feat(sqlite): add preupdate hook

[aschey]

Adds bindings for SQLite's preupdate hook.

This is exposed as a separate feature because the system SQLite version generally does not have this flag enabled, so using it with sqlite-unbundled may cause linker errors.

If we don't want to create a new feature flag in the main crate just for this, we could enable it by default with the sqlite (bundled) feature only. That would make the configuration a little simpler.

[abonander]

The sqlite3_value returned by this call and sqlite3_preupdate_new has weird semantics. It's a "protected" value so it's thread-safe, but at the same time the documentation also specifies:

The sqlite3_value that P points to will be destroyed when the preupdate callback returns.

We should handle this as a SqliteValueRef instead, using the same lifetime. The user can then use .to_owned() to get a fully independent value if they need it.

You'll need to add a new case here:

sqlx/sqlx-sqlite/src/value.rs

Line 19 in 1678b19

Value(&'r SqliteValue),

[abonander]

This should also check i against sqlite3_preupdate_count because the result is undefined if the index is out of bounds.

[aschey]

We should handle this as a SqliteValueRef instead, using the same lifetime. The user can then use .to_owned() to get a fully independent value if they need it.

SqliteValue::new calls sqlite3_value_dup which creates a copy of the sqlite3_value. I tested this out by storing the returned SqliteValue in a mutex and ensuring the returned value could still be decoded properly after the callback was completed.

I can add something to SqliteValueRef for this to avoid the call to sqlite3_value_dup, but I think that would require re-implementing a lot of the logic in SqliteValue or modifying it so that it can operate on both an owned or borrowed value. Let me know if I'm missing something.

EDIT: added this in a separate commit. I can revert it if this isn't what you had in mind.

[aschey]

This should also check i against sqlite3_preupdate_count because the result is undefined if the index is out of bounds.

I think the documentation might be a bit conservative here (or they don't want to make any guarantees) because it does check for these conditions and return an error, but I went ahead and added an explicit check here too.