Provide a test for variable existence in psql

"\if :{?variable_name}" will be translated to "\if TRUE" if the variable
exists and "\if FALSE" otherwise. Thus it will be possible to execute code
conditionally on the existence of the variable, regardless of its value.

Fabien Coelho, with some review by Robins Tharakan and some light text
editing by me.

Discussion: https://postgr.es/m/alpine.DEB.2.20.1708260835520.3627@lancre

1 files changed, 41 insertions, 1 deletions

diff --git a/src/fe_utils/psqlscan.l b/src/fe_utils/psqlscan.l
index 27689d72da8..4375142a007 100644
--- a/src/fe_utils/psqlscan.l
+++ b/src/fe_utils/psqlscan.l
@@ -745,9 +745,13 @@ other			.
 											 PQUOTE_SQL_IDENT);
 				}
 
+:\{\?{variable_char}+\}	{
+					psqlscan_test_variable(cur_state, yytext, yyleng);
+				}
+
 	/*
 	 * These rules just avoid the need for scanner backup if one of the
-	 * two rules above fails to match completely.
+	 * three rules above fails to match completely.
 	 */
 
 :'{variable_char}*	{
@@ -762,6 +766,17 @@ other			.
 					ECHO;
 				}
 
+:\{\?{variable_char}*	{
+					/* Throw back everything but the colon */
+					yyless(1);
+					ECHO;
+				}
+:\{	{
+					/* Throw back everything but the colon */
+					yyless(1);
+					ECHO;
+				}
+
 	/*
 	 * Back to backend-compatible rules.
 	 */
@@ -1442,3 +1457,28 @@ psqlscan_escape_variable(PsqlScanState state, const char *txt, int len,
 		psqlscan_emit(state, txt, len);
 	}
 }
+
+void
+psqlscan_test_variable(PsqlScanState state, const char *txt, int len)
+{
+	char	*varname;
+	char	*value;
+
+	varname = psqlscan_extract_substring(state, txt + 3, len - 4);
+	if (state->callbacks->get_variable)
+		value = state->callbacks->get_variable(varname, PQUOTE_PLAIN,
+											   state->cb_passthrough);
+	else
+		value = NULL;
+	free(varname);
+
+	if (value != NULL)
+	{
+		psqlscan_emit(state, "TRUE", 4);
+		free(value);
+	}
+	else
+	{
+		psqlscan_emit(state, "FALSE", 5);
+	}
+}