Check character offsets to prevent string indexing errors

CHANGELOG.md

@@ -9,6 +9,7 @@ All notable changes to the Docker Language Server will be documented in this fil
 - Compose
   - textDocument/completion
     - prevent errors if an empty JSON object is the content of the YAML file ([#330](https://github.com/docker/docker-language-server/issues/330))
+    - check character offset before processing to prevent errors ([#333](https://github.com/docker/docker-language-server/issues/333))
 
 ## [0.12.0] - 2025-06-12
 

internal/compose/completion.go

@@ -442,8 +442,9 @@ func buildTargetCompletionItems(params *protocol.CompletionParams, manager *docu
 				} else if prefix, ok := path[3].Value.(*ast.StringNode); ok {
 					if int(params.Position.Line) == path[3].Value.GetToken().Position.Line-1 {
 						offset := int(params.Position.Character) - path[3].Value.GetToken().Position.Column + 1
-						// offset can be greater than the length if there's just empty whitespace after the string value
-						if offset <= len(prefix.Value) {
+						// offset can be greater than the length if there's just empty whitespace after the string value,
+						// must be non-negative, if negative it suggests the cursor is in the whitespace before the attribute's value
+						if offset >= 0 && offset <= len(prefix.Value) {
 							return createBuildStageItems(params, manager, dockerfilePath, prefix.Value[0:offset], prefixLength), true
 						}
 					}

internal/compose/completion_test.go

@@ -4269,6 +4269,20 @@ services:
 				}
 			},
 		},
+		{
+			name:              "target attribute finds nothing",
+			dockerfileContent: "FROM scratch",
+			content: `
+services:
+  postgres:
+    build:
+      target:  base`,
+			line:      4,
+			character: 14,
+			list: func() *protocol.CompletionList {
+				return nil
+			},
+		},
 	}
 
 	composeFileURI := fmt.Sprintf("file:///%v", strings.TrimPrefix(filepath.ToSlash(filepath.Join(os.TempDir(), "compose.yaml")), "/"))