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Total rewrite.

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This commit is contained in:
Richard M. Stallman 1996-12-31 22:11:05 +00:00
parent 570591721b
commit 8dbbc384da
1 changed files with 256 additions and 296 deletions
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552
src/filelock.c
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@ -1,4 +1,4 @@
/* Copyright (C) 1985, 1986, 1987, 1993, 1994 Free Software Foundation, Inc.
/* Copyright (C) 1985, 86, 87, 93, 94, 96 Free Software Foundation, Inc.
This file is part of GNU Emacs.
@ -26,189 +26,297 @@ Boston, MA 02111-1307, USA. */
#include "vms-pwd.h"
#else
#include <pwd.h>
#endif
#endif /* not VMS */
#include <errno.h>
#include <sys/file.h>
#ifdef USG
#include <fcntl.h>
#include <string.h>
#endif /* USG */
#include "lisp.h"
#include <paths.h>
#include "buffer.h"
#ifdef SYSV_SYSTEM_DIR
#include <dirent.h>
#else /* not SYSV_SYSTEM_DIR */
#ifdef NONSYSTEM_DIR_LIBRARY
#include "ndir.h"
#else /* not NONSYSTEM_DIR_LIBRARY */
#ifdef MSDOS
#include <dirent.h>
#else
#include <sys/dir.h>
#endif
#endif /* not NONSYSTEM_DIR_LIBRARY */
#ifndef MSDOS
extern DIR *opendir ();
#endif /* not MSDOS */
#endif /* not SYSV_SYSTEM_DIR */
#include <errno.h>
#ifndef errno
extern int errno;
extern char *egetenv ();
extern char *strcpy ();
#ifdef DECLARE_GETPWUID_WITH_UID_T
extern struct passwd *getpwuid (uid_t);
#else
extern struct passwd *getpwuid ();
#endif
#ifdef CLASH_DETECTION
/* If system does not have symbolic links, it does not have lstat.
In that case, use ordinary stat instead. */
/* The strategy: to lock a file FN, create a symlink .#FN in FN's
directory, with link data `user@host.pid'. This avoids a single
mount (== failure) point for lock files.
#ifndef S_IFLNK
#define lstat stat
#endif
When the host in the lock data is the current host, we can check if
the pid is valid with kill.
Otherwise, we could look at a separate file that maps hostnames to
reboot times to see if the remote pid can possibly be valid, since we
don't want Emacs to have to communicate via pipes or sockets or
whatever to other processes, either locally or remotely; rms says
that's too unreliable. Hence the separate file, which could
theoretically be updated by daemons running separately -- but this
whole idea is unimplemented; in practice, at least in our
environment, it seems such stale locks arise fiarly infrequently, and
Emacs' standard methods of dealing with clashes suffice.
We use symlinks instead of normal files because (1) they can be
stored more efficiently on the filesystem, since the kernel knows
they will be small, and (2) all the info about the lock can be read
in a single system call (readlink). Although we could use regular
files to be useful on old systems lacking symlinks, noawdays
virtually all such systems are probably single-user anyway, so it
didn't seem worth the complication.
Similarly, we don't worry about a possible 14-character limit on
file names, because those are all the same systems that don't have
symlinks.
This is compatible with the locking scheme used by Interleaf (which
has contributed this implementation for Emacs), and was designed by
Ethan Jacobson, Kimbo Mundy, and others.
--karl@cs.umb.edu/karl@hq.ileaf.com. */
/* The name of the directory in which we keep lock files, with a '/'
appended. */
char *lock_dir;
/* Here is the structure that stores information about a lock. */
/* The name of the file in the lock directory which is used to
arbitrate access to the entire directory. */
#define SUPERLOCK_NAME "!!!SuperLock!!!"
/* The name of the superlock file. This is SUPERLOCK_NAME appended to
lock_dir. */
char *superlock_file;
/* Set LOCK to the name of the lock file for the filename FILE.
char *LOCK; Lisp_Object FILE; */
#ifndef HAVE_LONG_FILE_NAMES
#define MAKE_LOCK_NAME(lock, file) \
(lock = (char *) alloca (14 + strlen (lock_dir) + 1), \
fill_in_lock_short_file_name (lock, (file)))
fill_in_lock_short_file_name (lockfile, fn)
register char *lockfile;
register Lisp_Object fn;
typedef struct
{
register union
{
unsigned int word [2];
unsigned char byte [8];
} crc;
register unsigned char *p, new;
/* 7-bytes cyclic code for burst correction on byte-by-byte basis.
the used polynomial is D^7 + D^6 + D^3 +1. pot@cnuce.cnr.it */
char *user;
char *host;
int pid;
} lock_info_type;
crc.word[0] = crc.word[1] = 0;
/* When we read the info back, we might need this much more. */
#define LOCK_PID_MAX 21 /* enough for signed 64 bits plus null */
for (p = XSTRING (fn)->data; new = *p++; )
{
new += crc.byte[6];
crc.byte[6] = crc.byte[5] + new;
crc.byte[5] = crc.byte[4];
crc.byte[4] = crc.byte[3];
crc.byte[3] = crc.byte[2] + new;
crc.byte[2] = crc.byte[1];
crc.byte[1] = crc.byte[0];
crc.byte[0] = new;
}
sprintf (lockfile, "%s%.2x%.2x%.2x%.2x%.2x%.2x%.2x", lock_dir,
crc.byte[0], crc.byte[1], crc.byte[2], crc.byte[3],
crc.byte[4], crc.byte[5], crc.byte[6]);
}
/* Free the two dynamically-allocated pieces in PTR. */
#define FREE_LOCK_INFO(i) do { xfree ((i).user); xfree ((i).host); } while (0)
#else /* defined HAVE_LONG_FILE_NAMES */
/* Write the name of the lock file for FN into LFNAME. Length will be
that of FN plus two more for the leading `.#' plus one for the null. */
#define MAKE_LOCK_NAME(lock, file) \
(lock = (char *) alloca (XSTRING (file)->size + strlen (lock_dir) + 1), \
(lock = (char *) alloca (XSTRING (file)->size + 2 + 1), \
fill_in_lock_file_name (lock, (file)))
static void
fill_in_lock_file_name (lockfile, fn)
register char *lockfile;
register Lisp_Object fn;
{
register char *p;
strcpy (lockfile, lock_dir);
strcpy (lockfile, XSTRING (fn)->data);
p = lockfile + strlen (lockfile);
strcpy (p, XSTRING (fn)->data);
for (; *p; p++)
{
if (*p == '/')
*p = '!';
}
/* Shift the nondirectory part of the file name (including the null)
right two characters. Here is one of the places where we'd have to
do something to support 14-character-max file names. */
for (p = lockfile + strlen (lockfile); p != lockfile && *p != '/'; p--)
p[2] = *p;
/* Insert the `.#'. */
p[1] = '.';
p[2] = '#';
}
#endif /* !defined HAVE_LONG_FILE_NAMES */
static Lisp_Object
lock_file_owner_name (lfname)
/* Lock the lock file named LFNAME.
If FORCE is nonzero, we do so even if it is already locked.
Return 1 if successful, 0 if not. */
static int
lock_file_1 (lfname, force)
char *lfname;
int force;
{
register int err;
char *user_name = XSTRING (Fuser_login_name (Qnil))->data;
char *host_name = XSTRING (Fsystem_name ())->data;
char *lock_info_str = alloca (strlen (user_name) + strlen (host_name) + 21);
sprintf (lock_info_str, "%s@%s.%d", user_name, host_name, getpid ());
err = symlink (lock_info_str, lfname);
if (errno == EEXIST && force)
{
unlink (lfname);
err = symlink (lock_info_str, lfname);
}
return err == 0;
}
/* Return 0 if nobody owns the lock file LFNAME or the lock is obsolete,
1 if another process owns it (and set OWNER (if non-null) to info),
2 if the current process owns it,
or -1 if something is wrong with the locking mechanism. */
static int
current_lock_owner (owner, lfname)
lock_info_type *owner;
char *lfname;
{
struct stat s;
struct passwd *the_pw;
#ifndef index
extern char *rindex (), *index ();
#endif
int o, p, len, ret;
int local_owner = 0;
char *at, *dot;
char *lfinfo = 0;
int bufsize = 50;
/* Read arbitrarily-long contents of symlink. Similar code in
file-symlink-p in fileio.c. */
do
{
bufsize *= 2;
lfinfo = (char *) xrealloc (lfinfo, bufsize);
len = readlink (lfname, lfinfo, bufsize);
}
while (len >= bufsize);
/* If nonexistent lock file, all is well; otherwise, got strange error. */
if (len == -1)
{
xfree (lfinfo);
return errno == ENOENT ? 0 : -1;
}
if (lstat (lfname, &s) == 0)
the_pw = getpwuid (s.st_uid);
/* Link info exists, so `len' is its length. Null terminate. */
lfinfo[len] = 0;
/* Even if the caller doesn't want the owner info, we still have to
read it to determine return value, so allocate it. */
if (!owner)
{
owner = alloca (sizeof (lock_info_type));
local_owner = 1;
}
/* Parse USER@HOST.PID. If can't parse, return -1. */
/* The USER is everything before the first @. */
at = index (lfinfo, '@');
dot = rindex (lfinfo, '.');
if (!at || !dot) {
xfree (lfinfo);
return -1;
}
len = at - lfinfo;
owner->user = (char *) xmalloc (len + 1);
strncpy (owner->user, lfinfo, len);
owner->user[len] = 0;
/* The PID is everything after the last `.'. */
owner->pid = atoi (dot + 1);
/* The host is everything in between. */
len = dot - at - 1;
owner->host = (char *) xmalloc (len + 1);
strncpy (owner->host, at + 1, len);
owner->host[len] = 0;
/* We're done looking at the link info. */
xfree (lfinfo);
/* On current host? */
if (strcmp (owner->host, XSTRING (Fsystem_name ())->data) == 0)
{
if (owner->pid == getpid ())
ret = 2; /* We own it. */
if (owner->pid > 0
&& (kill (owner->pid, 0) >= 0 || errno == EPERM))
ret = 1; /* An existing process on this machine owns it. */
/* The owner process is dead or has a strange pid (<=0), so try to
zap the lockfile. */
if (unlink (lfname) < 0)
ret = -1;
ret = 0;
}
else
the_pw = 0;
return (the_pw == 0 ? Qnil : build_string (the_pw->pw_name));
{ /* If we wanted to support the check for stale locks on remote machines,
here's where we'd do it. */
ret = 1;
}
/* Avoid garbage. */
if (local_owner || ret <= 0)
{
FREE_LOCK_INFO (*owner);
}
return ret;
}
/* Lock the lock named LFNAME if possible.
Return 0 in that case.
Return positive if some other process owns the lock, and info about
that process in CLASHER.
Return -1 if cannot lock for any other reason. */
/* lock_file locks file fn,
static int
lock_if_free (clasher, lfname)
lock_info_type *clasher;
register char *lfname;
{
while (lock_file_1 (lfname, 0) == 0)
{
int locker;
if (errno != EEXIST)
return -1;
locker = current_lock_owner (clasher, lfname);
if (locker == 2)
{
FREE_LOCK_INFO (*clasher);
return 0; /* We ourselves locked it. */
}
else if (locker == 1)
return 1; /* Someone else has it. */
else if (locker == -1)
return -1; /* Something's wrong. */
/* If some other error, or no such lock, try to lock again. */
/* Is there a case where we loop forever? */
}
return 0;
}
/* lock_file locks file FN,
meaning it serves notice on the world that you intend to edit that file.
This should be done only when about to modify a file-visiting
buffer previously unmodified.
Do not (normally) call lock_buffer for a buffer already modified,
Do not (normally) call this for a buffer already modified,
as either the file is already locked, or the user has already
decided to go ahead without locking.
When lock_buffer returns, either the lock is locked for us,
When this returns, either the lock is locked for us,
or the user has said to go ahead without locking.
If the file is locked by someone else, lock_buffer calls
If the file is locked by someone else, this calls
ask-user-about-lock (a Lisp function) with two arguments,
the file name and the name of the user who did the locking.
the file name and info about the user who did the locking.
This function can signal an error, or return t meaning
take away the lock, or return nil meaning ignore the lock. */
/* The lock file name is the file name with "/" replaced by "!"
and put in the Emacs lock directory. */
/* (ie., /ka/king/junk.tex -> /!/!ka!king!junk.tex). */
/* If HAVE_LONG_FILE_NAMES is not defined, the lock file name is the hex
representation of a 14-bytes CRC generated from the file name
and put in the Emacs lock directory (not very nice, but it works).
(ie., /ka/king/junk.tex -> /!/12a82c62f1c6da). */
void
lock_file (fn)
register Lisp_Object fn;
register Lisp_Object fn;
{
register Lisp_Object attack, orig_fn;
register char *lfname;
register char *lfname, *locker;
lock_info_type lock_info;
orig_fn = fn;
fn = Fexpand_file_name (fn, Qnil);
/* Create the name of the lock-file for file fn */
MAKE_LOCK_NAME (lfname, fn);
/* See if this file is visited and has changed on disk since it was
@ -223,113 +331,27 @@ lock_file (fn)
}
/* Try to lock the lock. */
if (lock_if_free (lfname) <= 0)
/* Return now if we have locked it, or if lock dir does not exist */
if (lock_if_free (&lock_info, lfname) <= 0)
/* Return now if we have locked it, or if lock creation failed */
return;
/* Else consider breaking the lock */
attack = call2 (intern ("ask-user-about-lock"), fn,
lock_file_owner_name (lfname));
locker = alloca (strlen (lock_info.user) + strlen (lock_info.host)
+ LOCK_PID_MAX + 9);
sprintf (locker, "%s@%s (pid %d)", lock_info.user, lock_info.host,
lock_info.pid);
FREE_LOCK_INFO (lock_info);
attack = call2 (intern ("ask-user-about-lock"), fn, build_string (locker));
if (!NILP (attack))
/* User says take the lock */
{
lock_superlock (lfname);
lock_file_1 (lfname, O_WRONLY) ;
unlink (superlock_file);
lock_file_1 (lfname, 1);
return;
}
/* User says ignore the lock */
}
/* Lock the lock file named LFNAME.
If MODE is O_WRONLY, we do so even if it is already locked.
If MODE is O_WRONLY | O_EXCL | O_CREAT, we do so only if it is free.
Return 1 if successful, 0 if not. */
int
lock_file_1 (lfname, mode)
int mode; char *lfname;
{
register int fd;
char buf[20];
if ((fd = open (lfname, mode, 0666)) >= 0)
{
#ifdef USG
chmod (lfname, 0666);
#else
fchmod (fd, 0666);
#endif
sprintf (buf, "%d ", getpid ());
write (fd, buf, strlen (buf));
close (fd);
return 1;
}
else
return 0;
}
/* Lock the lock named LFNAME if possible.
Return 0 in that case.
Return positive if lock is really locked by someone else.
Return -1 if cannot lock for any other reason. */
int
lock_if_free (lfname)
register char *lfname;
{
register int clasher;
while (lock_file_1 (lfname, O_WRONLY | O_EXCL | O_CREAT) == 0)
{
if (errno != EEXIST)
return -1;
clasher = current_lock_owner (lfname);
if (clasher != 0)
if (clasher != getpid ())
return (clasher);
else return (0);
/* Try again to lock it */
}
return 0;
}
/* Return the pid of the process that claims to own the lock file LFNAME,
or 0 if nobody does or the lock is obsolete,
or -1 if something is wrong with the locking mechanism. */
int
current_lock_owner (lfname)
char *lfname;
{
int owner = current_lock_owner_1 (lfname);
if (owner == 0 && errno == ENOENT)
return (0);
/* Is it locked by a process that exists? */
if (owner != 0 && (kill (owner, 0) >= 0 || errno == EPERM))
return (owner);
if (unlink (lfname) < 0)
return (-1);
return (0);
}
int
current_lock_owner_1 (lfname)
char *lfname;
{
register int fd;
char buf[20];
int tem;
fd = open (lfname, O_RDONLY, 0666);
if (fd < 0)
return 0;
tem = read (fd, buf, sizeof buf);
close (fd);
return (tem <= 0 ? 0 : atoi (buf));
}
void
unlock_file (fn)
register Lisp_Object fn;
@ -340,77 +362,8 @@ unlock_file (fn)
MAKE_LOCK_NAME (lfname, fn);
lock_superlock (lfname);
if (current_lock_owner_1 (lfname) == getpid ())
if (current_lock_owner (0, lfname) == 2)
unlink (lfname);
unlink (superlock_file);
}
lock_superlock (lfname)
char *lfname;
{
register int i, fd;
DIR *lockdir;
struct stat first_stat, last_stat;
for (i = -20; i < 0;
i++)
{
fd = open (superlock_file,
O_WRONLY | O_EXCL | O_CREAT, 0666);
/* If we succeeded in creating the superlock, we win.
Fill in our info and return. */
if (fd >= 0)
{
#ifdef USG
chmod (superlock_file, 0666);
#else
fchmod (fd, 0666);
#endif
write (fd, lfname, strlen (lfname));
close (fd);
return;
}
/* If the problem is not just that it is already locked,
give up. */
if (errno != EEXIST)
return;
message ("Superlock file exists, retrying...");
if (i == -20)
stat (superlock_file, &first_stat);
if (i == -1)
stat (superlock_file, &last_stat);
/* This seems to be necessary to prevent Emacs from hanging when the
competing process has already deleted the superlock, but it's still
in the NFS cache. So we force NFS to synchronize the cache. */
lockdir = opendir (lock_dir);
if (lockdir)
closedir (lockdir);
sleep (1);
}
if (first_stat.st_ctime == last_stat.st_ctime)
{
int value;
value = unlink (superlock_file);
if (value != -1)
message ("Superlock file deleted");
else
message ("Failed to delete superlock file");
}
else
message ("Giving up on the superlock file");
}
void
@ -426,7 +379,6 @@ unlock_all_files ()
unlock_file (b->file_truename);
}
}
DEFUN ("lock-buffer", Flock_buffer, Slock_buffer,
0, 1, 0,
@ -458,7 +410,6 @@ if it should normally be locked.")
return Qnil;
}
/* Unlock the file visited in buffer BUFFER. */
unlock_buffer (buffer)
@ -475,20 +426,27 @@ t if it is locked by you, else a string of the name of the locker.")
(filename)
Lisp_Object filename;
{
Lisp_Object ret;
register char *lfname;
int owner;
lock_info_type locker;
filename = Fexpand_file_name (filename, Qnil);
MAKE_LOCK_NAME (lfname, filename);
owner = current_lock_owner (lfname);
owner = current_lock_owner (&locker, lfname);
if (owner <= 0)
return (Qnil);
else if (owner == getpid ())
return (Qt);
return (lock_file_owner_name (lfname));
ret = Qnil;
else if (owner == 2)
ret = Qt;
else
ret = build_string (locker.user);
if (owner > 0)
FREE_LOCK_INFO (locker);
return ret;
}
@ -496,6 +454,7 @@ t if it is locked by you, else a string of the name of the locker.")
init_filelock ()
{
#if 0
char *new_name;
lock_dir = egetenv ("EMACSLOCKDIR");
@ -515,6 +474,7 @@ init_filelock ()
+ sizeof (SUPERLOCK_NAME)));
strcpy (superlock_file, lock_dir);
strcat (superlock_file, SUPERLOCK_NAME);
#endif
}
syms_of_filelock ()