TextDocumentSignatureHelp throws IndexOutOfRangeException

[oskarskog]

I'm seeing a lot of these in my Neovim :messages, happens after I open a file and move the cursor around for a bit.

RPC[Error] code_name = InternalError, message = "System.IndexOutOfRangeException: Index was outside the bounds of the array.\
   at <StartupCode$FsAutoComplete-Core>.$FileSystem.TryGetPrevChar@248.Invoke(Position np) in /home/runner/work/FsAutoComplete/FsAutoComplete/src/FsAutoComplete.Core/FileSystem.fs:line 248\
   at Microsoft.FSharp.Core.OptionModule.Map[T,TResult](FSharpFunc`2 mapping, FSharpOption`1 option) in D:\\a\\_work\\1\\s\\src\\fsharp\\FSharp.Core\\option.fs:line 53\
   at FsAutoComplete.SignatureHelp.getSignatureHelpFor@215.Invoke(Unit unitVar) in /home/runner/work/FsAutoComplete/FsAutoComplete/src/FsAutoComplete.Core/SignatureHelp.fs:line 224\
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvoke[T,TResult](AsyncActivation`1 ctxt, TResult result1, FSharpFunc`2 part2) in D:\\a\\_work\\1\\s\\src\\fsharp\\FSharp.Core\\async.fs:line 447\
   at <StartupCode$fsautocomplete>.$AdaptiveFSharpLspServer.FsAutoComplete-Lsp-IFSharpLspServer-TextDocumentSignatureHelp@1890-3.Invoke(AsyncActivation`1 ctxt)\
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\\a\\_work\\1\\s\\src\\fsharp\\FSharp.Core\\async.fs:line 139"

Edit: I'm on 0.58.2

[vain0x]

Repro

Let Program.fs be:

().ToString(

)

Put the cursor on the second empty line and press ,.

Request is:

[Trace - 23:45:49] Sending request 'textDocument/signatureHelp - (137)'.
Params: {
    "textDocument": {
        "uri": "file:///home/owner/repo/fs-playground/Program.fs"
    },
    "position": {
        "line": 1,
        "character": 1
    },
    "context": {
        "isRetrigger": false,
        "triggerCharacter": ",",
        "triggerKind": 2
    }
}

Response is:

[23:45:49.052 ERR] [FsAutoComplete.Lsp.AdaptiveFSharpLspServer] TextDocumentSignatureHelp Request: {"TextDocument": {"Uri": "file:///home/owner/repo/fs-playground/Program.fs", "$type": "TextDocumentIdentifier"}, "Position": {"Line": 1, "Character": 1, "$type": "Position"}, "Context": {"Value": {"TriggerKind": "TriggerCharacter", "TriggerCharacter": {"Value": ",", "$type": "Some"}, "IsRetrigger": false, "ActiveSignatureHelp": null, "$type": "SignatureHelpContext"}, "$type": "Some"}, "$type": "SignatureHelpParams"}
System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.String.get_Chars(Int32 index)
   at <StartupCode$FsAutoComplete-Core>.$FileSystem.TryGetPrevChar@248.Invoke(Position np) in /home/runner/work/FsAutoComplete/FsAutoComplete/src/FsAutoComplete.Core/FileSystem.fs:line 248
   at Microsoft.FSharp.Core.OptionModule.Map[T,TResult](FSharpFunc`2 mapping, FSharpOption`1 option) in D:\a\_work\1\s\src\FSharp.Core\option.fs:line 98
   at FsAutoComplete.SignatureHelp.getSignatureHelpFor@214.Invoke(Unit unitVar) in /home/runner/work/FsAutoComplete/FsAutoComplete/src/FsAutoComplete.Core/SignatureHelp.fs:line 223
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvoke[T,TResult](AsyncActivation`1 ctxt, TResult result1, FSharpFunc`2 part2) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 510
   at FsToolkit.ErrorHandling.AsyncResult.zip@198-8.Invoke(AsyncActivation`1 ctxt)
   at FsToolkit.ErrorHandling.Async.zip@25-3.Invoke(AsyncActivation`1 ctxt)
   at <StartupCode$fsautocomplete>.$AdaptiveFSharpLspServer.FsAutoComplete-Lsp-IFSharpLspServer-TextDocumentSignatureHelp@2434-9.Invoke(AsyncActivation`1 ctxt)
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 148
[Trace - 23:45:49] Received response 'textDocument/signatureHelp - (137)' in 3ms. Request failed: System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.String.get_Chars(Int32 index)
   at <StartupCode$FsAutoComplete-Core>.$FileSystem.TryGetPrevChar@248.Invoke(Position np) in /home/runner/work/FsAutoComplete/FsAutoComplete/src/FsAutoComplete.Core/FileSystem.fs:line 248
   at Microsoft.FSharp.Core.OptionModule.Map[T,TResult](FSharpFunc`2 mapping, FSharpOption`1 option) in D:\a\_work\1\s\src\FSharp.Core\option.fs:line 98
   at FsAutoComplete.SignatureHelp.getSignatureHelpFor@214.Invoke(Unit unitVar) in /home/runner/work/FsAutoComplete/FsAutoComplete/src/FsAutoComplete.Core/SignatureHelp.fs:line 223
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvoke[T,TResult](AsyncActivation`1 ctxt, TResult result1, FSharpFunc`2 part2) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 510
   at FsToolkit.ErrorHandling.AsyncResult.zip@198-8.Invoke(AsyncActivation`1 ctxt)
   at FsToolkit.ErrorHandling.Async.zip@25-3.Invoke(AsyncActivation`1 ctxt)
   at <StartupCode$fsautocomplete>.$AdaptiveFSharpLspServer.FsAutoComplete-Lsp-IFSharpLspServer-TextDocumentSignatureHelp@2434-9.Invoke(AsyncActivation`1 ctxt)
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 148 (-32603).

Machine infos

• Operating system: Linux
• Arch: x64
• VSCode: 1.75.1
• UI Kind: Desktop
• Ionide: 7.5.1
• Runtime: netcore
• Dotnet version: 7.0.200

Cause

• The exception was raised from the getSignatureHelpFor function.
• PrevPos method is used to compute the "previous position" but it sometimes returns a position at the end of a line.
• The position is passed in GetCharUnsafe that assumes the position is at a character, which caused the error

Remarks:

• PrevPos probably needs to skip empty lines in a loop to find the previous character
• Column - 1 and mkPos (..) 0 in NextPos, and pos.Column = 0 in TryGetChar are suspicious since a column number should start with 1 here (I don't find any case to cause an actual error)

EDIT: Link fixed

[baronfel]

Excellent diagnosis - would you be interested in contributing a test + fix for this?

[vain0x]

Yes, I would like to try.

[baronfel]

I'd start by adding a test to SignatureHelpTests and debug that with dotnet run --filter "<fully qualified test name>" with the scenario you've provided above. Let me know if that doesn't work and we can look at other approaches!