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 n_dotlock() 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 - 2017 Steffen (Daode) Nurpmeso <steffen@sdaoden.eu>.
  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 n_dotlock_state a_dotlock_create(struct n_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 n_dotlock_state a_dotlock__create_excl(struct n_dotlock_info *dip,
  47                               char const *lname);
  48
  49 static enum n_dotlock_state
  50 a_dotlock_create(struct n_dotlock_info *dip){
  51    /* Use PATH_MAX not NAME_MAX to catch those "we proclaim the minimum value"
  52     * problems (SunOS), since the pathconf(3) value came too late! */
  53    char lname[PATH_MAX +1];
  54    sigset_t nset, oset;
  55    size_t tries;
  56    ssize_t w;
  57    enum n_dotlock_state rv, xrv;
  58
  59    /* (Callee ensured this doesn't end up as plain "di_lock_name") */
  60    snprintf(lname, sizeof lname, "%s.%s.%s",
  61       dip->di_lock_name, dip->di_hostname, dip->di_randstr);
  62
  63    sigfillset(&nset);
  64
  65    for(tries = 0;; ++tries){
  66       sigprocmask(SIG_BLOCK, &nset, &oset);
  67       rv = a_dotlock__create_excl(dip, lname);
  68       sigprocmask(SIG_SETMASK, &oset, NULL);
  69
  70       if(rv == n_DLS_NONE || (rv & n_DLS_ABANDON))
  71          break;
  72       if(dip->di_pollmsecs == 0 || tries >= DOTLOCK_TRIES){
  73          rv |= n_DLS_ABANDON;
  74          break;
  75       }
  76
  77       xrv = n_DLS_PING;
  78       w = write(STDOUT_FILENO, &xrv, sizeof xrv);
  79       if(w == -1 && errno == EPIPE){
  80          rv = n_DLS_DUNNO | n_DLS_ABANDON;
  81          break;
  82       }
  83       n_msleep(dip->di_pollmsecs, FAL0);
  84    }
  85    return rv;
  86 }
  87
  88 static enum n_dotlock_state
  89 a_dotlock__create_excl(struct n_dotlock_info *dip, char const *lname){
  90    struct stat stb;
  91    int fd;
  92    size_t tries;
  93    enum n_dotlock_state rv = n_DLS_NONE;
  94
  95    /* We try to create the unique filename */
  96    for(tries = 0;; ++tries){
  97       fd = open(lname,
  98 #ifdef O_SYNC
  99                (O_WRONLY | O_CREAT | O_EXCL | O_SYNC),
 100 #else
 101                (O_WRONLY | O_CREAT | O_EXCL),
 102 #endif
 103             S_IWUSR | S_IRUSR | S_IRGRP | S_IROTH);
 104       if(fd != -1){
 105 #ifdef n_PRIVSEP_SOURCE
 106          if(dip->di_stb != NULL &&
 107                fchown(fd, dip->di_stb->st_uid, dip->di_stb->st_gid)){
 108             int x = errno;
 109             close(fd);
 110             errno = x;
 111             goto jbados;
 112          }
 113 #endif
 114          close(fd);
 115          break;
 116       }else if(errno != EEXIST){
 117          rv = (errno == EROFS) ? n_DLS_ROFS | n_DLS_ABANDON : n_DLS_NOPERM;
 118          goto jleave;
 119       }else if(tries >= DOTLOCK_TRIES){
 120          rv = n_DLS_EXIST;
 121          goto jleave;
 122       }
 123    }
 124
 125    /* We link the name to the fname */
 126    if(link(lname, dip->di_lock_name) == -1)
 127       goto jbados;
 128
 129    /* Note that we stat our own exclusively created name, not the
 130     * destination, since the destination can be affected by others */
 131    if(stat(lname, &stb) == -1)
 132       goto jbados;
 133
 134    unlink(lname);
 135
 136    /* If the number of links was two (one for the unique file and one for
 137     * the lock), we've won the race */
 138    if(stb.st_nlink != 2)
 139       rv = n_DLS_EXIST;
 140 jleave:
 141    return rv;
 142 jbados:
 143    rv = (errno == EEXIST) ? n_DLS_EXIST : n_DLS_NOPERM | n_DLS_ABANDON;
 144    unlink(lname);
 145    goto jleave;
 146 }
 147
 148 /* s-it-mode */