fmt: fix handling of %% verb in Scanf

There were a couple of bugs, including not requiring a percent and
returning the wrong error for a bad format containing %%.

Both are addressed by fixing the first.

Fixes #34180.

Change-Id: If96c0c0258bcb95eec49871437d719cb9d399d9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/202879
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/fmt/scan.go

@@ -940,6 +940,15 @@ const (
 	uintptrBits = 32 << (^uintptr(0) >> 63)
 )
 
+// scanPercent scans a literal percent character.
+func (s *ss) scanPercent() {
+	s.SkipSpace()
+	s.notEOF()
+	if !s.accept("%") {
+		s.errorString("missing literal %")
+	}
+}
+
 // scanOne scans a single value, deriving the scanner from the type of the argument.
 func (s *ss) scanOne(verb rune, arg interface{}) {
 	s.buf = s.buf[:0]
@@ -1203,6 +1212,10 @@ func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err erro
 		if c != 'c' {
 			s.SkipSpace()
 		}
+		if c == '%' {
+			s.scanPercent()
+			continue // Do not consume an argument.
+		}
 		s.argLimit = s.limit
 		if f := s.count + s.maxWid; f < s.argLimit {
 			s.argLimit = f

src/fmt/scan_test.go

@@ -325,6 +325,8 @@ var scanfTests = []ScanfTest{
 	{"%c", " ", &uintVal, uint(' ')},      // %c must accept a blank.
 	{"%c", "\t", &uintVal, uint('\t')},    // %c must accept any space.
 	{"%c", "\n", &uintVal, uint('\n')},    // %c must accept any space.
+	{"%d%%", "23%\n", &uintVal, uint(23)}, // %% matches literal %.
+	{"%%%d", "%23\n", &uintVal, uint(23)}, // %% matches literal %.
 
 	// space handling
 	{"%d", "27", &intVal, 27},
@@ -467,6 +469,9 @@ var multiTests = []ScanfMultiTest{
 	{"X%d", "10X", args(&intVal), nil, "input does not match format"},
 	{"%d%", "42%", args(&intVal), args(42), "missing verb: % at end of format string"},
 	{"%d% ", "42%", args(&intVal), args(42), "too few operands for format '% '"}, // Slightly odd error, but correct.
+	{"%%%d", "xxx 42", args(&intVal), args(42), "missing literal %"},
+	{"%%%d", "x42", args(&intVal), args(42), "missing literal %"},
+	{"%%%d", "42", args(&intVal), args(42), "missing literal %"},
 
 	// Bad UTF-8: should see every byte.
 	{"%c%c%c", "\xc2X\xc2", args(&r1, &r2, &r3), args(utf8.RuneError, 'X', utf8.RuneError), ""},