X-Git-Url: https://git.hcoop.net/bpt/emacs.git/blobdiff_plain/28b6b84d4e91cf39c85f194c29b8e044ae6bac87..aa30fa6f3a4273adcfa1cba183a74a5aeb47b463:/src/filelock.c
diff --git a/src/filelock.c b/src/filelock.c
index 14b9d4aaca..df72eff595 100644
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -38,6 +38,8 @@ along with GNU Emacs. If not, see . */
#include
+#include
+
#include "lisp.h"
#include "character.h"
#include "buffer.h"
@@ -45,6 +47,7 @@ along with GNU Emacs. If not, see . */
#include "systime.h"
#ifdef WINDOWSNT
#include
+#include /* for fcntl */
#include "w32.h" /* for dostounix_filename */
#endif
@@ -64,7 +67,7 @@ along with GNU Emacs. If not, see . */
#define WTMP_FILE "/var/log/wtmp"
#endif
-/* On non-MS-Windows systems, use a symbolic link to represent a lock.
+/* Normally use a symbolic link to represent a lock.
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.
@@ -100,10 +103,21 @@ along with GNU Emacs. If not, see . */
--karl@cs.umb.edu/karl@hq.ileaf.com.
- On MS-Windows, symbolic links do not work well, so instead of a
- symlink .#FN -> 'user@host.pid', the lock is a regular file .#-FN
- with contents 'user@host.pid'. MS-Windows and non-MS-Windows
- versions of Emacs ignore each other's locks. */
+ On some file systems, notably those of MS-Windows, symbolic links
+ do not work well, so instead of a symlink .#FN -> 'user@host.pid',
+ the lock is a regular file .#FN with contents 'user@host.pid'. To
+ establish a lock, a nonce file is created and then renamed to .#FN.
+ On MS-Windows this renaming is atomic unless the lock is forcibly
+ acquired. On other systems the renaming is atomic if the lock is
+ forcibly acquired; if not, the renaming is done via hard links,
+ which is good enough for lock-file purposes.
+
+ To summarize, race conditions can occur with either:
+
+ * Forced locks on MS-Windows systems.
+
+ * Non-forced locks on non-MS-Windows systems that support neither
+ hard nor symbolic links. */
/* Return the time of the last system boot. */
@@ -167,7 +181,7 @@ get_boot_time (void)
since utmp is typically much smaller than wtmp.
Passing a null pointer causes get_boot_time_1
to inspect the default file, namely utmp. */
- get_boot_time_1 ((char *) 0, 0);
+ get_boot_time_1 (0, 0);
if (boot_time)
return boot_time;
@@ -243,18 +257,14 @@ void
get_boot_time_1 (const char *filename, bool newest)
{
struct utmp ut, *utp;
- int desc;
if (filename)
{
/* On some versions of IRIX, opening a nonexistent file name
is likely to crash in the utmp routines. */
- desc = emacs_open (filename, O_RDONLY, 0);
- if (desc < 0)
+ if (faccessat (AT_FDCWD, filename, R_OK, AT_EACCESS) != 0)
return;
- emacs_close (desc);
-
utmpname (filename);
}
@@ -284,30 +294,31 @@ get_boot_time_1 (const char *filename, bool newest)
}
#endif /* BOOT_TIME */
+/* An arbitrary limit on lock contents length. 8 K should be plenty
+ big enough in practice. */
+enum { MAX_LFINFO = 8 * 1024 };
+
/* Here is the structure that stores information about a lock. */
typedef struct
{
- char *user;
- char *host;
- pid_t pid;
- time_t boot_time;
-} lock_info_type;
-
-/* Free the two dynamically-allocated pieces in PTR. */
-#define FREE_LOCK_INFO(i) do { xfree ((i).user); xfree ((i).host); } while (0)
+ /* Location of '@', '.', ':' in USER. If there's no colon, COLON
+ points to the end of USER. */
+ char *at, *dot, *colon;
-#ifdef WINDOWSNT
-enum { defined_WINDOWSNT = 1 };
-#else
-enum { defined_WINDOWSNT = 0 };
-#endif
+ /* Lock file contents USER@HOST.PID with an optional :BOOT_TIME
+ appended. This memory is used as a lock file contents buffer, so
+ it needs room for MAX_LFINFO + 1 bytes. A string " (pid NNNN)"
+ may be appended to the USER@HOST while generating a diagnostic,
+ so make room for its extra bytes (as opposed to ".NNNN") too. */
+ char user[MAX_LFINFO + 1 + sizeof " (pid )" - sizeof "."];
+} lock_info_type;
/* Write the name of the lock file for FNAME into LOCKNAME. Length
- will be that of FNAME plus two more for the leading ".#",
- plus one for "-" if MS-Windows, plus one for the null. */
+ will be that of FNAME plus two more for the leading ".#", plus one
+ for the null. */
#define MAKE_LOCK_NAME(lockname, fname) \
- (lockname = SAFE_ALLOCA (SBYTES (fname) + 2 + defined_WINDOWSNT + 1), \
+ (lockname = SAFE_ALLOCA (SBYTES (fname) + 2 + 1), \
fill_in_lock_file_name (lockname, fname))
static void
@@ -319,70 +330,125 @@ fill_in_lock_file_name (char *lockfile, Lisp_Object fn)
memcpy (lockfile, SSDATA (fn), dirlen);
lockfile[dirlen] = '.';
lockfile[dirlen + 1] = '#';
- if (defined_WINDOWSNT)
- lockfile[dirlen + 2] = '-';
- strcpy (lockfile + dirlen + 2 + defined_WINDOWSNT, base);
+ strcpy (lockfile + dirlen + 2, base);
}
+/* For some reason Linux kernels return EPERM on file systems that do
+ not support hard or symbolic links. This symbol documents the quirk.
+ There is no way to tell whether a symlink call fails due to
+ permissions issues or because links are not supported, but luckily
+ the lock file code should work either way. */
+enum { LINKS_MIGHT_NOT_WORK = EPERM };
+
+/* Rename OLD to NEW. If FORCE, replace any existing NEW.
+ It is OK if there are temporarily two hard links to OLD.
+ Return 0 if successful, -1 (setting errno) otherwise. */
static int
-create_lock_file (char *lfname, char *lock_info_str, bool force)
+rename_lock_file (char const *old, char const *new, bool force)
{
- int err;
-
#ifdef WINDOWSNT
- /* Symlinks are supported only by latest versions of Windows, and
- creating them is a privileged operation that often triggers UAC
- elevation prompts. Therefore, instead of using symlinks, we
- create a regular file with the lock info written as its
- contents. */
- {
- /* Deny everybody else any kind of access to the file until we are
- done writing it and close the handle. This makes the entire
- open/write/close operation atomic, as far as other WINDOWSNT
- processes are concerned. */
- int fd = _sopen (lfname,
- _O_WRONLY | _O_BINARY | _O_CREAT | _O_EXCL | _O_NOINHERIT,
- _SH_DENYRW, S_IREAD | S_IWRITE);
-
- if (fd < 0 && errno == EEXIST && force)
- fd = _sopen (lfname, _O_WRONLY | _O_BINARY | _O_TRUNC |_O_NOINHERIT,
- _SH_DENYRW, S_IREAD | S_IWRITE);
- if (fd >= 0)
- {
- ssize_t lock_info_len = strlen (lock_info_str);
+ return sys_rename_replace (old, new, force);
+#else
+ if (! force)
+ {
+ struct stat st;
- err = 0;
- if (emacs_write (fd, lock_info_str, lock_info_len) != lock_info_len)
- err = -1;
- if (emacs_close (fd))
- err = -1;
- }
- else
- err = -1;
- }
+ if (link (old, new) == 0)
+ return unlink (old) == 0 || errno == ENOENT ? 0 : -1;
+ if (errno != ENOSYS && errno != LINKS_MIGHT_NOT_WORK)
+ return -1;
+
+ /* 'link' does not work on this file system. This can occur on
+ a GNU/Linux host mounting a FAT32 file system. Fall back on
+ 'rename' after checking that NEW does not exist. There is a
+ potential race condition since some other process may create
+ NEW immediately after the existence check, but it's the best
+ we can portably do here. */
+ if (lstat (new, &st) == 0 || errno == EOVERFLOW)
+ {
+ errno = EEXIST;
+ return -1;
+ }
+ if (errno != ENOENT)
+ return -1;
+ }
+
+ return rename (old, new);
+#endif
+}
+
+/* Create the lock file LFNAME with contents LOCK_INFO_STR. Return 0 if
+ successful, an errno value on failure. If FORCE, remove any
+ existing LFNAME if necessary. */
+
+static int
+create_lock_file (char *lfname, char *lock_info_str, bool force)
+{
+#ifdef WINDOWSNT
+ /* Symlinks are supported only by later versions of Windows, and
+ creating them is a privileged operation that often triggers
+ User Account Control elevation prompts. Avoid the problem by
+ pretending that 'symlink' does not work. */
+ int err = ENOSYS;
#else
- err = symlink (lock_info_str, lfname);
- if (err != 0 && errno == EEXIST && force)
+ int err = symlink (lock_info_str, lfname) == 0 ? 0 : errno;
+#endif
+
+ if (err == EEXIST && force)
{
unlink (lfname);
- err = symlink (lock_info_str, lfname);
+ err = symlink (lock_info_str, lfname) == 0 ? 0 : errno;
+ }
+
+ if (err == ENOSYS || err == LINKS_MIGHT_NOT_WORK || err == ENAMETOOLONG)
+ {
+ static char const nonce_base[] = ".#-emacsXXXXXX";
+ char *last_slash = strrchr (lfname, '/');
+ ptrdiff_t lfdirlen = last_slash + 1 - lfname;
+ USE_SAFE_ALLOCA;
+ char *nonce = SAFE_ALLOCA (lfdirlen + sizeof nonce_base);
+ int fd;
+ memcpy (nonce, lfname, lfdirlen);
+ strcpy (nonce + lfdirlen, nonce_base);
+
+ fd = mkostemp (nonce, O_BINARY | O_CLOEXEC);
+ if (fd < 0)
+ err = errno;
+ else
+ {
+ ptrdiff_t lock_info_len;
+ if (! O_CLOEXEC)
+ fcntl (fd, F_SETFD, FD_CLOEXEC);
+ lock_info_len = strlen (lock_info_str);
+ err = 0;
+ /* Use 'write', not 'emacs_write', as garbage collection
+ might signal an error, which would leak FD. */
+ if (write (fd, lock_info_str, lock_info_len) != lock_info_len
+ || fchmod (fd, S_IRUSR | S_IRGRP | S_IROTH) != 0)
+ err = errno;
+ /* There is no need to call fsync here, as the contents of
+ the lock file need not survive system crashes. */
+ if (emacs_close (fd) != 0)
+ err = errno;
+ if (!err && rename_lock_file (nonce, lfname, force) != 0)
+ err = errno;
+ if (err)
+ unlink (nonce);
+ }
+
+ SAFE_FREE ();
}
-#endif
return err;
}
/* Lock the lock file named LFNAME.
If FORCE, do so even if it is already locked.
- Return true if successful. */
+ Return 0 if successful, an error number on failure. */
-static bool
+static int
lock_file_1 (char *lfname, bool force)
{
- int err;
- int symlink_errno;
- USE_SAFE_ALLOCA;
-
/* Call this first because it can GC. */
printmax_t boot = get_boot_time ();
@@ -390,20 +456,16 @@ lock_file_1 (char *lfname, bool force)
char const *user_name = STRINGP (luser_name) ? SSDATA (luser_name) : "";
Lisp_Object lhost_name = Fsystem_name ();
char const *host_name = STRINGP (lhost_name) ? SSDATA (lhost_name) : "";
- ptrdiff_t lock_info_size = (strlen (user_name) + strlen (host_name)
- + 2 * INT_STRLEN_BOUND (printmax_t)
- + sizeof "@.:");
- char *lock_info_str = SAFE_ALLOCA (lock_info_size);
+ char lock_info_str[MAX_LFINFO + 1];
printmax_t pid = getpid ();
- esprintf (lock_info_str, boot ? "%s@%s.%"pMd":%"pMd : "%s@%s.%"pMd,
- user_name, host_name, pid, boot);
- err = create_lock_file (lfname, lock_info_str, force);
+ if (sizeof lock_info_str
+ <= snprintf (lock_info_str, sizeof lock_info_str,
+ boot ? "%s@%s.%"pMd":%"pMd : "%s@%s.%"pMd,
+ user_name, host_name, pid, boot))
+ return ENAMETOOLONG;
- symlink_errno = errno;
- SAFE_FREE ();
- errno = symlink_errno;
- return err == 0;
+ return create_lock_file (lfname, lock_info_str, force);
}
/* Return true if times A and B are no more than one second apart. */
@@ -414,32 +476,45 @@ within_one_second (time_t a, time_t b)
return (a - b >= -1 && a - b <= 1);
}
-static Lisp_Object
-read_lock_data (char *lfname)
-{
-#ifndef WINDOWSNT
- return emacs_readlinkat (AT_FDCWD, lfname);
-#else
- int fd = emacs_open (lfname, O_RDONLY | O_BINARY, S_IREAD);
- ssize_t nbytes;
- /* 256 chars for user, 1024 chars for host, 10 digits for each of 2 int's. */
- enum { MAX_LFINFO = 256 + 1024 + 10 + 10 + 2 };
- char lfinfo[MAX_LFINFO + 1];
+/* On systems lacking ELOOP, test for an errno value that shouldn't occur. */
+#ifndef ELOOP
+# define ELOOP (-1)
+#endif
- if (fd < 0)
- return Qnil;
+/* Read the data for the lock file LFNAME into LFINFO. Read at most
+ MAX_LFINFO + 1 bytes. Return the number of bytes read, or -1
+ (setting errno) on error. */
- nbytes = emacs_read (fd, lfinfo, MAX_LFINFO);
- emacs_close (fd);
+static ptrdiff_t
+read_lock_data (char *lfname, char lfinfo[MAX_LFINFO + 1])
+{
+ ptrdiff_t nbytes;
- if (nbytes > 0)
+ while ((nbytes = readlinkat (AT_FDCWD, lfname, lfinfo, MAX_LFINFO + 1)) < 0
+ && errno == EINVAL)
{
- lfinfo[nbytes] = '\0';
- return build_string (lfinfo);
+ int fd = emacs_open (lfname, O_RDONLY | O_BINARY | O_NOFOLLOW, 0);
+ if (0 <= fd)
+ {
+ /* Use read, not emacs_read, since FD isn't unwind-protected. */
+ ptrdiff_t read_bytes = read (fd, lfinfo, MAX_LFINFO + 1);
+ int read_errno = errno;
+ if (emacs_close (fd) != 0)
+ return -1;
+ errno = read_errno;
+ return read_bytes;
+ }
+
+ if (errno != ELOOP)
+ return -1;
+
+ /* readlinkat saw a non-symlink, but emacs_open saw a symlink.
+ The former must have been removed and replaced by the latter.
+ Try again. */
+ QUIT;
}
- else
- return Qnil;
-#endif
+
+ return nbytes;
}
/* Return 0 if nobody owns the lock file LFNAME or the lock is obsolete,
@@ -451,83 +526,78 @@ static int
current_lock_owner (lock_info_type *owner, char *lfname)
{
int ret;
- ptrdiff_t len;
lock_info_type local_owner;
- intmax_t n;
- char *at, *dot, *colon;
- Lisp_Object lfinfo_object = read_lock_data (lfname);
- char *lfinfo;
- struct gcpro gcpro1;
-
- /* If nonexistent lock file, all is well; otherwise, got strange error. */
- if (NILP (lfinfo_object))
- return errno == ENOENT ? 0 : -1;
- lfinfo = SSDATA (lfinfo_object);
+ ptrdiff_t lfinfolen;
+ intmax_t pid, boot_time;
+ char *at, *dot, *lfinfo_end;
/* Even if the caller doesn't want the owner info, we still have to
read it to determine return value. */
if (!owner)
owner = &local_owner;
+ /* If nonexistent lock file, all is well; otherwise, got strange error. */
+ lfinfolen = read_lock_data (lfname, owner->user);
+ if (lfinfolen < 0)
+ return errno == ENOENT ? 0 : -1;
+ if (MAX_LFINFO < lfinfolen)
+ return -1;
+ owner->user[lfinfolen] = 0;
+
/* Parse USER@HOST.PID:BOOT_TIME. If can't parse, return -1. */
/* The USER is everything before the last @. */
- at = strrchr (lfinfo, '@');
- dot = strrchr (lfinfo, '.');
- if (!at || !dot)
+ owner->at = at = memrchr (owner->user, '@', lfinfolen);
+ if (!at)
+ return -1;
+ owner->dot = dot = strrchr (at, '.');
+ if (!dot)
return -1;
- len = at - lfinfo;
- GCPRO1 (lfinfo_object);
- owner->user = xmalloc (len + 1);
- memcpy (owner->user, lfinfo, len);
- owner->user[len] = 0;
/* The PID is everything from the last `.' to the `:'. */
+ if (! c_isdigit (dot[1]))
+ return -1;
errno = 0;
- n = strtoimax (dot + 1, NULL, 10);
- owner->pid =
- ((0 <= n && n <= TYPE_MAXIMUM (pid_t)
- && (TYPE_MAXIMUM (pid_t) < INTMAX_MAX || errno != ERANGE))
- ? n : 0);
+ pid = strtoimax (dot + 1, &owner->colon, 10);
+ if (errno == ERANGE)
+ pid = -1;
- colon = strchr (dot + 1, ':');
/* After the `:', if there is one, comes the boot time. */
- n = 0;
- if (colon)
+ switch (owner->colon[0])
{
- errno = 0;
- n = strtoimax (colon + 1, NULL, 10);
+ case 0:
+ boot_time = 0;
+ lfinfo_end = owner->colon;
+ break;
+
+ case ':':
+ if (! c_isdigit (owner->colon[1]))
+ return -1;
+ boot_time = strtoimax (owner->colon + 1, &lfinfo_end, 10);
+ break;
+
+ default:
+ return -1;
}
- owner->boot_time =
- ((0 <= n && n <= TYPE_MAXIMUM (time_t)
- && (TYPE_MAXIMUM (time_t) < INTMAX_MAX || errno != ERANGE))
- ? n : 0);
-
- /* The host is everything in between. */
- len = dot - at - 1;
- owner->host = xmalloc (len + 1);
- memcpy (owner->host, at + 1, len);
- owner->host[len] = 0;
-
- /* We're done looking at the link info. */
- UNGCPRO;
+ if (lfinfo_end != owner->user + lfinfolen)
+ return -1;
/* On current host? */
- if (STRINGP (Fsystem_name ())
- && strcmp (owner->host, SSDATA (Fsystem_name ())) == 0)
+ if (STRINGP (Vsystem_name)
+ && dot - (at + 1) == SBYTES (Vsystem_name)
+ && memcmp (at + 1, SSDATA (Vsystem_name), SBYTES (Vsystem_name)) == 0)
{
- if (owner->pid == getpid ())
+ if (pid == getpid ())
ret = 2; /* We own it. */
- else if (owner->pid > 0
- && (kill (owner->pid, 0) >= 0 || errno == EPERM)
- && (owner->boot_time == 0
- || within_one_second (owner->boot_time, get_boot_time ())))
+ else if (0 < pid && pid <= TYPE_MAXIMUM (pid_t)
+ && (kill (pid, 0) >= 0 || errno == EPERM)
+ && (boot_time == 0
+ || (boot_time <= TYPE_MAXIMUM (time_t)
+ && within_one_second (boot_time, get_boot_time ()))))
ret = 1; /* An existing process on this machine owns it. */
- /* The owner process is dead or has a strange pid (<=0), so try to
+ /* The owner process is dead or has a strange pid, so try to
zap the lockfile. */
- else if (unlink (lfname) < 0)
- ret = -1;
else
- ret = 0;
+ return unlink (lfname);
}
else
{ /* If we wanted to support the check for stale locks on remote machines,
@@ -535,11 +605,6 @@ current_lock_owner (lock_info_type *owner, char *lfname)
ret = 1;
}
- /* Avoid garbage. */
- if (owner == &local_owner || ret <= 0)
- {
- FREE_LOCK_INFO (*owner);
- }
return ret;
}
@@ -551,29 +616,25 @@ current_lock_owner (lock_info_type *owner, char *lfname)
Return -1 if cannot lock for any other reason. */
static int
-lock_if_free (lock_info_type *clasher, register char *lfname)
+lock_if_free (lock_info_type *clasher, char *lfname)
{
- while (! lock_file_1 (lfname, 0))
+ int err;
+ while ((err = lock_file_1 (lfname, 0)) == EEXIST)
{
- 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; /* current_lock_owner returned strange error. */
+ switch (current_lock_owner (clasher, lfname))
+ {
+ case 2:
+ return 0; /* We ourselves locked it. */
+ case 1:
+ return 1; /* Someone else has it. */
+ case -1:
+ return -1; /* current_lock_owner returned strange error. */
+ }
/* We deleted a stale lock; try again to lock the file. */
}
- return 0;
+
+ return err ? -1 : 0;
}
/* lock_file locks file FN,
@@ -645,17 +706,16 @@ lock_file (Lisp_Object fn)
if (0 < lock_if_free (&lock_info, lfname))
{
/* Someone else has the lock. Consider breaking it. */
- ptrdiff_t locker_size = (strlen (lock_info.user) + strlen (lock_info.host)
- + INT_STRLEN_BOUND (printmax_t)
- + sizeof "@ (pid )");
- char *locker = SAFE_ALLOCA (locker_size);
- printmax_t pid = lock_info.pid;
Lisp_Object attack;
- esprintf (locker, "%s@%s (pid %"pMd")",
- lock_info.user, lock_info.host, pid);
- FREE_LOCK_INFO (lock_info);
-
- attack = call2 (intern ("ask-user-about-lock"), fn, build_string (locker));
+ char *dot = lock_info.dot;
+ ptrdiff_t pidlen = lock_info.colon - (dot + 1);
+ static char const replacement[] = " (pid ";
+ int replacementlen = sizeof replacement - 1;
+ memmove (dot + replacementlen, dot + 1, pidlen);
+ strcpy (dot + replacementlen + pidlen, ")");
+ memcpy (dot, replacement, replacementlen);
+ attack = call2 (intern ("ask-user-about-lock"), fn,
+ build_string (lock_info.user));
/* Take the lock if the user said so. */
if (!NILP (attack))
lock_file_1 (lfname, 1);
@@ -685,16 +745,15 @@ unlock_file (Lisp_Object fn)
void
unlock_all_files (void)
{
- register Lisp_Object tail;
+ register Lisp_Object tail, buf;
register struct buffer *b;
- for (tail = Vbuffer_alist; CONSP (tail); tail = XCDR (tail))
+ FOR_EACH_LIVE_BUFFER (tail, buf)
{
- b = XBUFFER (XCDR (XCAR (tail)));
- if (STRINGP (BVAR (b, file_truename)) && BUF_SAVE_MODIFF (b) < BUF_MODIFF (b))
- {
- unlock_file (BVAR (b, file_truename));
- }
+ b = XBUFFER (buf);
+ if (STRINGP (BVAR (b, file_truename))
+ && BUF_SAVE_MODIFF (b) < BUF_MODIFF (b))
+ unlock_file (BVAR (b, file_truename));
}
}
@@ -760,10 +819,7 @@ t if it is locked by you, else a string saying which user has locked it. */)
else if (owner == 2)
ret = Qt;
else
- ret = build_string (locker.user);
-
- if (owner > 0)
- FREE_LOCK_INFO (locker);
+ ret = make_string (locker.user, locker.at - locker.user);
SAFE_FREE ();
return ret;