database/sql: add Null[T]

Generic version of NullString, NullInt64, etc.

Fixes #60370

Change-Id: I166a05a6126e8b8571db5cbb026303bb6551d56b
GitHub-Last-Rev: 3c8d2d5
GitHub-Pull-Request: #60677
Reviewed-on: https://go-review.googlesource.com/c/go/+/501700
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

api/next/60370.txt

@@ -0,0 +1,5 @@
+pkg database/sql, method (*Null[$0]) Scan(interface{}) error #60370
+pkg database/sql, method (Null[$0]) Value() (driver.Value, error) #60370
+pkg database/sql, type Null[$0 interface{}] struct #60370
+pkg database/sql, type Null[$0 interface{}] struct, Valid bool #60370
+pkg database/sql, type Null[$0 interface{}] struct, V $0 #60370

src/database/sql/sql.go

@@ -391,6 +391,39 @@ func (n NullTime) Value() (driver.Value, error) {
 	return n.Time, nil
 }
 
+// Null represents a value that may be null.
+// Null implements the Scanner interface so
+// it can be used as a scan destination:
+//
+//	var s Null[string]
+//	err := db.QueryRow("SELECT name FROM foo WHERE id=?", id).Scan(&s)
+//	...
+//	if s.Valid {
+//	   // use s.V
+//	} else {
+//	   // NULL value
+//	}
+type Null[T any] struct {
+	V     T
+	Valid bool
+}
+
+func (n *Null[T]) Scan(value any) error {
+	if value == nil {
+		n.V, n.Valid = *new(T), false
+		return nil
+	}
+	n.Valid = true
+	return convertAssign(&n.V, value)
+}
+
+func (n Null[T]) Value() (driver.Value, error) {
+	if !n.Valid {
+		return nil, nil
+	}
+	return n.V, nil
+}
+
 // Scanner is an interface used by Scan.
 type Scanner interface {
 	// Scan assigns a value from a database driver.

src/database/sql/sql_test.go

@@ -1803,6 +1803,18 @@ func TestNullStringParam(t *testing.T) {
 	nullTestRun(t, spec)
 }
 
+func TestGenericNullStringParam(t *testing.T) {
+	spec := nullTestSpec{"nullstring", "string", [6]nullTestRow{
+		{Null[string]{"aqua", true}, "", Null[string]{"aqua", true}},
+		{Null[string]{"brown", false}, "", Null[string]{"", false}},
+		{"chartreuse", "", Null[string]{"chartreuse", true}},
+		{Null[string]{"darkred", true}, "", Null[string]{"darkred", true}},
+		{Null[string]{"eel", false}, "", Null[string]{"", false}},
+		{"foo", Null[string]{"black", false}, nil},
+	}}
+	nullTestRun(t, spec)
+}
+
 func TestNullInt64Param(t *testing.T) {
 	spec := nullTestSpec{"nullint64", "int64", [6]nullTestRow{
 		{NullInt64{31, true}, 1, NullInt64{31, true}},
@@ -1916,8 +1928,9 @@ func nullTestRun(t *testing.T, spec nullTestSpec) {
 	}
 
 	// Can't put null val into non-null col
-	if _, err := stmt.Exec(6, "bob", spec.rows[5].nullParam, spec.rows[5].notNullParam); err == nil {
-		t.Errorf("expected error inserting nil val with prepared statement Exec")
+	row5 := spec.rows[5]
+	if _, err := stmt.Exec(6, "bob", row5.nullParam, row5.notNullParam); err == nil {
+		t.Errorf("expected error inserting nil val with prepared statement Exec: NULL=%#v, NOT-NULL=%#v", row5.nullParam, row5.notNullParam)
 	}
 
 	_, err = db.Exec("INSERT|t|id=?,name=?,nullf=?", 999, nil, nil)