The previous logic was systematically attempting to retrieve last_insert_id
even for models without an AutoField primary key when they had a GeneratedField
on backends that can't return columns from INSERT.
The issue affected MySQL, SQLite < 3.35, and Oracle when the use_returning_into
option was disabled and could result in either crashes when the non-auto
primary key wasn't an IntegerField subclass or silent misassignment of bogus
insert ids (0 or the previous auto primary key insert value) to the first
defined generated field value.
This logic could only be exercised if the composite primary key included an
AutoField but it's not allowed yet (refs #35957).
It was also slightly broken as it expected the AutoField to always be the first
member of returning_fields.
This avoids many awkward checks against NOT_PROVIDED and provides symmetry
with Field.has_default() which is also the reason why it wasn't made a
property.
Thanks to Bhuvnesh Sharma and Adam Johnson for mentoring this Google
Summer of Code 2024 project. Thanks to Sarah Boyce, David Smith, Jacob
Walls and Natalia Bidart for reviews.
To use the simple `filename="..."` form, the value must conform to the
official grammar from RFC6266[^1]:
filename-parm = "filename" "=" value
value = <value, defined in [RFC2616], Section 3.6>
; token | quoted-string
The `quoted-string` definition comes from RFC 9110[^2]:
```
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
The backslash octet ("\") can be used as a single-octet quoting
mechanism within quoted-string and comment constructs. Recipients that
process the value of a quoted-string MUST handle a quoted-pair as if
it were replaced by the octet following the backslash.
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
A sender SHOULD NOT generate a quoted-pair in a quoted-string except
where necessary to quote DQUOTE and backslash octets occurring within
that string.
```
That is, quoted strings are able to express horizontal tabs, space
characters, and everything in the range from 0x21 to 0x7e, expect for
0x22 (`"`) and 0x5C (`\`), which can still be expressed but must be
escaped with their own `\`.
We ignore the case of `obs-text`, which is defined as the range
0x80-0xFF, since its presence is there for permissive parsing of
accidental high-bit characters, and it should not be generated by
conforming implementations.
Transform this character range into a regex and apply it in addition
to the "is ASCII" check. This ensures that all simple filenames are
expressed in the simple format, and that all filenames with newlines
and other control characters are properly expressed with the
percent-encoded `filename*=...`form.
[^1]: https://datatracker.ietf.org/doc/html/rfc6266#section-4.1
[^2]: https://datatracker.ietf.org/doc/html/rfc9110#name-quoted-strings
BigAutoField is the default type for primary keys. In models.txt, the linked
anchor shows that the default primary key is a BigAutoField, so it now defers
to that section instead of duplicating an (incorrect) type.
