fs/reiser4/plugin/file/symfile.c

   1 /* Copyright 2001, 2002, 2003 by Hans Reiser, licensing governed by reiser4/README */
   2
   3 /* Symfiles are a generalization of Unix symlinks.
   4
   5    A symfile when read behaves as though you took its contents and
   6    substituted them into the reiser4 naming system as the right hand side
   7    of an assignment, and then read that which you had assigned to it.
   8
   9    A key issue for symfiles is how to implement writes through to
  10    subfiles.  In general, one must have some method of determining what
  11    of that which is written to the symfile is written to what subfile.
  12    This can be done by use of custom plugin methods written by users, or
  13    by using a few general methods we provide for those willing to endure
  14    the insertion of delimiters into what is read.
  15
  16    Writing to symfiles without delimiters to denote what is written to
  17    what subfile is not supported by any plugins we provide in this
  18    release.  Our most sophisticated support for writes is that embodied
  19    by the invert plugin (see invert.c).
  20
  21    A read only version of the /etc/passwd file might be
  22    constructed as a symfile whose contents are as follows:
  23
  24    /etc/passwd/userlines/*
  25
  26    or
  27
  28    /etc/passwd/userlines/demidov+/etc/passwd/userlines/edward+/etc/passwd/userlines/reiser+/etc/passwd/userlines/root
  29
  30    or
  31
  32    /etc/passwd/userlines/(demidov+edward+reiser+root)
  33
  34    A symfile with contents
  35
  36    /filenameA+"(some text stored in the uninvertable symfile)+/filenameB
  37
  38    will return when read
  39
  40    The contents of filenameAsome text stored in the uninvertable symfileThe contents of filenameB
  41
  42    and write of what has been read will not be possible to implement as
  43    an identity operation because there are no delimiters denoting the
  44    boundaries of what is to be written to what subfile.
  45
  46    Note that one could make this a read/write symfile if one specified
  47    delimiters, and the write method understood those delimiters delimited
  48    what was written to subfiles.
  49
  50    So, specifying the symfile in a manner that allows writes:
  51
  52    /etc/passwd/userlines/demidov+"(
  53    )+/etc/passwd/userlines/edward+"(
  54    )+/etc/passwd/userlines/reiser+"(
  55    )+/etc/passwd/userlines/root+"(
  56    )
  57
  58    or
  59
  60    /etc/passwd/userlines/(demidov+"(
  61    )+edward+"(
  62    )+reiser+"(
  63    )+root+"(
  64    ))
  65
  66    and the file demidov might be specified as:
  67
  68    /etc/passwd/userlines/demidov/username+"(:)+/etc/passwd/userlines/demidov/password+"(:)+/etc/passwd/userlines/demidov/userid+"(:)+/etc/passwd/userlines/demidov/groupid+"(:)+/etc/passwd/userlines/demidov/gecos+"(:)+/etc/passwd/userlines/demidov/home+"(:)+/etc/passwd/userlines/demidov/shell
  69
  70    or
  71
  72    /etc/passwd/userlines/demidov/(username+"(:)+password+"(:)+userid+"(:)+groupid+"(:)+gecos+"(:)+home+"(:)+shell)
  73
  74    Notice that if the file demidov has a carriage return in it, the
  75    parsing fails, but then if you put carriage returns in the wrong place
  76    in a normal /etc/passwd file it breaks things also.
  77
  78    Note that it is forbidden to have no text between two interpolations
  79    if one wants to be able to define what parts of a write go to what
  80    subfiles referenced in an interpolation.
  81
  82    If one wants to be able to add new lines by writing to the file, one
  83    must either write a custom plugin for /etc/passwd that knows how to
  84    name an added line, or one must use an invert, or one must use a more
  85    sophisticated symfile syntax that we are not planning to write for
  86    version 4.0.
  87 */