pgbench: increase the maximum number of variables/arguments

pgbench's arbitrary limit of 10 arguments for SQL statements or
metacommands is far too low. Increase it to 256.

This results in a very modest increase in memory usage, not enough to
worry about.

The maximum includes the SQL statement or metacommand. This is reflected
in the comments and revised TAP tests.

Simon Riggs and Dagfinn Ilmari Mannsåker with some light editing by me.
Reviewed by: David Rowley and Fabien Coelho

Discussion: https://postgr.es/m/CANP8+jJiMJOAf-dLoHuR-8GENiK+eHTY=Omw38Qx7j2g0NDTXA@mail.gmail.com

doc/src/sgml/ref/pgbench.sgml

@@ -916,6 +916,8 @@ pgbench <optional> <replaceable>options</replaceable> </optional> <replaceable>d
    value can be inserted into a SQL command by writing
    <literal>:</literal><replaceable>variablename</replaceable>.  When running more than
    one client session, each session has its own set of variables.
+   <application>pgbench</application> supports up to 255 variable uses in one
+   statement.
   </para>
 
    <table id="pgbench-automatic-variables">

src/bin/pgbench/pgbench.c

@@ -476,7 +476,12 @@ typedef struct
  */
 #define SQL_COMMAND		1
 #define META_COMMAND	2
-#define MAX_ARGS		10
+
+/*
+ * max number of backslash command arguments or SQL variables,
+ * including the command or SQL statement itself
+ */
+#define MAX_ARGS		256
 
 typedef enum MetaCommand
 {
@@ -4124,6 +4129,10 @@ parseQuery(Command *cmd)
 			continue;
 		}
 
+		/*
+		 * cmd->argv[0] is the SQL statement itself, so the max number of
+		 * arguments is one less than MAX_ARGS
+		 */
 		if (cmd->argc >= MAX_ARGS)
 		{
 			fprintf(stderr, "statement has too many arguments (maximum is %d): %s\n",
@@ -4461,6 +4470,10 @@ process_backslash_command(PsqlScanState sstate, const char *source)
 	/* For all other commands, collect remaining words. */
 	while (expr_lex_one_word(sstate, &word_buf, &word_offset))
 	{
+		/*
+		 * my_command->argv[0] is the command itself, so the max number of
+		 * arguments is one less than MAX_ARGS
+		 */
 		if (j >= MAX_ARGS)
 			syntax_error(source, lineno, my_command->first_line, my_command->argv[0],
 						 "too many arguments", NULL, -1);

src/bin/pgbench/t/001_pgbench_with_server.pl

@@ -597,11 +597,10 @@ my @errors = (
 }
 	],
 	[
-		'sql too many args', 1, [qr{statement has too many arguments.*\b9\b}],
-		q{-- MAX_ARGS=10 for prepared
+		'sql too many args', 1, [qr{statement has too many arguments.*\b255\b}],
+		q{-- MAX_ARGS=256 for prepared
 \set i 0
-SELECT LEAST(:i, :i, :i, :i, :i, :i, :i, :i, :i, :i, :i);
-}
+SELECT LEAST(}.join(', ', (':i') x 256).q{)}
 	],
 
 	# SHELL
@@ -619,25 +618,8 @@ SELECT LEAST(:i, :i, :i, :i, :i, :i, :i, :i, :i, :i, :i);
 	[ 'shell missing command', 1, [qr{missing command }], q{\shell} ],
 	[
 		'shell too many args', 1, [qr{too many arguments in command "shell"}],
-		q{-- 257 arguments to \shell
-\shell echo \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F \
- 0 1 2 3 4 5 6 7 8 9 A B C D E F
-}
+		q{-- 256 arguments to \shell
+\shell echo }.join(' ', ('arg') x 255)
 	],
 
 	# SET