fix: don't reference unloaded buffers

[prettymuchbryce]

Problem

Currently when checking if a buffer is valid, we use the function nvim_buf_is_valid, however there is an edge case here described in the documentation where a buffer could have been unloaded, but still be "valid". If a buffer is unloaded and valid then referencing buflisted will result in a (UNKNOWN PLUGIN): Error executing lua: attempt to call a number value error.

Solution

Instead of nvim_buf_is_valid we use nvim_buf_is_loaded which checks both whether a buffer is valid and loaded. This way we never attempt to reference buflisted of unloaded or invalid buffers.

This PR should at least partially address #869. Applying the patch removes these errors for my config when invoking Neovim via nvim .. Because this error is so generic, it's possible that there could be more than one cause.

[akinsho]

Thanks, happy to give this a shot

[stevenxxiu]

I just updated my plugins, and this commit causes bufferline to stop working with possession. With this commit, only the last used tab gets loaded.

[latipun7]

I just updated my plugins, and this commit causes bufferline to stop working with possession. With this commit, only the last used tab gets loaded.

I use different session plugin and have same issue.
I guess all sessions are loaded, but only the last used buffer is showing on bufferline. I can't switch it with :BufferLineCyclePrev or :BufferLineCycleNext. I can switch to 2nd last buffer with :e #, and then bufferline shows two buffer.

[akinsho]

Sigh 😔 well worth a shot will revert for now, too many obscure requirements to get this working

[prettymuchbryce]

Sigh 😔 well worth a shot will revert for now, too many obscure requirements to get this working

Rolling back is the right call for now. Clearly this PR causes some unexpected side effects for session plugin users. I am happy to install possession and see if I can further root cause the issue. My initial hunch is that these plugins must intentionally utilize unloaded buffers in a way that I didn't initially anticipate.

[akinsho]

Someone suggested swapping the order of the exists check as a potential solution which I did. @prettymuchbryce can you see if that made a difference for you?

[prettymuchbryce]

Someone suggested swapping the order of the exists check as a potential solution which I did. @prettymuchbryce can you see if that made a difference for you?

Unfortunately that solution does not resolve the UNKNOWN PLUGIN errors because that solution still attempts to call .buflisted on an unloaded buffer.

I have opened a new PR which I believe contains a working solution. That is— it resolves the UNKNOWN PLUGIN errors, while also maintaining support for session plugins that utilize hidden buffers. Let me know your thoughts.

#931