[LUCY-98] Clownfish::Type qualifier "nullable"

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Component/s: Clownfish
• Assignee: Marvin Humphrey
• Reporter: Marvin Humphrey
• Created: 31/Jan/10 05:52
• Updated: 08/Feb/10 18:05
• Resolved: 08/Feb/10 18:05

Description

Most Lucy methods which return objects may not return NULL. Internal Lucy
code does not NULL-check the return values of such methods before using them
– for efficiency's sake, it trusts that the source has upheld the API
contract and returned a real object of the correct type.

If we screw up our C code and return NULL from such a method we'll get a
segfault, but that's OK – we're C developers and we know how to deal with
segfaults.

The problem arises when a host-language user creates a custom subclass and
overrides a method that's not supposed to return NULL, but screws up and e.g.
returns undef from Perl. Their invalid undef gets translated to NULL at the
binding boundary, hits our C code and triggers a segfault – but this time,
the user is not prepared to troubleshoot segfaults.

We don't want to litter every single method invocation with NULL checks –
especially since only a small minority of methods which return pointers might
legally return NULL.

The solution is to introduce a "nullable" type qualifier to Clownfish.

    /** Open an InStream, or set Err_error and return NULL on failure.
     * 
     * @param path A relative filepath.
     * @return an InStream.
     */
    public incremented nullable InStream*
    Open_In(Folder *self, const CharBuf *path);

The callback wrappers we auto-generate can then know that they should perform
NULL-checking of return values when the return type does not have a "nullable"
qualifier – and throw an exception before inner Lucy code has the chance to
deref the NULL pointer and segfault.