plpgsql: correctly handle parsing errors

[DrewKimball]

plpgsql: correctly handle parsing errors

This patch ensures that PLpgSQL parsing errors are correctly propagated
in all cases. Previously, there were a few cases (like variable declaration
type parsing) where an error didn't halt parsing. The contract for
GetTypeFromValidSQLSyntax is also clarified, since it is ok to call with
an invalid type name as long as the error is properly handled.

Informs #105254

Release note: None

plpgsql: handle multiple expressions when one expression is expected

Previously, the PLpgSQL parser could panic when the user supplied more
than one expression in a location where only one was expected, for example,
in a return statement. This was because the PLpgSQL parser delegated to
the SQL parser's ParseExpr function, which expects exactly one input
expression. This commit returns a syntax error instead of the panic by
switching to use ParseExprs, which can handle multiple input expressions.

Informs #109342

Release note: None

plpgsql: return correct error for invalid parantheses and missing expression

This patch fixes error messages in the PLpgSQL parser for the case when
the parenthesis nesting is invalid, and for the case when no expression
(or statement) is supplied. Previously, invalid parentheses would cause
a panic without an error code, and a missing expression had the incorrect
message, since it wasn't checked until the SQL parser attempted to read
an empty string. Now, both cases are checked immediately by the PLpgSQL
parser and the correct error is propagated.

Fixes #109342

Release note: None

[cockroach-teamcity]

This change is 

[rharding6373]

Nice work! Only one very minor test suggestion.

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball and @mgartner)

---

pkg/sql/plpgsql/parser/testdata/stmt_assign line 68 at r3 (raw file):

DECLARE
BEGIN
  a := 1, (2, 3, 4, 5);

nit: have one of these tests return 3+ columns.

[DrewKimball]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

---

pkg/sql/plpgsql/parser/testdata/stmt_assign line 68 at r3 (raw file):

Previously, rharding6373 (Rachael Harding) wrote…

nit: have one of these tests return 3+ columns.

Done.

[DrewKimball]

TFTR!

[mgartner]

Nice finds!

Reviewed 2 of 3 files at r1, 7 of 7 files at r4, 2 of 2 files at r5, 7 of 7 files at r6, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (and 1 stale) (waiting on @DrewKimball)

[DrewKimball]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)