How to debug language server

Hi

How can users of the language server debug it?

I have a valid R file which seems to reliably cause a textDocument/documentSymbol to time out without a response.

I would like to try to debug a running language server, but I am struggling a bit to figure out the workflow.

I started to use tests/testthat/test-symbol.R as a template and I was able to observe this in the log:

[2021-02-16 22:43:16.866] did save: {
  "uri": "file:///home/miles/code/r/blogdown_test/content/post/test.R"
}
[2021-02-16 22:43:17.089] parse_callback called: {
  "uri": "file:///home/miles/code/r/blogdown_test/content/post/test.R",
  "version": null
}
[2021-02-16 22:43:17.641] resolve_callback called: {
  "uri": "file:///home/miles/code/r/blogdown_test/content/post/test.R",
  "version": null
}
[2021-02-16 22:43:18.153] received:  Content-Length: 159
[2021-02-16 22:43:18.164] received payload.
[2021-02-16 22:43:18.165] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:18.165] document definitions found:  0
[2021-02-16 22:43:18.166] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:18.572] received:  Content-Length: 159
[2021-02-16 22:43:18.584] received payload.
[2021-02-16 22:43:18.585] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:18.586] document definitions found:  0
[2021-02-16 22:43:18.588] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:18.896] received:  Content-Length: 159
[2021-02-16 22:43:18.907] received payload.
[2021-02-16 22:43:18.907] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:18.907] document definitions found:  0
[2021-02-16 22:43:18.908] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:19.216] received:  Content-Length: 159
[2021-02-16 22:43:19.228] received payload.
[2021-02-16 22:43:19.229] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:19.230] document definitions found:  0
[2021-02-16 22:43:19.233] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:19.645] received:  Content-Length: 159
[2021-02-16 22:43:19.657] received payload.
[2021-02-16 22:43:19.658] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:19.659] document definitions found:  0
[2021-02-16 22:43:19.660] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:20.071] received:  Content-Length: 159
[2021-02-16 22:43:20.083] received payload.
[2021-02-16 22:43:20.084] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:20.086] document definitions found:  0
[2021-02-16 22:43:20.088] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:20.396] received:  Content-Length: 159
[2021-02-16 22:43:20.408] received payload.
[2021-02-16 22:43:20.409] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:20.410] document definitions found:  0
[2021-02-16 22:43:20.413] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:20.822] received:  Content-Length: 159
[2021-02-16 22:43:20.835] received payload.
[2021-02-16 22:43:20.836] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:20.837] document definitions found:  0
[2021-02-16 22:43:20.839] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:21.252] received:  Content-Length: 160
[2021-02-16 22:43:21.264] received payload.
[2021-02-16 22:43:21.265] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:21.266] document definitions found:  0
[2021-02-16 22:43:21.269] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:21.681] received:  Content-Length: 160
[2021-02-16 22:43:21.693] received payload.
[2021-02-16 22:43:21.693] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:21.694] document definitions found:  0
[2021-02-16 22:43:21.696] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:22.107] received:  Content-Length: 160
[2021-02-16 22:43:22.119] received payload.
[2021-02-16 22:43:22.120] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:22.121] document definitions found:  0
[2021-02-16 22:43:22.122] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:22.534] received:  Content-Length: 160
[2021-02-16 22:43:22.546] received payload.
[2021-02-16 22:43:22.547] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:22.549] document definitions found:  0
[2021-02-16 22:43:22.550] deliver:  ["Response", "Message", "R6"]
[2021-02-16 22:43:22.962] received:  Content-Length: 160
[2021-02-16 22:43:22.974] received payload.
[2021-02-16 22:43:22.975] handling request:  textDocument/documentSymbol
[2021-02-16 22:43:22.976] document definitions found:  0

loop continues...

So it seems there is some issue in the symbol parsing setup. I would like to stick a browser() call in document_symbol_reply() to try to figure it out, but then need to be running the languageserver in an interactive session. So I can't figure out how to start a server interactively that I can address e.g. in my interactive session I tried:

languageserver::run(debug = "~/repos/languageserver/logs/log.txt", port = 8888)
Error in socketConnection(host = host, port = port, open = "r+b") : 
  cannot open the connection
In addition: Warning message:
In socketConnection(host = host, port = port, open = "r+b") :
  localhost:8888 cannot be opened

Is this supposed to work? Looking into the code... wouldn't it need socketConnection(server = TRUE)?

Is there some way I can use the stdio mode and connect a language server client to a language server that isn't in a child process?

A bit lost. I am sure you have a good workflow for this. Please share!

Start with tests/testthat/test-symbol.R, document_symbol_reply(), and languageserver::run() to understand the reported documentSymbol loop and current debug setup. Investigate the stdio and socket connection paths described in the issue. Done means documenting a reproducible workflow for interactively debugging a running language server, including how to connect a client and inspect symbol parsing.