Merge remote-tracking branch 'github/pr/34'
[exim.git] / 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 ****