Make psql reject attempts to set special variables to invalid values.
Previously, if the user set a special variable such as ECHO to an
unrecognized value, psql would bleat but store the new value anyway, and
then fall back to a default setting for the behavior controlled by the
variable. This was agreed to be a not particularly good idea. With
this patch, invalid values result in an error message and no change in
state.
(But this applies only to variables that affect psql's behavior; purely
informational variables such as ENCODING can still be set to random
values.)
To do this, modify the API for psql's assign-hook functions so that they
can return an OK/not OK result, and give them the responsibility for
printing error messages when they reject a value. Adjust the APIs for
ParseVariableBool and ParseVariableNum to support the new behavior
conveniently.
In passing, document the variable VERSION, which had somehow escaped that.
And improve the quite-inadequate commenting in psql/variables.c.
Daniel Vérité, reviewed by Rahila Syed, some further tweaking by me
Discussion: https://postgr.es/m/
7356e741-fa59-4146-a8eb-
cf95fd6b21fb@mm