asm: handle EOF better

Add some error catches to prevent looping at EOF.
Also give better diagnostics.
Also add tests for these cases.

Fixes #12656.

Change-Id: I1355fc149b71c868e740bfa53de29c25d160777d
Reviewed-on: https://go-review.googlesource.com/14710Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/asm/internal/lex/input.go

@@ -63,7 +63,12 @@ func predefine(defines flags.MultiFlag) map[string]*Macro {
 	return macros
 }
 
+var panicOnError bool // For testing.
+
 func (in *Input) Error(args ...interface{}) {
+	if panicOnError {
+		panic(fmt.Errorf("%s:%d: %s", in.File(), in.Line(), fmt.Sprintln(args...)))
+	}
 	fmt.Fprintf(os.Stderr, "%s:%d: %s", in.File(), in.Line(), fmt.Sprintln(args...))
 	os.Exit(1)
 }
@@ -113,6 +118,10 @@ func (in *Input) Next() ScanToken {
 			}
 			fallthrough
 		default:
+			if tok == scanner.EOF && len(in.ifdefStack) > 0 {
+				// We're skipping text but have run out of input with no #endif.
+				in.Error("unclosed #ifdef or #ifndef")
+			}
 			in.beginningOfLine = tok == '\n'
 			if in.enabled() {
 				in.text = in.Stack.Text()
@@ -251,6 +260,9 @@ func (in *Input) macroDefinition(name string) ([]string, []Token) {
 	var tokens []Token
 	// Scan to newline. Backslashes escape newlines.
 	for tok != '\n' {
+		if tok == scanner.EOF {
+			in.Error("missing newline in macro definition for %q\n", name)
+		}
 		if tok == '\\' {
 			tok = in.Stack.Next()
 			if tok != '\n' && tok != '\\' {

src/cmd/asm/internal/lex/lex_test.go

@@ -258,3 +258,76 @@ func drain(input *Input) string {
 		buf.WriteString(input.Text())
 	}
 }
+
+type badLexTest struct {
+	input string
+	error string
+}
+
+var badLexTests = []badLexTest{
+	{
+		"3 #define foo bar\n",
+		"'#' must be first item on line",
+	},
+	{
+		"#ifdef foo\nhello",
+		"unclosed #ifdef or #ifndef",
+	},
+	{
+		"#ifndef foo\nhello",
+		"unclosed #ifdef or #ifndef",
+	},
+	{
+		"#ifdef foo\nhello\n#else\nbye",
+		"unclosed #ifdef or #ifndef",
+	},
+	{
+		"#define A() A()\nA()",
+		"recursive macro invocation",
+	},
+	{
+		"#define A a\n#define A a\n",
+		"redefinition of macro",
+	},
+	{
+		"#define A a",
+		"no newline after macro definition",
+	},
+}
+
+func TestBadLex(t *testing.T) {
+	for _, test := range badLexTests {
+		input := NewInput(test.error)
+		input.Push(NewTokenizer(test.error, strings.NewReader(test.input), nil))
+		err := firstError(input)
+		if err == nil {
+			t.Errorf("%s: got no error", test.error)
+			continue
+		}
+		if !strings.Contains(err.Error(), test.error) {
+			t.Errorf("got error %q expected %q", err.Error(), test.error)
+		}
+	}
+}
+
+// firstError returns the first error value triggered by the input.
+func firstError(input *Input) (err error) {
+	panicOnError = true
+	defer func() {
+		panicOnError = false
+		switch e := recover(); e := e.(type) {
+		case nil:
+		case error:
+			err = e
+		default:
+			panic(e)
+		}
+	}()
+
+	for {
+		tok := input.Next()
+		if tok == scanner.EOF {
+			return
+		}
+	}
+}