parser: allow multiple DECLARE keywords in function definitions

Previously, UDFs with multiple DECLARE statements would cause the parser
to panic. This patch allows us to accept these statements, which is what
Postgres does.

Fixes: cockroachdb#129285
Release note (bug fix): Using more than one DECLARE statment in the
definition of a user defined function now correctly declares additional
variables.

pkg/ccl/logictestccl/testdata/logic_test/plpgsql_block

@@ -693,3 +693,40 @@ BEGIN
   END IF;
 END;
 $$ LANGUAGE PLpgSQL;
+
+subtest multi_declare
+
+statement ok
+DROP FUNCTION IF EXISTS f;
+
+statement ok
+CREATE FUNCTION f() RETURNS INT AS $$
+DECLARE
+  x INT := 0;
+DECLARE
+  y INT := x + 1;
+BEGIN
+  RAISE NOTICE '% %', x, y;
+  RETURN 0;
+END;
+$$ LANGUAGE PLpgSQL;
+
+query T noticetrace
+SELECT f();
+----
+NOTICE: 0 1
+
+statement ok
+DROP FUNCTION IF EXISTS f;
+
+statement error pgcode 42601 pq: duplicate declaration at or near "x"
+CREATE FUNCTION f() RETURNS INT AS $$
+DECLARE
+  x INT := 0;
+DECLARE
+  x INT := 1;
+BEGIN
+  RAISE NOTICE '%', x;
+  RETURN 0;
+END;
+$$ LANGUAGE PLpgSQL;

pkg/sql/plpgsql/parser/plpgsql.y

@@ -344,7 +344,7 @@ func (u *plpgsqlSymUnion) sqlStatement() tree.Statement {
 %type <plpgsqltree.Statement>	stmt_commit stmt_rollback
 %type <plpgsqltree.Statement>	stmt_case stmt_foreach_a
 
-%type <plpgsqltree.Statement> decl_stmt decl_statement
+%type <plpgsqltree.Statement> decl_statement
 %type <[]plpgsqltree.Statement> decl_sect opt_decl_stmts decl_stmts
 
 %type <[]plpgsqltree.Exception> exception_sect proc_exceptions
@@ -419,36 +419,23 @@ opt_decl_stmts: decl_stmts
   }
 ;
 
-decl_stmts: decl_stmts decl_stmt
+opt_declare: DECLARE {}
+| {}
+;
+
+decl_stmts: decl_stmts opt_declare decl_statement
   {
     decs := $1.statements()
-    dec := $2.statement()
+    dec := $3.statement()
     $$.val = append(decs, dec)
   }
-| decl_stmt
+| decl_statement
   {
     dec := $1.statement()
     $$.val = []plpgsqltree.Statement{dec}
 	}
 ;
 
-decl_stmt	: decl_statement
-  {
-    $$.val = $1.statement()
-  }
-| DECLARE
-  {
-    // This is to allow useless extra "DECLARE" keywords in the declare section.
-    $$.val = (plpgsqltree.Statement)(nil)
-  }
-// TODO(drewk): turn this block on and throw useful error if user
-// tries to put the block label just before BEGIN instead of before
-// DECLARE.
-//| LESS_LESS any_identifier GREATER_GREATER
-//  {
-//  }
-;
-
 decl_statement: decl_varname decl_const decl_datatype decl_collate decl_notnull decl_defval
   {
     $$.val = &plpgsqltree.Declaration{