Support multiple folders per workspace

[radeksimko]

Depends on hashicorp/vscode-terraform#627

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Current Version

0.16.1

Problem Definition

Typically folders represent Terraform modules and the LS needs to obtain certain metadata about each module, before providing much of the functionality, such as completion, hover or semantic highlighting.

Obtaining such metadata can be CPU/memory intensive, which in itself shouldn't be problem as we cap CPU consumption by limiting number of tasks that can run at any given time based on available CPU.

However such caps don't apply when multiple instances of LS run at the same time as it is the client which orchestrates launching these instances and the instances do not know about each other.

Attempted Solutions

Currently the LS does not support multiple folders per workspace. As described above this may cause CPU spikes.

Proposal

Support multiple folders natively via LSP, as per https://microsoft.github.io/language-server-protocol/specifications/specification-current/#workspace_workspaceFolders
so that clients can run just a single instance of LS and just pass list of folders.

For clients that support this, we may also consider disabling the module walker.

References

hashicorp/vscode-terraform#591

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