LOCK_EX, nonblk = 0;
+	pid_t pid;
+
+	ARGBEGIN {
+	case 'n':
+		nonblk = LOCK_NB;
+		break;
+	case 's':
+		flags = LOCK_SH;
+		break;
+	case 'u':
+		flags = LOCK_UN;
+		break;
+	case 'x':
+		/* for compat */
+		break;
+	default:
+		usage();
+	} ARGEND;
+
+	if (argc < 2)
+		usage();
+
+	if ((fd = open(*argv, O_RDONLY | O_CREAT)) < 0)
+		eprintf("open %s:", *argv);
+
+	if (flock(fd, flags | nonblk)) {
+		if (nonblk && errno == EWOULDBLOCK)
+			return 1;
+		eprintf("flock:");
+	}
+
+	switch ((pid = fork())) {
+	case -1:
+		eprintf("fork:");
+	case 0:
+		argv++;
+		execvp(*argv, argv);
+		savederrno = errno;
+		weprintf("execvp %s:", *argv);
+		_exit(126 + (savederrno == ENOENT));
+	default:
+		break;
+	}
+	waitpid(pid, &status, 0);
+
+        if (WIFSIGNALED(status))
+                return 128 + WTERMSIG(status);
+        if (WIFEXITED(status))
+		return WEXITSTATUS(status);
+
+	if (close(fd) < 0)
+		eprintf("close:");
+
+	return 0;
+}