Handle null input in the presence of errors

Signed-off-by: Moritz Hoffmann <mh@materialize.com>

Author: antiguru

src/expr/src/scalar.rs

@@ -1241,10 +1241,18 @@ impl MirScalarExpr {
                                         BinaryFunc::RegexpReplace { regex, limit },
                                     )
                                 }
-                                Err(err) => MirScalarExpr::literal(
-                                    Err(err),
-                                    e.typ(column_types).scalar_type,
-                                ),
+                                Err(err) => {
+                                    let mut exprs = mem::take(exprs);
+                                    let source = exprs.swap_remove(0);
+                                    let scalar_type = e.typ(column_types).scalar_type;
+                                    // We need to return `NULL` on `NULL` input, and error otherwise.
+                                    source
+                                        .call_is_null()
+                                        .if_then_else(
+                                            MirScalarExpr::literal_null(scalar_type.clone()),
+                                            MirScalarExpr::literal(Err(err), scalar_type),
+                                        )
+                                }
                             };
                         } else if *func == VariadicFunc::RegexpSplitToArray
                             && exprs[1].is_literal()

test/sqllogictest/regex.slt

@@ -717,6 +717,11 @@ Target cluster: multiprocess
 
 EOF
 
+query T
+SELECT regexp_replace(null, 'b(..', 'X', 'ig');
+----
+NULL
+
 query error invalid regular expression: regex parse error:
 SELECT regexp_replace('foobarbaz', 'b(..', 'X', 'ig');
 
@@ -734,8 +739,8 @@ EXPLAIN OPTIMIZED PLAN WITH (humanized expressions) AS VERBOSE TEXT FOR
 SELECT s, regexp_replace(s, 'b(..', replacement, 'ig') FROM e;
 ----
 Explained Query:
-  Map (error("invalid regular expression: regex parse error:\n    b(..\n     ^\nerror: unclosed group"))
-    Project (#0{s})
+  Project (#0{s}, #3)
+    Map (case when (#0{s}) IS NULL then null else error("invalid regular expression: regex parse error:\n    b(..\n     ^\nerror: unclosed group") end)
       ReadStorage materialize.public.e
 
 Source materialize.public.e