1 $PostgreSQL: pgsql/src/backend/utils/misc/README,v 1.10 2008/03/20 17:55:15 momjian Exp $
3 Guc Implementation Notes
4 ========================
6 The GUC (Grand Unified Configuration) module implements configuration
7 variables of multiple types (currently boolean, enum, int, float, and string).
8 Variable settings can come from various places, with a priority ordering
9 determining which setting is used.
15 Each variable known to GUC can optionally have an assign_hook and/or
16 a show_hook to provide customized behavior. Assign hooks are used to
17 perform validity checking on variable values (above and beyond what
18 GUC can do). They are also used to update any derived state that needs
19 to change when a GUC variable is set. Show hooks are used to modify
20 the default SHOW display for a variable.
22 If an assign_hook is provided, it points to a function of the signature
23 bool assign_hook(newvalue, bool doit, GucSource source)
24 where the type of "newvalue" matches the kind of variable. This function
25 is called immediately before actually setting the variable's value (so it
26 can look at the actual variable to determine the old value). If the
27 function returns "true" then the assignment is completed; if it returns
28 "false" then newvalue is considered invalid and the assignment is not
29 performed. If "doit" is false then the function should simply check
30 validity of newvalue and not change any derived state. The "source" parameter
31 indicates where the new value came from. If it is >= PGC_S_INTERACTIVE,
32 then we are performing an interactive assignment (e.g., a SET command), and
33 ereport(ERROR) is safe to do. But when source < PGC_S_INTERACTIVE, we are
34 reading a non-interactive option source, such as postgresql.conf. In this
35 case the assign_hook should *not* ereport but should just return false if it
36 doesn't like the newvalue.
38 If an assign_hook returns false then guc.c will report a generic "invalid
39 value for option FOO" error message. If you feel the need to provide a more
40 specific error message, ereport() it using "GUC_complaint_elevel(source)"
41 as the error level. Note that this might return either ERROR or a lower level
42 such as LOG, so the ereport call might or might not return. If it does
43 return, return false out of the assign_hook.
45 For string variables, the signature for assign hooks is a bit different:
46 const char *assign_hook(const char *newvalue,
49 The meanings of the parameters are the same as for the other types of GUC
50 variables, but the return value is handled differently:
51 NULL --- assignment fails (like returning false for other datatypes)
52 newvalue --- assignment succeeds, assign the newvalue as-is
53 malloc'd (not palloc'd!!!) string --- assign that value instead
54 The third choice is allowed in case the assign_hook wants to return a
55 "canonical" version of the new value. For example, the assign_hook for
56 datestyle always returns a string that includes both output and input
57 datestyle options, although the input might have specified only one.
59 Note that a string variable's assign_hook will NEVER be called with a NULL
60 value for newvalue, since there would be no way to distinguish success
61 and failure returns. If the boot_val or reset_val for a string variable
62 is NULL, it will just be assigned without calling the assign_hook.
63 Therefore, a NULL boot_val should never be used in combination with an
64 assign_hook that has side-effects, as the side-effects wouldn't happen
65 during a RESET that re-institutes the boot-time setting.
67 If a show_hook is provided, it points to a function of the signature
68 const char *show_hook(void)
69 This hook allows variable-specific computation of the value displayed
73 Saving/Restoring Guc Variable Values
74 ------------------------------------
76 Prior values of configuration variables must be remembered in order to deal
77 with several special cases: RESET (a/k/a SET TO DEFAULT), rollback of SET
78 on transaction abort, rollback of SET LOCAL at transaction end (either
79 commit or abort), and save/restore around a function that has a SET option.
80 RESET is defined as selecting the value that would be effective had there
81 never been any SET commands in the current session.
83 To handle these cases we must keep track of many distinct values for each
84 variable. The primary values are:
86 * actual variable contents always the current effective value
88 * reset_val the value to use for RESET
90 (Each GUC entry also has a boot_val which is the wired-in default value.
91 This is assigned to the reset_val and the actual variable during
92 InitializeGUCOptions(). The boot_val is also consulted to restore the
93 correct reset_val if SIGHUP processing discovers that a variable formerly
94 specified in postgresql.conf is no longer set there.)
96 In addition to the primary values, there is a stack of former effective
97 values that might need to be restored in future. Stacking and unstacking
98 is controlled by the GUC "nest level", which is zero when outside any
99 transaction, one at top transaction level, and incremented for each
100 open subtransaction or function call with a SET option. A stack entry
101 is made whenever a GUC variable is first modified at a given nesting level.
102 (Note: the reset_val need not be stacked because it is only changed by
103 non-transactional operations.)
105 A stack entry has a state, a prior value of the GUC variable, a remembered
106 source of that prior value, and depending on the state may also have a
107 "masked" value. The masked value is needed when SET followed by SET LOCAL
108 occur at the same nest level: the SET's value is masked but must be
109 remembered to restore after transaction commit.
111 During initialization we set the actual value and reset_val based on
112 whichever non-interactive source has the highest priority. They will
115 The possible transactional operations on a GUC value are:
117 Entry to a function with a SET option:
119 Push a stack entry with the prior variable value and state SAVE,
120 then set the variable.
124 If no stack entry of current level:
125 Push new stack entry w/prior value and state SET
126 else if stack entry's state is SAVE, SET, or LOCAL:
127 change stack state to SET, don't change saved value
128 (here we are forgetting effects of prior set action)
129 else (entry must have state SET+LOCAL):
130 discard its masked value, change state to SET
131 (here we are forgetting effects of prior SET and SET LOCAL)
136 If no stack entry of current level:
137 Push new stack entry w/prior value and state LOCAL
138 else if stack entry's state is SAVE or LOCAL or SET+LOCAL:
139 no change to stack entry
140 (in SAVE case, SET LOCAL will be forgotten at func exit)
141 else (entry must have state SET):
142 put current active into its masked slot, set state SET+LOCAL
145 Transaction or subtransaction abort:
147 Pop stack entries, restoring prior value, until top < subxact depth
149 Transaction or subtransaction commit (incl. successful function exit):
151 While stack entry level >= subxact depth
153 if entry's state is SAVE:
154 pop, restoring prior value
155 else if level is 1 and entry's state is SET+LOCAL:
156 pop, restoring *masked* value
157 else if level is 1 and entry's state is SET:
158 pop, discarding old value
159 else if level is 1 and entry's state is LOCAL:
160 pop, restoring prior value
161 else if there is no entry of exactly level N-1:
162 decrement entry's level, no other state change
164 merge entries of level N-1 and N as specified below
166 The merged entry will have level N-1 and prior = older prior, so easiest
167 to keep older entry and free newer. There are 12 possibilities since
168 we already handled level N state = SAVE:
172 SAVE SET discard top prior, set state SET
173 SAVE LOCAL discard top prior, no change to stack entry
174 SAVE SET+LOCAL discard top prior, copy masked, state S+L
176 SET SET discard top prior, no change to stack entry
177 SET LOCAL copy top prior to masked, state S+L
178 SET SET+LOCAL discard top prior, copy masked, state S+L
180 LOCAL SET discard top prior, set state SET
181 LOCAL LOCAL discard top prior, no change to stack entry
182 LOCAL SET+LOCAL discard top prior, copy masked, state S+L
184 SET+LOCAL SET discard top prior and second masked, state SET
185 SET+LOCAL LOCAL discard top prior, no change to stack entry
186 SET+LOCAL SET+LOCAL discard top prior, copy masked, state S+L
189 RESET is executed like a SET, but using the reset_val as the desired new
190 value. (We do not provide a RESET LOCAL command, but SET LOCAL TO DEFAULT
191 has the same behavior that RESET LOCAL would.) The source associated with
192 the reset_val also becomes associated with the actual value.
194 If SIGHUP is received, the GUC code rereads the postgresql.conf
195 configuration file (this does not happen in the signal handler, but at
196 next return to main loop; note that it can be executed while within a
197 transaction). New values from postgresql.conf are assigned to actual
198 variable, reset_val, and stacked actual values, but only if each of
199 these has a current source priority <= PGC_S_FILE. (It is thus possible
200 for reset_val to track the config-file setting even if there is
201 currently a different interactive value of the actual variable.)
203 The assign_hook and show_hook routines work only with the actual variable,
204 and are not directly aware of the additional values maintained by GUC.
205 This is not a problem for normal usage, since we can assign first to the
206 actual variable and then (if that succeeds) to the additional values as
207 needed. However, for SIGHUP rereads we may not want to assign to the
208 actual variable. Our procedure in that case is to call the assign_hook
209 with doit = false so that the value is validated, but no derived state is
213 String Memory Handling
214 ----------------------
216 String option values are allocated with strdup, not with the
217 pstrdup/palloc mechanisms. We would need to keep them in a permanent
218 context anyway, and strdup gives us more control over handling
219 out-of-memory failures.
221 We allow a string variable's actual value, reset_val, boot_val, and stacked
222 values to point at the same storage. This makes it slightly harder to free
223 space (we must test whether a value to be freed isn't equal to any of the
224 other pointers in the GUC entry or associated stack items). The main
225 advantage is that we never need to strdup during transaction commit/abort,
226 so cannot cause an out-of-memory failure there.