fix(lsp): guard Locate against out-of-bounds ranges

lang/lsp/lsp_methods.go

@@ -311,8 +311,15 @@ func (cli *LSPClient) Locate(id Location) (string, error) {
 
 	text := f.Text
 	// get block text of range
+	if id.Range.Start.Line < 0 || id.Range.Start.Line >= len(f.LineCounts) ||
+		id.Range.End.Line < 0 || id.Range.End.Line >= len(f.LineCounts) {
+		return "", fmt.Errorf("range %s out of bounds for %s", id.Range, id.URI)
+	}
 	start := f.LineCounts[id.Range.Start.Line] + id.Range.Start.Character
 	end := f.LineCounts[id.Range.End.Line] + id.Range.End.Character
+	if start < 0 || end > len(text) || start > end {
+		return "", fmt.Errorf("range %s out of bounds for %s", id.Range, id.URI)
+	}
 	return text[start:end], nil
 }
 

lang/lsp/lsp_methods_test.go

@@ -0,0 +1,63 @@
+// Copyright 2025 CloudWeGo Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package lsp
+
+import (
+	"testing"
+
+	"github.com/cloudwego/abcoder/lang/utils"
+)
+
+func TestLocateOutOfBounds(t *testing.T) {
+	const uri = DocumentURI("file:///tmp/sample.py")
+	text := "def f():\n    return 1\n"
+	cli := &LSPClient{
+		files: map[DocumentURI]*TextDocumentItem{
+			uri: {
+				URI:        uri,
+				Text:       text,
+				LineCounts: utils.CountLines(text),
+			},
+		},
+	}
+
+	loc := func(sl, sc, el, ec int) Location {
+		l := Location{URI: uri}
+		l.Range.Start.Line, l.Range.Start.Character = sl, sc
+		l.Range.End.Line, l.Range.End.Character = el, ec
+		return l
+	}
+
+	got, err := cli.Locate(loc(0, 0, 0, 3))
+	if err != nil {
+		t.Fatalf("valid range returned error: %v", err)
+	}
+	if got != "def" {
+		t.Fatalf("valid range = %q, want %q", got, "def")
+	}
+
+	cases := map[string]Location{
+		"line past EOF":      loc(13, 0, 13, 6),
+		"inverted offsets":   loc(1, 6, 0, 1),
+		"character past EOF": loc(0, 0, 1, 1000),
+	}
+	for name, l := range cases {
+		t.Run(name, func(t *testing.T) {
+			if _, err := cli.Locate(l); err == nil {
+				t.Fatalf("expected error for %s, got nil", name)
+			}
+		})
+	}
+}