Fix panic in parser when first token is invalid

newParser was calling p.next(), which could trigger a scanner error
that results in a panic.  Move the first p.next() into parse(p), which
correctly converts the paanic into a reportable error.

Bug: 254831383
Test: TestParserError
Change-Id: I2a427010379bb8dd5087550c7f159499cbb84066

parser/parser.go

@@ -70,6 +70,7 @@ func parse(p *parser) (file *File, errs []error) {
 		}
 	}()
 
+	p.next()
 	defs := p.parseDefinitions()
 	p.accept(scanner.EOF)
 	errs = p.errors
@@ -100,6 +101,7 @@ func Parse(filename string, r io.Reader, scope *Scope) (file *File, errs []error
 
 func ParseExpression(r io.Reader) (value Expression, errs []error) {
 	p := newParser(r, NewScope(nil))
+	p.next()
 	value = p.parseExpression()
 	p.accept(scanner.EOF)
 	errs = p.errors
@@ -124,7 +126,6 @@ func newParser(r io.Reader, scope *Scope) *parser {
 	}
 	p.scanner.Mode = scanner.ScanIdents | scanner.ScanInts | scanner.ScanStrings |
 		scanner.ScanRawStrings | scanner.ScanComments
-	p.next()
 	return p
 }
 

parser/parser_test.go

@@ -1214,7 +1214,36 @@ func TestParseValidInput(t *testing.T) {
 	}
 }
 
-// TODO: Test error strings
+func TestParserError(t *testing.T) {
+	testcases := []struct {
+		name  string
+		input string
+		err   string
+	}{
+		{
+			name:  "invalid first token",
+			input: "\x00",
+			err:   "invalid character NUL",
+		},
+		// TODO: test more parser errors
+	}
+
+	for _, tt := range testcases {
+		t.Run(tt.name, func(t *testing.T) {
+			r := bytes.NewBufferString(tt.input)
+			_, errs := ParseAndEval("", r, NewScope(nil))
+			if len(errs) == 0 {
+				t.Fatalf("missing expected error")
+			}
+			if g, w := errs[0], tt.err; !strings.Contains(g.Error(), w) {
+				t.Errorf("expected error %q, got %q", w, g)
+			}
+			for _, err := range errs[1:] {
+				t.Errorf("got unexpected extra error %q", err)
+			}
+		})
+	}
+}
 
 func TestParserEndPos(t *testing.T) {
 	in := `