I'm tired of seeing this done incorrectly and non-portably, so add a

flopen(3) function which reliably opens and locks a file.

MFC after:	3 weeks

4 files changed, 177 insertions, 6 deletions

diff --git a/lib/libutil/Makefile b/lib/libutil/Makefile
index 9ce21c0..ce64153 100644
--- a/lib/libutil/Makefile
+++ b/lib/libutil/Makefile
@@ -8,11 +8,11 @@ SHLIBDIR?= /lib
 LIB=	util
 SHLIB_MAJOR= 6
 
-SRCS=	_secure_path.c auth.c fparseln.c humanize_number.c kld.c login.c \
-	login_auth.c login_cap.c login_class.c login_crypt.c login_ok.c \
-	login_times.c login_tty.c logout.c logwtmp.c \
-	pidfile.c property.c pty.c pw_util.c realhostname.c stub.c \
-	trimdomain.c uucplock.c
+SRCS=	_secure_path.c auth.c flopen.c fparseln.c humanize_number.c \
+	kld.c login.c login_auth.c login_cap.c login_class.c \
+	login_crypt.c login_ok.c login_times.c login_tty.c logout.c \
+	logwtmp.c pidfile.c property.c pty.c pw_util.c realhostname.c \
+	stub.c trimdomain.c uucplock.c
 INCS=	libutil.h login_cap.h
 
 CFLAGS+= -DLIBC_SCCS
@@ -27,7 +27,7 @@ MAN+=	kld.3 login.3 login_auth.3 login_tty.3 logout.3 logwtmp.3 pty.3 \
 	login_cap.3 login_class.3 login_times.3 login_ok.3 \
 	_secure_path.3 uucplock.3 property.3 auth.3 realhostname.3 \
 	realhostname_sa.3 trimdomain.3 fparseln.3 humanize_number.3 \
-	pidfile.3
+	pidfile.3 flopen.3
 MAN+=	login.conf.5 auth.conf.5
 MLINKS+= kld.3 kld_isloaded.3 kld.3 kld_load.3
 MLINKS+= property.3 properties_read.3  property.3 properties_free.3
diff --git a/lib/libutil/flopen.3 b/lib/libutil/flopen.3
new file mode 100644
index 0000000..53a2132
--- /dev/null
+++ b/lib/libutil/flopen.3
@@ -0,0 +1,79 @@
+.\"-
+.\" Copyright (c) 2007 Dag-Erling Coïdan Smørgrav
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd May 10, 2007
+.Dt FLOPEN 3
+.Os
+.Sh NAME
+.Nm flopen
+.Nd "Reliably open and lock a file"
+.Sh LIBRARY
+.Lb libutil
+.Sh SYNOPSIS
+.In sys/fcntl.h
+.In libutil.h
+.Ft int
+.Fn flopen "const char *path" "int flags"
+.Ft int
+.Fn flopen "const char *path" "int flags" "mode_t mode"
+.Sh DESCRIPTION
+The
+.Fn flopen
+function opens or creates a file and acquires an exclusive lock on it.
+It is essentially equivalent with calling
+.Fn open
+with the same parameters followed by
+.Fn flock
+with an
+.Va operation
+argument of
+.Dv LOCK_EX ,
+except that
+.Fn flopen
+will attempt to detect and handle races that may occur between opening
+/ creating the file and locking it.
+Thus, it is well suited for opening lock files, PID files, spool
+files, mailboxes and other kinds of files which are used for
+synchronization between processes.
+.Pp
+As with
+.Fn flopen ,
+the additional
+.Va mode
+argument is required if
+.Va flags
+includes
+.Dv O_CREAT .
+.Sh SEE ALSO
+.Xr flock 2 ,
+.Xr open 2
+.Sh AUTHORS
+.An -nosplit
+The
+.Nm
+function and this manual page were written by
+.An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org .
diff --git a/lib/libutil/flopen.c b/lib/libutil/flopen.c
new file mode 100644
index 0000000..66d4e47
--- /dev/null
+++ b/lib/libutil/flopen.c
@@ -0,0 +1,91 @@
+/*-
+ * Copyright (c) 2007 Dag-Erling Coïdan Smørgrav
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer
+ *    in this position and unchanged.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $Id$
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/stat.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdarg.h>
+
+int
+flopen(const char *path, int flags, ...)
+{
+	struct stat sb, fsb;
+	mode_t mode;
+	int fd, serrno;
+
+#ifdef O_EXLOCK
+	flags &= ~O_EXLOCK;
+#endif
+
+	if (flags & O_CREAT) {
+		va_list ap;
+
+		va_start(ap, flags);
+		mode = va_arg(ap, int); /* mode_t promoted to int */
+		va_end(ap);
+	} else {
+		mode = 0;
+	}
+
+	for (;;) {
+		if ((fd = open(path, flags, mode)) == -1)
+			/* non-existent or no access */
+			return (-1);
+		if (flock(fd, LOCK_EX) == -1) {
+			/* unsupported or interrupted */
+			serrno = errno;
+			close(fd);
+			errno = serrno;
+			return (-1);
+		}
+		if (stat(path, &sb) == -1) {
+			/* disappeared from under our feet */
+			close(fd);
+			continue;
+		}
+		if (fstat(fd, &fsb) == -1) {
+			/* can't happen [tm] */
+			serrno = errno;
+			close(fd);
+			errno = serrno;
+			return (-1);
+		}
+		if (sb.st_dev != fsb.st_dev ||
+		    sb.st_ino != fsb.st_ino) {
+			/* changed under our feet */
+			close(fd);
+			continue;
+		}
+		return (fd);
+	}
+}
diff --git a/lib/libutil/libutil.h b/lib/libutil/libutil.h
index aefff0c..4a89856 100644
--- a/lib/libutil/libutil.h
+++ b/lib/libutil/libutil.h
@@ -70,6 +70,7 @@ void	clean_environment(const char * const *_white,
 	    const char * const *_more_white);
 int	extattr_namespace_to_string(int _attrnamespace, char **_string);
 int	extattr_string_to_namespace(const char *_string, int *_attrnamespace);
+int	flopen(const char *_path, int _flags, mode_t _mode);
 void	login(struct utmp *_ut);
 int	login_tty(int _fd);
 int	logout(const char *_line);