Fixed #14094 -- Added support for unlimited models.CharField on PostgreSQL.

[Elkasitu]

This commit introduced a new database feature flag
supports_unlimited_varchar which denotes which database backends
support defining VARCHAR columns with no/unlimited size.

When this feature flag is enabled for a given backend, CharField
definitions may rely on the default max_length value of None instead
of having to explicitly define one. It is however still possible to
explicitly pass a max_length and this will be enforced at the database
level.

This commit also implemented the necessary changes for the feature to
apply to the postgres backend.

[carltongibson]

Just a query, on the ticket there was [a mention of inspectdb's output here]https://code.djangoproject.com/ticket/14094#comment:3). Did you happen to look into that at all?

[Elkasitu]

@carltongibson I didn't, I wasn't aware of this Django feature until just now nor what Joe meant by "much smoother" (faster? less code complexity? etc.).

I have just tested it in a dummy Django project and the result is incorrect Django code (max_length=-1), I will take a look at it later today and see what I can do.

Thanks for the heads up!

[Elkasitu]

@carltongibson it should work now, I am not exactly happy with the implementation (it assumes that a negative internal_size for a varchar column means that the varchar is of unlimited size, which works for pg but unsure about other backends) but I'm not sure that there's a better alternative given the way that inspectdb sets field attributes.

[Elkasitu]

I could define a constant in the backend's instropection class e.g. UNLIMITED_VARCHAR_ISIZE = -1 (for pg) and use that in inspectdb to determine whether it is in fact a varchar of unlimited size

[Elkasitu]

Rebased on top of the changes introduced by #16368, some tests are failing but the test that fails seems unrelated to my changes

[felixxm]

@Elkasitu Thanks for this patch 👍 Please check my comments.

[felixxm]

Also, a release note in the Models section is missing.

[Elkasitu]

@felixxm I believe I've addressed all of your concerns now, there's one test failing but it seems like it fails randomly (previously it had failed on 3.11 and passed on 3.8)

[felixxm]

@Elkasitu Thanks 👍

@charettes Do you want to take a look?

[felixxm]

@Elkasitu Thanks for updates 👍

I pushed small edits and fixed CharField.description.

[felixxm]

Suggested change

	supports_unlimited_varchar = False
	supports_unlimited_charfield = False