Crashing String.slice after editing file/module endings

[florianb]

Environment

• Erlang/OTP 26 [erts-14.0.2] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]
• Elixir 1.15.4 (compiled with Erlang/OTP 26)
• Elixir Language Server version: 0.17.0
• Operating system: Alpine Linux v3.18; Linux 4d993fe017da 6.4.16-linuxkit # 1 SMP PREEMPT_DYNAMIC Sat Sep 23 13:39:52 UTC 2023 x86_64 Linux
• VSCode: 1.83.0
• ElixirLS: 0.17.0

Current behavior

The LS seems to crash when adding/removing the last end in this module:

Change

defmodule T do
  defdelegate, to:
  def t(), do: :smth e
  defp u do

  end
end

to

defmodule T do
  defdelegate, to:
  def t(), do: :smth e
  defp u do

  end

It might be, that the appearance/disappearance of the last line/last "newline" plays a role here. I noticed my Editor currently seems to strip the last line and this happens a lot when quickly edditing in the last section of the file.

Log

[Error - 7:42:30 AM] GenServer ElixirLS.LanguageServer.Server terminating
** (FunctionClauseError) no function clause matching in String.slice/2
    (elixir 1.15.4) lib/string.ex:2241: String.slice(nil, 0..0)
    (language_server 0.17.0) lib/language_server/source_file.ex:316: ElixirLS.LanguageServer.SourceFile.elixir_character_to_lsp/2
    (language_server 0.17.0) lib/language_server/diagnostics.ex:324: ElixirLS.LanguageServer.Diagnostics.range/2
    (language_server 0.17.0) lib/language_server/diagnostics.ex:210: anonymous fn/2 in ElixirLS.LanguageServer.Diagnostics.publish_file_diagnostics/3
    (elixir 1.15.4) lib/enum.ex:1693: Enum."-map/2-lists^map/1-1-"/2
    (elixir 1.15.4) lib/enum.ex:1693: Enum."-map/2-lists^map/1-1-"/2
    (language_server 0.17.0) lib/language_server/diagnostics.ex:192: ElixirLS.LanguageServer.Diagnostics.publish_file_diagnostics/3
    (language_server 0.17.0) lib/language_server/server.ex:1187: anonymous fn/4 in ElixirLS.LanguageServer.Server.publish_diagnostics/3

[lukaszsamson]

That's most likely race conditions between publishing diagnostics and source file changes. This line assumes the source file has the line with error https://github.com/elixir-lsp/elixir-ls/blob/42f16b83708bd2068f8c54b3631b53edc99d7525/apps/language_server/lib/language_server/diagnostics.ex#L322

[florianb]

Shouldn't the file already being read at this point, how can this differ?

[lukaszsamson]

Async parsing and async text document synchronisation. When the diagnostic is being published the line is not there anymore and we need the line to convert UTF8 code points to UTF16. I know what needs fixing here :)

[florianb]

Thank you very much for your work! 🙏