dotlock.h

   1 /*@ S-nail - a mail user agent derived from Berkeley Mail.
   2  *@ Creation of an exclusive "dotlock" file.  This is (potentially) shared
   3  *@ in between dot_lock() and the privilege-separated "dotlocker"..
   4  *@ (Which is why it doesn't use NYD or other utilities.)
   5  *@ The code assumes it has been chdir(2)d into the target directory and
   6  *@ that SIGPIPE is ignored (we react upon EPIPE).
   7  *@ It furtherly assumes that it can create a file name that is at least one
   8  *@ byte longer than the dotlock file's name!
   9  *
  10  * Copyright (c) 2012 - 2015 Steffen (Daode) Nurpmeso <sdaoden@users.sf.net>.
  11  */
  12 /*
  13  * Copyright (c) 1996 Christos Zoulas.  All rights reserved.
  14  *
  15  * Redistribution and use in source and binary forms, with or without
  16  * modification, are permitted provided that the following conditions
  17  * are met:
  18  * 1. Redistributions of source code must retain the above copyright
  19  *    notice, this list of conditions and the following disclaimer.
  20  * 2. Redistributions in binary form must reproduce the above copyright
  21  *    notice, this list of conditions and the following disclaimer in the
  22  *    documentation and/or other materials provided with the distribution.
  23  *
  24  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  25  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  26  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  27  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  28  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  29  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  30  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  31  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  32  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  33  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  34  */
  35
  36 /* Jump in */
  37 static enum dotlock_state _dotlock_create(struct dotlock_info *dip);
  38
  39 /* Create a unique file. O_EXCL does not really work over NFS so we follow
  40  * the following trick (inspired by S.R. van den Berg):
  41  * - make a mostly unique filename and try to create it
  42  * - link the unique filename to our target
  43  * - get the link count of the target
  44  * - unlink the mostly unique filename
  45  * - if the link count was 2, then we are ok; else we've failed */
  46 static enum dotlock_state  __dotlock_create_excl(struct dotlock_info *dip,
  47                               char const *lname);
  48
  49 static enum dotlock_state
  50 _dotlock_create(struct dotlock_info *dip)
  51 {
  52    /* Use PATH_MAX not NAME_MAX to catch those "we proclaim the minimum value"
  53     * problems (SunOS), since the pathconf(3) value came too late! */
  54    char lname[PATH_MAX +1];
  55    sigset_t nset, oset;
  56    size_t tries;
  57    ssize_t w;
  58    enum dotlock_state rv, xrv;
  59
  60    /* (Callee ensured this doesn't end up as plain "di_lock_name") */
  61    snprintf(lname, sizeof lname, "%s.%s.%s",
  62       dip->di_lock_name, dip->di_hostname, dip->di_randstr);
  63
  64    sigemptyset(&nset);
  65    sigaddset(&nset, SIGHUP);
  66    sigaddset(&nset, SIGINT);
  67    sigaddset(&nset, SIGQUIT);
  68    sigaddset(&nset, SIGTERM);
  69    sigaddset(&nset, SIGTTIN);
  70    sigaddset(&nset, SIGTTOU);
  71    sigaddset(&nset, SIGTSTP);
  72
  73    for (tries = 0;; ++tries) {
  74       sigprocmask(SIG_BLOCK, &nset, &oset);
  75       rv = __dotlock_create_excl(dip, lname);
  76       sigprocmask(SIG_SETMASK, &oset, NULL);
  77
  78       if (rv == DLS_NONE || (rv & DLS_ABANDON))
  79          break;
  80       if (dip->di_pollmsecs == 0 || tries >= DOTLOCK_TRIES) {
  81          rv |= DLS_ABANDON;
  82          break;
  83       }
  84
  85       xrv = DLS_PING;
  86       w = write(STDOUT_FILENO, &xrv, sizeof xrv);
  87       if (w == -1 && errno == EPIPE) {
  88          rv = DLS_DUNNO | DLS_ABANDON;
  89          break;
  90       }
  91       sleep(1); /* TODO pollmsecs -> use finer grain */
  92    }
  93    return rv;
  94 }
  95
  96 static enum dotlock_state
  97 __dotlock_create_excl(struct dotlock_info *dip, char const *lname)
  98 {
  99    struct stat stb;
 100    int fd;
 101    size_t tries;
 102    enum dotlock_state rv = DLS_NONE;
 103
 104    /* We try to create the unique filename */
 105    for (tries = 0;; ++tries) {
 106       fd = open(lname,
 107 #ifdef O_SYNC
 108                (O_WRONLY | O_CREAT | O_TRUNC | O_EXCL | O_SYNC),
 109 #else
 110                (O_WRONLY | O_CREAT | O_TRUNC | O_EXCL),
 111 #endif
 112             S_IWUSR | S_IRUSR | S_IRGRP | S_IROTH);
 113       if (fd != -1) {
 114 #ifdef n_PRIVSEP_SOURCE
 115          if (dip->di_stb != NULL &&
 116                fchown(fd, dip->di_stb->st_uid, dip->di_stb->st_gid)) {
 117             int x = errno;
 118             close(fd);
 119             errno = x;
 120             goto jbados;
 121          }
 122 #endif
 123          close(fd);
 124          break;
 125       } else if (errno != EEXIST) {
 126          rv = (errno == EROFS) ? DLS_ROFS | DLS_ABANDON : DLS_NOPERM;
 127          goto jleave;
 128       } else if (tries >= DOTLOCK_TRIES) {
 129          rv = DLS_EXIST;
 130          goto jleave;
 131       }
 132    }
 133
 134    /* We link the name to the fname */
 135    if (link(lname, dip->di_lock_name) == -1)
 136       goto jbados;
 137
 138    /* Note that we stat our own exclusively created name, not the
 139     * destination, since the destination can be affected by others */
 140    if (stat(lname, &stb) == -1)
 141       goto jbados;
 142
 143    unlink(lname);
 144
 145    /* If the number of links was two (one for the unique file and one for
 146     * the lock), we've won the race */
 147    if (stb.st_nlink != 2)
 148       rv = DLS_EXIST;
 149 jleave:
 150    return rv;
 151 jbados:
 152    rv = (errno == EEXIST) ? DLS_EXIST : DLS_NOPERM | DLS_ABANDON;
 153    unlink(lname);
 154    goto jleave;
 155 }
 156
 157 /* s-it-mode */