Allow subscribing to updates of module.providers data

[radeksimko]

Use-cases

#712 introduced a custom command to allow clients obtain data about required and installed providers. Clients may call this command repeatedly at any time and it's relatively cheap to do so. However the client may not necessarily know when is the right time to do so.

Clients should be able to "subscribe" to receive data updates whenever this data changes.

Attempted Solutions

Clients ask repeatedly - can lead to either excessive amount of requests and/or outdated info.

Proposal

The server already has mechanisms allowing it to detect state changes which affect the data returned from module.providers via hooks

terraform-ls/internal/state/hooks.go

Lines 3 to 11 in 2027413

	type ModuleChangeHook func(oldMod, newMod *Module)
	type ModuleChangeHooks []ModuleChangeHook
	func (mh ModuleChangeHooks) notifyModuleChange(oldMod, newMod *Module) {
	for _, h := range mh {
	h(oldMod, newMod)
	}
	}

We can introduce additional optional argument update-command-id alongside uri

terraform-ls/internal/langserver/handlers/command/module_providers.go

Line 29 in 2027413

func ModuleProvidersHandler(ctx context.Context, args cmd.CommandArgs) (interface{}, error) {

which clients can use to register a particular client-side command ID which the server will call and pass updated data that way.

Alternatively we could follow the LSP spec a bit more closely and use that command just to trigger client-side re-reading of data, i.e. have client call module.providers again. This does require another RPC roundtrip, but aligns more with e.g. semantic tokens or code lens refresh.

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