src/src/lookups/README

   1 $Cambridge: exim/src/src/lookups/README,v 1.1 2004/10/07 13:10:01 ph10 Exp $
   2
   3 LOOKUPS
   4 -------
   5
   6 Each lookup type is implemented by 6 functions, xxx_open(), xxx_check(),
   7 xxx_find(), xxx_close(), xxx_tidy(), and xxx_quote(), where xxx is the name of
   8 the lookup type (e.g. lsearch, dbm, or whatever).
   9
  10 The xxx_check(), xxx_close(), xxx_tidy(), and xxx_quote() functions need not
  11 exist. There is a table in drtables.c which links the lookup names to the
  12 various sets of functions, with NULL entries for any that don't exist. When
  13 the code for a lookup type is omitted from the binary, all its entries are
  14 NULL.
  15
  16 One of the fields in the table contains flags describing the kind of lookup.
  17 These are
  18
  19   lookup_querystyle
  20
  21 This is a "query style" lookup without a file name, as opposed to the "single
  22 key" style, where the key is associated with a "file name".
  23
  24   lookup_absfile
  25
  26 For single key lookups, this means that the file name must be an absolute path.
  27 It is set for lsearch and dbm, but not for NIS, for example.
  28
  29 When a single-key lookup file is opened, the handle returned by the xxx_open()
  30 function is saved, along with the file name and lookup type, in a tree. The
  31 xxx_close() function is not called when the first lookup is completed. If there
  32 are subsequent lookups of the same type that quote the same file name,
  33 xxx_open() isn't called; instead the cached handle is re-used.
  34
  35 Exim calls the function search_tidyup() at strategic points in its processing
  36 (e.g. after all routing and directing has been done) and this function walks
  37 the tree and calls the xxx_close() functions for all the cached handles.
  38
  39 Query-style lookups don't have the concept of an open file that can be cached
  40 this way. Of course, the local code for the lookup can manage its own caching
  41 information in any way it pleases. This means that the xxx_close()
  42 function, even it it exists, is never called. However, if an xxx_tidy()
  43 function exists, it is called once whenever Exim calls search_tidyup().
  44
  45 A single-key lookup type may also have an xxx_tidy() function, which is called
  46 by search_tidyup() after all cached handles have been closed via the
  47 xxx_close() function.
  48
  49 The lookup functions are wrapped into a special store pool (POOL_SEARCH). You
  50 can safely use store_get to allocate store for your handle caching. The store
  51 will be reset after all xxx_tidy() functions are called.
  52
  53 The function interfaces are as follows:
  54
  55
  56 xxx_open()
  57 ----------
  58
  59 This function is called to initiate the lookup. For things that involve files
  60 it should do a real open; for other kinds of lookup it may do nothing at all.
  61 The arguments are:
  62
  63   uschar *filename    the name of the "file" to open, for non-query-style
  64                         lookups; NULL for query-style lookups
  65   uschar **errmsg     where to put an error message if there is a problem
  66
  67 The yield of xxx_open() is a void * value representing the open file or
  68 database. For real files is is normally the FILE or DBM value. For other
  69 kinds of lookup, if there is no natural value to use, (-1) is recommended.
  70 The value should not be NULL (or 0) as that is taken to indicate failure of
  71 the xxx_open() function. For single-key lookups, the handle is cached along
  72 with the filename and type, and may be used for several lookups.
  73
  74
  75 xxx_check()
  76 -----------
  77
  78 If this function exists, it is called after a successful open to check on the
  79 ownership and mode of the file(s). The arguments are:
  80
  81   void *handle        the handle passed back from xxx_open()
  82   uschar *filename    the filename passed to xxx_open()
  83   int modemask        mode bits that must not be set
  84   int *owners         permitted owners of the file(s)
  85   int *owngroups      permitted group owners of the file(s)
  86   uschar **errmsg     where to put an error message if there is a problem
  87
  88 In the owners and owngroups vectors, the first element is the count of the
  89 remaining elements. There is a common function that can be called to check
  90 a file:
  91
  92 int search_check_file(int fd, char *filename, int modemask, int *owners,
  93   int *owngroups, char *type, char **errmsg);
  94
  95 If fd is >= 0, it is checked using fstat(), and filename is used only in
  96 error messages. If fd is < 0 then filename is checked using stat(). The yield
  97 is zero if all is well, +1 if the mode or uid or gid is wrong, or -1 if the
  98 stat() fails.
  99
 100 The yield of xxx_check() is TRUE if all is well, FALSE otherwise. The
 101 function should not close the file(s) on failure. That is done from outside
 102 by calling the xxx_close() function.
 103
 104
 105 xxx_find()
 106 ----------
 107
 108 This is called to search an open file/database. The result is OK, FAIL, or
 109 DEFER. The arguments are:
 110
 111   void *handle        the handle passed back from xxx_open()
 112   uschar *filename    the filename passed to xxx_open() (NULL for querystyle)
 113   uschar *keyquery    the key to look up, or query to process, zero-terminated
 114   int  length         the length of the key
 115   uschar **result     point to the yield, in dynamic store, on success
 116   uschar **errmsg     where to put an error message on failure;
 117                       this is initially set to "", and should be left
 118                       as that for a standard "entry not found" error
 119   BOOL *do_cache      the lookup should set this to FALSE when it changes data.
 120                       This is TRUE by default. When set to FALSE the cache tree
 121                       of the current search handle will be cleaned and the
 122                       current result will NOT be cached. Currently the mysql
 123                       and pgsql lookups use this when UPDATE/INSERT queries are
 124                       executed.
 125
 126 Even though the key is zero-terminated, the length is passed because in the
 127 common case it has been computed already and is often needed.
 128
 129
 130 xxx_close()
 131 -----------
 132
 133 This is called for single-key lookups when a file is finished with. There is no
 134 yield, and the only argument is the handle that was passed back from
 135 xxx_open(). It is not called for query style lookups.
 136
 137
 138 xxx_tidy()
 139 ----------
 140
 141 This function is called once at the end of search_tidyup() for every lookup
 142 type for which it exists.
 143
 144
 145 xxx_quote()
 146 -----------
 147
 148 This is called by the string expansion code for expansions of the form
 149 ${quote_xxx:<string>}, if it exists. If not, the expansion code makes no change
 150 to the string. The function must apply any quoting rules that are specific to
 151 the lookup, and return a pointer to the revised string. If quoting is not
 152 needed, it can return its single argument, which is a uschar *. This function
 153 does NOT use the POOL_SEARCH store, because it's usually never called from any
 154 lookup code.
 155
 156 ****