Add db/errors.go, move error definitions to it

pkg/db/errors.go

@@ -0,0 +1,75 @@
+package db
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/mattn/go-sqlite3"
+)
+
+var (
+	ErrNotInDB               = errors.New("not in db")
+	ErrItemIsDeleted         = errors.New("item is deleted")
+	ErrForeignKeyViolation   = errors.New("foreign key constraint failed")
+	ErrDatabaseAlreadyExists = errors.New("target already exists")
+)
+
+type ForeignKey interface {
+	~uint64 | ~string
+}
+
+type ForeignKeyError[T ForeignKey] struct {
+	Field       string
+	TargetTable string
+	FkValue     T
+}
+
+func NewForeignKeyError[T ForeignKey](field, table string, fkValue T) ForeignKeyError[T] {
+	return ForeignKeyError[T]{
+		Field:       field,
+		TargetTable: table,
+		FkValue:     fkValue,
+	}
+}
+
+func (e ForeignKeyError[T]) Error() string {
+	return fmt.Sprintf(`%s: fk field %q (to %q) with value "%v"`,
+		ErrForeignKeyViolation.Error(), e.Field, e.TargetTable, e.FkValue,
+	)
+}
+
+// Unwrap returns the sentinel error-- permits the use of errors.Is for convenience
+func (e ForeignKeyError[T]) Unwrap() error {
+	return ErrForeignKeyViolation
+}
+
+// -------------
+// SQLite errors
+// -------------
+
+// IsSqliteFkError checks whether an error is a SQLite foreign key constraint violation.
+func IsSqliteFkError(err error) bool {
+	var sqliteErr sqlite3.Error
+	if !errors.As(err, &sqliteErr) {
+		return false
+	}
+	return errors.Is(sqliteErr.ExtendedCode, sqlite3.ErrConstraintForeignKey)
+}
+
+// IsSqliteUniqError checks whether an error is a SQLite unique constraint violation.
+func IsSqliteUniqError(err error) bool {
+	var sqliteErr sqlite3.Error
+	if !errors.As(err, &sqliteErr) {
+		return false
+	}
+	return errors.Is(sqliteErr.ExtendedCode, sqlite3.ErrConstraintUnique)
+}
+
+// IsSqliteNotNullError checks whether an error is a SQLite not null constraint violation.
+func IsSqliteNotNullError(err error) bool {
+	var sqliteErr sqlite3.Error
+	if !errors.As(err, &sqliteErr) {
+		return false
+	}
+	return errors.Is(sqliteErr.ExtendedCode, sqlite3.ErrConstraintNotNull)
+}