x/tools/gopls: Completion: OOB panic in server.toProtocolCompletionItems

[adonovan]

#!stacks
"goPanicSliceB" && "completion.Selection.Suffix" && "toProtocolCompletionItems"

Issue created by stacks.

func (p Selection) PrefixRange() (protocol.Range, error) {
	return p.mapper.PosRange(p.tokFile, p.start, p.cursor)
}

func (p Selection) Prefix() string {
	return p.content[:p.cursor-p.start]
}

func (p Selection) Suffix() string {
	return p.content[p.cursor-p.start:]  // <--- out of bounds panic, after a successful call to PrefixRange 
}

panicSliceB indicates failure of the p.cursor-p.start <= len(p.content) assertion.

This stack BigXFw was reported by telemetry:

• crash/crash
• runtime.gopanic:+69
• runtime.goPanicSliceB:+2
• golang.org/x/tools/gopls/internal/golang/completion.Selection.Suffix:=367
• golang.org/x/tools/gopls/internal/server.toProtocolCompletionItems:+9
• golang.org/x/tools/gopls/internal/server.(*server).Completion:+52
• golang.org/x/tools/gopls/internal/protocol.serverDispatch:+193
• golang.org/x/tools/gopls/internal/lsprpc.(*streamServer).ServeStream.ServerHandler.func3:+5
• golang.org/x/tools/gopls/internal/lsprpc.(*streamServer).ServeStream.handshaker.func4:+52
• golang.org/x/tools/gopls/internal/protocol.Handlers.MustReplyHandler.func1:+2
• golang.org/x/tools/gopls/internal/protocol.Handlers.AsyncHandler.func2.2:+3
• runtime.goexit:+0

golang.org/x/tools/gopls@v0.16.0 go1.23.12 darwin/amd64 gopls.client=vscode (5)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=darwin GOARCH=amd64 GOTOOLCHAIN=go1.23.12 go install golang.org/x/tools/gopls@v0.16.0 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less

[gabyhelp]

Related Issues

• x/tools/gopls: Completion: nil deref panic in addFieldItems #72828 (closed)
• x/tools/gopls: OOB index crash in completion.expectedReturnStmtType #70636 (closed)
• x/tools/gopls: out-of-bounds slice panic in bug in frob.(*reader).bytes #71244
• x/tools/gopls: slice OOB panic in fixAST on array type #70745 (closed)
• x/tools/gopls: impossible crash in strings.HasPrefix #76627
• x/tools/gopls: Completion: SEGV in deepSearch (cand slice has invalid data pointer) #73263
• x/tools/gopls: "slice bounds out of range" crash in ExtractToNewFile #70553
• x/tools/gopls: Completion: "ZeroString for unexpected type" panic #70744 (closed)
• x/tools/gopls: crash in completion.Selection.Suffix #68169 (closed)
• x/tools/gopls: nil Signature (?) deref in Completion #70634 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[adonovan]

This construction of a Selection in golang/completion/definition.go seems to violate the invariant. (It is merely the first one I found; there may be others. A little audit is needed.)

func definition(path []ast.Node, obj types.Object, pgf *parsego.File) ([]CompletionItem, *Selection) {
...
	start := path[0].Pos()
	end := path[0].End()
	sel := &Selection{
		content: "",
		cursor:  start,
		tokFile: pgf.Tok,
		start:   start,
		end:     end,
		mapper:  pgf.Mapper,
	}

[gopherbot]

Change https://go.dev/cl/733640 mentions this issue: gopls/completion: check Selection invariant