$PostgreSQL: pgsql/src/backend/utils/misc/README,v 1.4 2004/01/19 19:04:40 tgl Exp $


GUC IMPLEMENTATION NOTES

The GUC (Grand Unified Configuration) module implements configuration
variables of multiple types (currently boolean, int, float, and string).
Variable settings can come from various places, with a priority ordering
determining which setting is used.


PER-VARIABLE HOOKS

Each variable known to GUC can optionally have an assign_hook and/or
a show_hook to provide customized behavior.  Assign hooks are used to
perform validity checking on variable values (above and beyond what
GUC can do).  They are also used to update any derived state that needs
to change when a GUC variable is set.  Show hooks are used to modify
the default SHOW display for a variable.

If an assign_hook is provided, it points to a function of the signature
	bool assign_hook(newvalue, bool doit, GucSource source)
where the type of "newvalue" matches the kind of variable.  This function
is called immediately before actually setting the variable's value (so it
can look at the actual variable to determine the old value).  If the
function returns "true" then the assignment is completed; if it returns
"false" then newvalue is considered invalid and the assignment is not
performed.  If "doit" is false then the function should simply check
validity of newvalue and not change any derived state.  The "source" parameter
indicates where the new value came from.  If it is >= PGC_S_INTERACTIVE,
then we are performing an interactive assignment (e.g., a SET command).
In such cases it is okay for the assign_hook to raise an error via ereport().
If the function returns false for an interactive assignment then guc.c will
report a generic "invalid value" error message.  (An internal ereport() in
an assign_hook is only needed if you want to generate a specialized error
message.)  But when source < PGC_S_INTERACTIVE, we are reading a
non-interactive option source, such as postgresql.conf.  In this case the
assign_hook should *not* ereport but should just return false if it doesn't
like the newvalue.  (An ereport(LOG) call would be acceptable if you feel a
need for a custom complaint in this situation.)

For string variables, the signature for assign hooks is a bit different:
	const char *assign_hook(const char *newvalue,
				bool doit,
				GucSource source)
The meanings of the parameters are the same as for the other types of GUC
variables, but the return value is handled differently:
	NULL --- assignment fails (like returning false for other datatypes)
	newvalue --- assignment succeeds, assign the newvalue as-is
	malloc'd (not palloc'd!!!) string --- assign that value instead
The third choice is allowed in case the assign_hook wants to return a
"canonical" version of the new value.  For example, the assign_hook for
datestyle always returns a string that includes both output and input
datestyle options, although the input might have specified only one.

If a show_hook is provided, it points to a function of the signature
	const char *show_hook(void)
This hook allows variable-specific computation of the value displayed
by SHOW.


SAVING/RESTORING GUC VARIABLE VALUES

Prior values of configuration variables must be remembered in order to
deal with three special cases: RESET (a/k/a SET TO DEFAULT), rollback of
SET on transaction abort, and rollback of SET LOCAL at transaction end
(either commit or abort).  RESET is defined as selecting the value that
would be effective had there never been any SET commands in the current
session.

To handle these cases we must keep track of as many as four distinct
values for each variable.  They are:

* actual variable contents	always the current effective value

* reset_value			the value to use for RESET

* session_value			the "committed" setting for the session

* tentative_value		the uncommitted result of SET

During initialization we set the first three of these (actual, reset_value,
and session_value) based on whichever non-interactive source has the
highest priority.  All three will have the same value.

A SET LOCAL command sets the actual variable (and nothing else).  At
transaction end, the session_value is used to restore the actual variable
to its pre-transaction value.

A SET (or SET SESSION) command sets the actual variable, and if no error,
then sets the tentative_value.  If the transaction commits, the
tentative_value is assigned to the session_value and the actual variable
(which could by now be different, if the SET was followed by SET LOCAL).
If the transaction aborts, the tentative_value is discarded and the
actual variable is restored from the session_value.

RESET is executed like a SET, but using the reset_value as the desired new
value.  (We do not provide a RESET LOCAL command, but SET LOCAL TO DEFAULT
has the same behavior that RESET LOCAL would.)  The source associated with
the reset_value also becomes associated with the actual and session values.

If SIGHUP is received, the GUC code rereads the postgresql.conf
configuration file (this does not happen in the signal handler, but at
next return to main loop; note that it can be executed while within a
transaction).  New values from postgresql.conf are assigned to actual
variable, reset_value, and session_value, but only if each of these has a
current source priority <= PGC_S_FILE.  (It is thus possible for
reset_value to track the config-file setting even if there is currently
a different interactive value of the actual variable.)

Note that tentative_value is unused and undefined except between a SET
command and the end of the transaction.  Also notice that we must track
the source associated with each of the four values.

The assign_hook and show_hook routines work only with the actual variable,
and are not directly aware of the additional values maintained by GUC.
This is not a problem for normal usage, since we can assign first to the
actual variable and then (if that succeeds) to the additional values as
needed.  However, for SIGHUP rereads we may not want to assign to the
actual variable.  Our procedure in that case is to call the assign_hook
with doit = false so that the value is validated, but no derived state is
changed.


STRING MEMORY HANDLING

String option values are allocated with strdup, not with the
pstrdup/palloc mechanisms.  We would need to keep them in a permanent
context anyway, and strdup gives us more control over handling
out-of-memory failures.

We allow a variable's actual value, reset_val, session_val, and
tentative_val to point at the same storage.  This makes it slightly harder
to free space (must test that the value to be freed isn't equal to any of
the other three pointers).  The main advantage is that we never need to
strdup during transaction commit/abort, so cannot cause an out-of-memory
failure there.
