signatureHelpProvider returns empty response

[jemag]

Server Version

v.0.28.1

Terraform Version

Terraform v0.14.10

Client Version

Neovim (NVIM v0.8.0-dev+537-gffbed8d6c)

Log Output

https://gist.github.com/cb9737660e034ef2b4430b7bf5702c45

Expected Behavior

SignatureHelpProvider capabilities returns nothing

Actual Behavior

SignatureHelpProvider returns empty response

Steps to Reproduce

1. Open any terraform file
2. Notice SignatureHelpProvider capability response

Notes

It seems that currently signature help is not supported (#857). However, the server currently seems to return an empty response instead of simply returning nothing.

This makes it hard for clients of the server to guess if the capability is actually supported or not. See : neovim/neovim#18939

From that issue, to add context:

See microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_formatting for an example. That also makes me think that an empty dict must not be treated as falsy because DocumentFormattingOptions doesn't even have any properties

[radeksimko]

Hi @jemag
Thanks for the report.

This looks like an effect of the Go bindings for LSP which we leverage from gopls, or rather the unfortunate effect of these bindings being generated from TypeScript where a field/variable can freely change type while still communicating the subset of types that are supported.

terraform-ls/internal/protocol/protocol.go

Lines 4319 to 4322 in 14e71fd

	/**
	* The server provides signature help support.
	*/
	SignatureHelpProvider SignatureHelpOptions `json:"signatureHelpProvider,omitempty"`

This of course doesn't translate that well into any strongly typed language such as Go, so gopls maintainers already used interface{} (any type) to account for this in other instances, e.g.

terraform-ls/internal/protocol/protocol.go

Lines 4391 to 4394 in 14e71fd

	/**
	* The server provides folding provider support.
	*/
	FoldingRangeProvider interface{}/* bool | FoldingRangeOptions | FoldingRangeRegistrationOptions*/ `json:"foldingRangeProvider,omitempty"`

The only reason this likely doesn't affect gopls is because it already implements that signatureHelp capability, so that struct is never empty.

We could either send a patch upstream to gopls, to fix the logic which generates the Go bindings, and/or re-implement the whole ServerCapabilities struct internally.

As for a better long-term solution, LSP 3.17 finally introduced machine-readable spec. AFAIK nobody has yet tried to generate Go binding from that, but it's likely going to be much easier, cleaner and safer and it may eradicate problems like this.

---

The same problem likely applies to onTypeFormatting capability which we don't implement (yet)

terraform-ls/internal/protocol/protocol.go

Lines 4381 to 4384 in 14e71fd

	/**
	* The server provides document formatting on typing.
	*/
	DocumentOnTypeFormattingProvider DocumentOnTypeFormattingOptions `json:"documentOnTypeFormattingProvider,omitempty"`

[radeksimko]

Signature help was implemented as part of #1077 and a number of other supporting PRs, so this should no longer be an issue. This is now part of the latest v0.31.0 release.

The other mentioned problem with onTypeFormatting is being addressed as another side effect of #1256

[github-actions]

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