Support \if ... \elif ... \else ... \endif in psql scripting.

This patch adds nestable conditional blocks to psql.  The control
structure feature per se is complete, but the boolean expressions
understood by \if and \elif are pretty primitive; basically, after
variable substitution and backtick expansion, the result has to be
"true" or "false" or one of the other standard spellings of a boolean
value.  But that's enough for many purposes, since you can always
do the heavy lifting on the server side; and we can extend it later.

Along the way, pay down some of the technical debt that had built up
around psql/command.c:
* Refactor exec_command() into a function per command, instead of
being a 1500-line monstrosity.  This makes the file noticeably longer
because of repetitive function header/trailer overhead, but it seems
much more readable.
* Teach psql_get_variable() and psqlscanslash.l to suppress variable
substitution and backtick expansion on the basis of the conditional
stack state, thereby allowing removal of the OT_NO_EVAL kluge.
* Fix the no-doubt-once-expedient hack of sometimes silently substituting
mainloop.c's previous_buf for query_buf when calling HandleSlashCmds.
(It's a bit remarkable that commands like \r worked at all with that.)
Recall of a previous query is now done explicitly in the slash commands
where that should happen.

Corey Huinker, reviewed by Fabien Coelho, further hacking by me

Discussion: https://postgr.es/m/CADkLM=c94OSRTnat=LX0ivNq4pxDNeoomFfYvBKM5N_xfmLtAA@mail.gmail.com

1 files changed, 6 insertions, 1 deletions

diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index e9d4fe67866..b06ae9779d5 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -121,7 +121,8 @@ setQFout(const char *fname)
  * (Failure in escaping should lead to returning NULL.)
  *
  * "passthrough" is the pointer previously given to psql_scan_set_passthrough.
- * psql currently doesn't use this.
+ * In psql, passthrough points to a ConditionalStack, which we check to
+ * determine whether variable expansion is allowed.
  */
 char *
 psql_get_variable(const char *varname, bool escape, bool as_ident,
@@ -130,6 +131,10 @@ psql_get_variable(const char *varname, bool escape, bool as_ident,
 	char	   *result;
 	const char *value;
 
+	/* In an inactive \if branch, suppress all variable substitutions */
+	if (passthrough && !conditional_active((ConditionalStack) passthrough))
+		return NULL;
+
 	value = GetVariable(pset.vars, varname);
 	if (!value)
 		return NULL;