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

doc/src/sgml/ref/psql-ref.sgml

@@ -783,6 +783,10 @@ testdb=>
     The forms <literal>:'<replaceable>variable_name</>'</literal> and
     <literal>:"<replaceable>variable_name</>"</literal> described there
     work as well.
+    The <literal>:{?<replaceable>variable_name</>}</> syntax allows
+    testing whether a variable is defined. It is substituted by
+    TRUE or FALSE.
+    Escaping the colon with a backslash protects it from substitution.
     </para>
 
     <para>
@@ -3938,6 +3942,12 @@ testdb=&gt; <userinput>INSERT INTO my_table VALUES (:'content');</userinput>
     can escape a colon with a backslash to protect it from substitution.
     </para>
 
+    <para>
+    The <literal>:{?<replaceable>name</>}</> special syntax returns TRUE
+    or FALSE depending on whether the variable exists or not, and is thus
+    always substituted, unless the colon is backslash-escaped.
+    </para>
+
     <para>
     The colon syntax for variables is standard <acronym>SQL</acronym> for
     embedded query languages, such as <application>ECPG</application>.

src/bin/psql/psqlscanslash.l

@@ -281,6 +281,10 @@ other			.
 					unquoted_option_chars = 0;
 				}
 
+:\{\?{variable_char}+\}	{
+					psqlscan_test_variable(cur_state, yytext, yyleng);
+				}
+
 :'{variable_char}*	{
 					/* Throw back everything but the colon */
 					yyless(1);
@@ -295,6 +299,20 @@ other			.
 					ECHO;
 				}
 
+:\{\?{variable_char}*	{
+					/* Throw back everything but the colon */
+					yyless(1);
+					unquoted_option_chars++;
+					ECHO;
+				}
+
+:\{		{
+					/* Throw back everything but the colon */
+					yyless(1);
+					unquoted_option_chars++;
+					ECHO;
+				}
+
 {other}			{
 					unquoted_option_chars++;
 					ECHO;

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);
+	}
+}

src/include/fe_utils/psqlscan_int.h

@@ -142,5 +142,7 @@ extern char *psqlscan_extract_substring(PsqlScanState state,
 extern void psqlscan_escape_variable(PsqlScanState state,
 						 const char *txt, int len,
 						 PsqlScanQuoteType quote);
+extern void psqlscan_test_variable(PsqlScanState state,
+								   const char *txt, int len);
 
 #endif							/* PSQLSCAN_INT_H */

src/test/regress/expected/psql.out

@@ -3014,6 +3014,32 @@ bar 'bar' "bar"
 	\echo 'should print #8-1'
 should print #8-1
 \endif
+-- :{?...} defined variable test
+\set i 1
+\if :{?i}
+  \echo '#9-1 ok, variable i is defined'
+#9-1 ok, variable i is defined
+\else
+  \echo 'should not print #9-2'
+\endif
+\if :{?no_such_variable}
+  \echo 'should not print #10-1'
+\else
+  \echo '#10-2 ok, variable no_such_variable is not defined'
+#10-2 ok, variable no_such_variable is not defined
+\endif
+SELECT :{?i} AS i_is_defined;
+ i_is_defined 
+--------------
+ t
+(1 row)
+
+SELECT NOT :{?no_such_var} AS no_such_var_is_not_defined;
+ no_such_var_is_not_defined 
+----------------------------
+ t
+(1 row)
+
 -- SHOW_CONTEXT
 \set SHOW_CONTEXT never
 do $$

src/test/regress/sql/psql.sql

@@ -572,6 +572,24 @@ select \if false \\ (bogus \else \\ 42 \endif \\ forty_two;
 	\echo 'should print #8-1'
 \endif
 
+-- :{?...} defined variable test
+\set i 1
+\if :{?i}
+  \echo '#9-1 ok, variable i is defined'
+\else
+  \echo 'should not print #9-2'
+\endif
+
+\if :{?no_such_variable}
+  \echo 'should not print #10-1'
+\else
+  \echo '#10-2 ok, variable no_such_variable is not defined'
+\endif
+
+SELECT :{?i} AS i_is_defined;
+
+SELECT NOT :{?no_such_var} AS no_such_var_is_not_defined;
+
 -- SHOW_CONTEXT
 
 \set SHOW_CONTEXT never