src/src/lookups/README

   1 LOOKUPS
   2 -------
   3
   4 Each lookup type is implemented by 6 functions, xxx_open(), xxx_check(),
   5 xxx_find(), xxx_close(), xxx_tidy(), and xxx_quote(), where xxx is the name of
   6 the lookup type (e.g. lsearch, dbm, or whatever). In addition, there is
   7 a version reporting function used to trace compile-vs-runtime conflicts and
   8 to help administrators ensure that the modules from the correct build are
   9 in use by the main binary.
  10
  11 The xxx_check(), xxx_close(), xxx_tidy(), and xxx_quote() functions need not
  12 exist. There is a table in drtables.c which links the lookup names to the
  13 various sets of functions, with NULL entries for any that don't exist. When
  14 the code for a lookup type is omitted from the binary, all its entries are
  15 NULL.
  16
  17 One of the fields in the table contains flags describing the kind of lookup.
  18 These are
  19
  20   lookup_querystyle
  21
  22 This is a "query style" lookup without a file name, as opposed to the "single
  23 key" style, where the key is associated with a "file name".
  24
  25   lookup_absfile
  26
  27 For single key lookups, this means that the file name must be an absolute path.
  28 It is set for lsearch and dbm, but not for NIS, for example.
  29
  30   lookup_absfilequery
  31
  32 This is a query-style lookup that must start with an absolute file name. For
  33 example, the sqlite lookup is of this type.
  34
  35 When a single-key or absfilequery lookup file is opened, the handle returned by
  36 the xxx_open() function is saved, along with the file name and lookup type, in
  37 a tree. The xxx_close() function is not called when the first lookup is
  38 completed. If there are subsequent lookups of the same type that quote the same
  39 file name, xxx_open() isn't called; instead the cached handle is re-used.
  40
  41 Exim calls the function search_tidyup() at strategic points in its processing
  42 (e.g. after all routing and directing has been done) and this function walks
  43 the tree and calls the xxx_close() functions for all the cached handles.
  44
  45 Query-style lookups don't have the concept of an open file that can be cached
  46 this way. Of course, the local code for the lookup can manage its own caching
  47 information in any way it pleases. This means that the xxx_close()
  48 function, even it it exists, is never called. However, if an xxx_tidy()
  49 function exists, it is called once whenever Exim calls search_tidyup().
  50
  51 A single-key lookup type may also have an xxx_tidy() function, which is called
  52 by search_tidyup() after all cached handles have been closed via the
  53 xxx_close() function.
  54
  55 The lookup functions are wrapped into a special store pool (POOL_SEARCH). You
  56 can safely use store_get to allocate store for your handle caching. The store
  57 will be reset after all xxx_tidy() functions are called.
  58
  59 The function interfaces are as follows:
  60
  61
  62 xxx_open()
  63 ----------
  64
  65 This function is called to initiate the lookup. For things that involve files
  66 it should do a real open; for other kinds of lookup it may do nothing at all.
  67 The arguments are:
  68
  69   uschar *filename    the name of the "file" to open, for non-query-style
  70                         lookups; NULL for query-style lookups
  71   uschar **errmsg     where to put an error message if there is a problem
  72
  73 The yield of xxx_open() is a void * value representing the open file or
  74 database. For real files is is normally the FILE or DBM value. For other
  75 kinds of lookup, if there is no natural value to use, (-1) is recommended.
  76 The value should not be NULL (or 0) as that is taken to indicate failure of
  77 the xxx_open() function. For single-key lookups, the handle is cached along
  78 with the filename and type, and may be used for several lookups.
  79
  80
  81 xxx_check()
  82 -----------
  83
  84 If this function exists, it is called after a successful open to check on the
  85 ownership and mode of the file(s). The arguments are:
  86
  87   void *handle        the handle passed back from xxx_open()
  88   uschar *filename    the filename passed to xxx_open()
  89   int modemask        mode bits that must not be set
  90   int *owners         permitted owners of the file(s)
  91   int *owngroups      permitted group owners of the file(s)
  92   uschar **errmsg     where to put an error message if there is a problem
  93
  94 In the owners and owngroups vectors, the first element is the count of the
  95 remaining elements. There is a common function that can be called to check
  96 a file:
  97
  98 int search_check_file(int fd, char *filename, int modemask, int *owners,
  99   int *owngroups, char *type, char **errmsg);
 100
 101 If fd is >= 0, it is checked using fstat(), and filename is used only in
 102 error messages. If fd is < 0 then filename is checked using stat(). The yield
 103 is zero if all is well, +1 if the mode or uid or gid is wrong, or -1 if the
 104 stat() fails.
 105
 106 The yield of xxx_check() is TRUE if all is well, FALSE otherwise. The
 107 function should not close the file(s) on failure. That is done from outside
 108 by calling the xxx_close() function.
 109
 110
 111 xxx_find()
 112 ----------
 113
 114 This is called to search an open file/database. The result is OK, FAIL, or
 115 DEFER. The arguments are:
 116
 117   void *handle        the handle passed back from xxx_open()
 118   uschar *filename    the filename passed to xxx_open() (NULL for querystyle)
 119   uschar *keyquery    the key to look up, or query to process, zero-terminated
 120   int  length         the length of the key
 121   uschar **result     point to the yield, in dynamic store, on success
 122   uschar **errmsg     where to put an error message on failure;
 123                       this is initially set to "", and should be left
 124                       as that for a standard "entry not found" error
 125   uint *do_cache      the lookup should set this to 0 when it changes data.
 126                       This is MAXINT by default. When set to 0 the cache tree
 127                       of the current search handle will be cleaned and the
 128                       current result will NOT be cached. Currently the mysql
 129                       and pgsql lookups use this when UPDATE/INSERT queries are
 130                       executed.
 131                       If set to a nonzero number of seconds, the cached value
 132                       becomes unusable after this time. Currently the dnsdb
 133                       lookup uses this to support the TTL value.
 134
 135 Even though the key is zero-terminated, the length is passed because in the
 136 common case it has been computed already and is often needed.
 137
 138
 139 xxx_close()
 140 -----------
 141
 142 This is called for single-key lookups when a file is finished with. There is no
 143 yield, and the only argument is the handle that was passed back from
 144 xxx_open(). It is not called for query style lookups.
 145
 146
 147 xxx_tidy()
 148 ----------
 149
 150 This function is called once at the end of search_tidyup() for every lookup
 151 type for which it exists.
 152
 153
 154 xxx_quote()
 155 -----------
 156
 157 This is called by the string expansion code for expansions of the form
 158 ${quote_xxx:<string>}, if it exists. If not, the expansion code makes no change
 159 to the string. The function must apply any quoting rules that are specific to
 160 the lookup, and return a pointer to the revised string. If quoting is not
 161 needed, it can return its single argument, which is a uschar *. This function
 162 does NOT use the POOL_SEARCH store, because it's usually never called from any
 163 lookup code.
 164
 165 xxx_report_version()
 166 --------------------
 167
 168 This is called to report diagnostic information to a file stream.
 169 Typically it would report both compile-time and run-time version information.
 170 The arguments are:
 171
 172   FILE *stream    where to fprintf() the data to
 173
 174
 175 ****