Missing Gdal dataset open flags about gdal

[rmanoka]

The bitflags approach is very cool; I'd opened a similar issue but this is more comprehensive, so I'm closing it. There is one issue with the open_ex options: GDAL_OF_SHARED that needs to be discussed (see below).

Sharing Issue

GDAL since 2.4 allows Dataset to be accessed from different threads, as long as only one thread accesses it at any time (ref. discussion). In this crate, we impl Send on Dataset to reflect that we can pass it to another thread, but not Sync so it is not accessed from two threads at the same time.

However, the GDAL_OF_SHARED option allows this to be subverted. We might open the same dataset twice with the shared option, and then pass one of the Dataset handles to another thread. This could lead to undefined behaviours as we are accessing a non-thread-safe data-structure from different threads simultaneously.

The simplest solution I could think of is to remove GDAL_OF_SHARED from the bitflags, thus not allowing it in the flags. Then, we could add a flag in DatasetOptions which can only be set via an unsafe method. This way, we allow shared open, if the user knows what they're doing, and there's no way to get UB via safe rust.

[jdroenner]

wow this is a really cool solution. I think @rmanoka is right. we should remove the shared bitfag and add an unsafe method to DatasetOptions to enable this feature.

[dmarteau]

I do agree that GDAL_OF_SHARED defeat the purpose of multithreading safety and I'm also in favor te remove it.

[dmarteau]

Then, we could add a flag in DatasetOptions which can only be set via an unsafe method

Amha, this is even not necessary and it would be better to rely on rust safe way of handling shared access to dataset.

[jdroenner]

i guess we can add a special method for shared later ( if we need it)

[rmanoka]

Amha, this is even not necessary and it would be better to rely on rust safe way of handling shared access to dataset.

Sounds good; bitflags already has an unsafe fn from_bits_unchecked that allows this anyway.