Allow subscribing to updates of module.calls data

[radeksimko]

Depends on #723

---

Use-cases

#632 introduced a custom command to allow clients obtain data about module calls. 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Clients ask repeatedly - can lead to either excessive amount of requests and/or outdated info.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.calls 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_calls.go

Line 32 in 2027413

func ModuleCallsHandler(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.calls again. This does require another RPC roundtrip, but aligns more with e.g. semantic tokens or code lens refresh.

Implementation Notes

We can try to calculate the response before sending and compare hash that client could also calculate, to avoid sending the duplicate data repeatedly.

[jpogran]

While we can hook this up quickly because of 722, until #725 is done we won't detect new information to show anyway.

[github-actions]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.