src/src/lookups/README

   1 $Cambridge: exim/src/src/lookups/README,v 1.2 2005/08/01 13:20:28 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   lookup_absfilequery
  30
  31 This is a query-style lookup that must start with an absolute file name. For
  32 example, the sqlite lookup is of this type.
  33
  34 When a single-key or absfilequery lookup file is opened, the handle returned by
  35 the xxx_open() function is saved, along with the file name and lookup type, in
  36 a tree. The xxx_close() function is not called when the first lookup is
  37 completed. If there are subsequent lookups of the same type that quote the same
  38 file name, xxx_open() isn't called; instead the cached handle is re-used.
  39
  40 Exim calls the function search_tidyup() at strategic points in its processing
  41 (e.g. after all routing and directing has been done) and this function walks
  42 the tree and calls the xxx_close() functions for all the cached handles.
  43
  44 Query-style lookups don't have the concept of an open file that can be cached
  45 this way. Of course, the local code for the lookup can manage its own caching
  46 information in any way it pleases. This means that the xxx_close()
  47 function, even it it exists, is never called. However, if an xxx_tidy()
  48 function exists, it is called once whenever Exim calls search_tidyup().
  49
  50 A single-key lookup type may also have an xxx_tidy() function, which is called
  51 by search_tidyup() after all cached handles have been closed via the
  52 xxx_close() function.
  53
  54 The lookup functions are wrapped into a special store pool (POOL_SEARCH). You
  55 can safely use store_get to allocate store for your handle caching. The store
  56 will be reset after all xxx_tidy() functions are called.
  57
  58 The function interfaces are as follows:
  59
  60
  61 xxx_open()
  62 ----------
  63
  64 This function is called to initiate the lookup. For things that involve files
  65 it should do a real open; for other kinds of lookup it may do nothing at all.
  66 The arguments are:
  67
  68   uschar *filename    the name of the "file" to open, for non-query-style
  69                         lookups; NULL for query-style lookups
  70   uschar **errmsg     where to put an error message if there is a problem
  71
  72 The yield of xxx_open() is a void * value representing the open file or
  73 database. For real files is is normally the FILE or DBM value. For other
  74 kinds of lookup, if there is no natural value to use, (-1) is recommended.
  75 The value should not be NULL (or 0) as that is taken to indicate failure of
  76 the xxx_open() function. For single-key lookups, the handle is cached along
  77 with the filename and type, and may be used for several lookups.
  78
  79
  80 xxx_check()
  81 -----------
  82
  83 If this function exists, it is called after a successful open to check on the
  84 ownership and mode of the file(s). The arguments are:
  85
  86   void *handle        the handle passed back from xxx_open()
  87   uschar *filename    the filename passed to xxx_open()
  88   int modemask        mode bits that must not be set
  89   int *owners         permitted owners of the file(s)
  90   int *owngroups      permitted group owners of the file(s)
  91   uschar **errmsg     where to put an error message if there is a problem
  92
  93 In the owners and owngroups vectors, the first element is the count of the
  94 remaining elements. There is a common function that can be called to check
  95 a file:
  96
  97 int search_check_file(int fd, char *filename, int modemask, int *owners,
  98   int *owngroups, char *type, char **errmsg);
  99
 100 If fd is >= 0, it is checked using fstat(), and filename is used only in
 101 error messages. If fd is < 0 then filename is checked using stat(). The yield
 102 is zero if all is well, +1 if the mode or uid or gid is wrong, or -1 if the
 103 stat() fails.
 104
 105 The yield of xxx_check() is TRUE if all is well, FALSE otherwise. The
 106 function should not close the file(s) on failure. That is done from outside
 107 by calling the xxx_close() function.
 108
 109
 110 xxx_find()
 111 ----------
 112
 113 This is called to search an open file/database. The result is OK, FAIL, or
 114 DEFER. The arguments are:
 115
 116   void *handle        the handle passed back from xxx_open()
 117   uschar *filename    the filename passed to xxx_open() (NULL for querystyle)
 118   uschar *keyquery    the key to look up, or query to process, zero-terminated
 119   int  length         the length of the key
 120   uschar **result     point to the yield, in dynamic store, on success
 121   uschar **errmsg     where to put an error message on failure;
 122                       this is initially set to "", and should be left
 123                       as that for a standard "entry not found" error
 124   BOOL *do_cache      the lookup should set this to FALSE when it changes data.
 125                       This is TRUE by default. When set to FALSE the cache tree
 126                       of the current search handle will be cleaned and the
 127                       current result will NOT be cached. Currently the mysql
 128                       and pgsql lookups use this when UPDATE/INSERT queries are
 129                       executed.
 130
 131 Even though the key is zero-terminated, the length is passed because in the
 132 common case it has been computed already and is often needed.
 133
 134
 135 xxx_close()
 136 -----------
 137
 138 This is called for single-key lookups when a file is finished with. There is no
 139 yield, and the only argument is the handle that was passed back from
 140 xxx_open(). It is not called for query style lookups.
 141
 142
 143 xxx_tidy()
 144 ----------
 145
 146 This function is called once at the end of search_tidyup() for every lookup
 147 type for which it exists.
 148
 149
 150 xxx_quote()
 151 -----------
 152
 153 This is called by the string expansion code for expansions of the form
 154 ${quote_xxx:<string>}, if it exists. If not, the expansion code makes no change
 155 to the string. The function must apply any quoting rules that are specific to
 156 the lookup, and return a pointer to the revised string. If quoting is not
 157 needed, it can return its single argument, which is a uschar *. This function
 158 does NOT use the POOL_SEARCH store, because it's usually never called from any
 159 lookup code.
 160
 161 ****