Re: \if, \elseif, \else, \endif (was Re: PSQL commands:\quit_if, \quit_unless)

Patches do not apply cleanly.
Part 1 gets:
 error: patch failed: src/test/regress/parallel_schedule:89
 error: src/test/regress/parallel_schedule: patch does not apply

There is still the useless file, ok it is removed by part2. Could have been just one patch...

ISTM that PQExpBuffer is partially a memory leak. Something should need to be freed?

I think that you should use appendPQExpBufferChar and Str instead of relying on the format variant which is probably expensive. Something like:

  if (num_options > 0)
    append...Char(buf, ' ');
  append...Str(buf, ...);

is_true_boolean_expression: "return (success) ? tf : false;"
Is this simply: "return success && tf;"?

Some functions have opt1, opt2, but some start at opt0. This does not look too consistent, although the inconsistency may be preexisting from your patch. Basically, there is a need for some more restructuring in "command.c".