Fix build target completion crash from line number differences

CHANGELOG.md

@@ -27,6 +27,7 @@ All notable changes to the Docker Language Server will be documented in this fil
 - Compose
   - textDocument/completion
     - fix panic in code completion in an empty file ([#196](https://github.com/docker/docker-language-server/issues/196))
+    - fix line number assumption issues when using code completion for build targets ([#210](https://github.com/docker/docker-language-server/issues/210))
   - textDocument/hover
     - ensure results are returned even if the file has CRLFs ([#205](https://github.com/docker/docker-language-server/issues/205))
 - Bake

internal/compose/completion.go

@@ -360,8 +360,10 @@ func buildTargetCompletionItems(params *protocol.CompletionParams, manager *docu
 				if _, ok := path[3].Value.(*ast.NullNode); ok {
 					return createBuildStageItems(params, manager, dockerfilePath, "", prefixLength), true
 				} else if prefix, ok := path[3].Value.(*ast.StringNode); ok {
-					offset := int(params.Position.Character) - path[3].Value.GetToken().Position.Column + 1
-					return createBuildStageItems(params, manager, dockerfilePath, prefix.Value[0:offset], prefixLength), true
+					if int(params.Position.Line) == path[3].Value.GetToken().Position.Line-1 {
+						offset := int(params.Position.Character) - path[3].Value.GetToken().Position.Column + 1
+						return createBuildStageItems(params, manager, dockerfilePath, prefix.Value[0:offset], prefixLength), true
+					}
 				}
 			}
 		}

internal/compose/completion_test.go

@@ -3410,6 +3410,44 @@ services:
 				}
 			},
 		},
+		{
+			name:              "completion on line different from the target attribute",
+			dockerfileContent: "FROM alpine as astage",
+			content: `
+services:
+  postgres:
+    build:
+      target:
+        `,
+			line:      5,
+			character: 8,
+			list: func() *protocol.CompletionList {
+				return &protocol.CompletionList{
+					Items: []protocol.CompletionItem{
+						{
+							Label:         "astage",
+							Documentation: "alpine",
+							TextEdit:      textEdit("astage", 5, 8, 0),
+						},
+					},
+				}
+			},
+		},
+		{
+			name:              "completion on a different line from target that already has content",
+			dockerfileContent: "FROM scratch AS stage",
+			content: `
+services:
+  postgres:
+    build:
+      target: ab
+        `,
+			line:      5,
+			character: 8,
+			list: func() *protocol.CompletionList {
+				return nil
+			},
+		},
 		{
 			name:              "no build stages suggested if dockerfile_inline used",
 			dockerfileContent: "FROM scratch AS base",