ctxbackground: fix false negative and unsafe autofix for closures

pkg/linters/ctxbackground/ctxbackground.go

@@ -40,12 +40,17 @@ func run(pass *analysis.Pass) (any, error) {
 			continue
 		}
 
-		for encl := range cur.Enclosing((*ast.FuncDecl)(nil)) {
-			fn, ok := encl.Node().(*ast.FuncDecl)
-			if !ok {
+		for encl := range cur.Enclosing((*ast.FuncDecl)(nil), (*ast.FuncLit)(nil)) {
+			var ftype *ast.FuncType
+			switch fn := encl.Node().(type) {
+			case *ast.FuncDecl:
+				ftype = fn.Type
+			case *ast.FuncLit:
+				ftype = fn.Type
+			default:
 				continue
 			}
-			ctxParamName, ok := contextParamName(pass, fn)
+			ctxParamName, ok := contextParamName(pass, ftype)
 			if !ok {
 				break
 			}
@@ -95,15 +100,15 @@ func isContextBackgroundCall(pass *analysis.Pass, call *ast.CallExpr) bool {
 }
 
 // contextParamName returns the first non-blank context.Context parameter name.
-func contextParamName(pass *analysis.Pass, fn *ast.FuncDecl) (string, bool) {
-	if fn.Type.Params == nil {
+func contextParamName(pass *analysis.Pass, ftype *ast.FuncType) (string, bool) {
+	if ftype == nil || ftype.Params == nil {
 		return "", false
 	}
 	ctxType := contextType(pass)
 	if ctxType == nil {
 		return "", false
 	}
-	for _, field := range fn.Type.Params.List {
+	for _, field := range ftype.Params.List {
 		t := pass.TypesInfo.TypeOf(field.Type)
 		if t == nil {
 			continue

pkg/linters/ctxbackground/testdata/src/ctxbackground/ctxbackground.go

@@ -41,3 +41,21 @@ func DoWorkShadowedContext(ctx context.Context) {
 	_ = context.Background()
 	_ = ctx
 }
+
+// flagged: closure whose own signature receives ctx
+func RegisterHandler() {
+	handle(func(ctx context.Context) {
+		_ = context.Background() // want `use the context.Context parameter instead of context.Background\(\)`
+	})
+}
+
+// not flagged: closure with no ctx param nested inside a function that does receive ctx;
+// the nearest enclosing function (the closure) has no ctx, so we stop there.
+func DoWorkWithDetachedClosure(ctx context.Context) {
+	go func() {
+		_ = context.Background()
+	}()
+	_ = ctx
+}
+
+func handle(f func(context.Context)) { f(nil) }

pkg/linters/ctxbackground/testdata/src/ctxbackground/ctxbackground.go.golden

@@ -41,3 +41,21 @@ func DoWorkShadowedContext(ctx context.Context) {
 	_ = context.Background()
 	_ = ctx
 }
+
+// flagged: closure whose own signature receives ctx
+func RegisterHandler() {
+	handle(func(ctx context.Context) {
+		_ = ctx // want `use the context.Context parameter instead of context.Background\(\)`
+	})
+}
+
+// not flagged: closure with no ctx param nested inside a function that does receive ctx;
+// the nearest enclosing function (the closure) has no ctx, so we stop there.
+func DoWorkWithDetachedClosure(ctx context.Context) {
+	go func() {
+		_ = context.Background()
+	}()
+	_ = ctx
+}
+
+func handle(f func(context.Context)) { f(nil) }