base64: fix bug that decoder fails to detect corruption

Encoding.Decode() failed to detect trailing garbages if input contains "==" followed by garbage smaller than 3 bytes (for example, it failed to detect "x" in "AA==x"). This patch fixes the bug and adds a few tests.

LGTM=nigeltao
R=golang-codereviews, bradfitz, nigeltao
CC=golang-codereviews
https://golang.org/cl/75340044

src/pkg/encoding/base64/base64.go

@@ -224,21 +224,33 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		var dbuf [4]byte
 		dlen := 4
 
-		for j := 0; j < 4; {
+		for j := range dbuf {
 			if len(src) == 0 {
 				return n, false, CorruptInputError(olen - len(src) - j)
 			}
 			in := src[0]
 			src = src[1:]
-			if in == '=' && j >= 2 && len(src) < 4 {
+			if in == '=' {
 				// We've reached the end and there's padding
-				if len(src)+j < 4-1 {
-					// not enough padding
-					return n, false, CorruptInputError(olen)
-				}
-				if len(src) > 0 && src[0] != '=' {
+				switch j {
+				case 0, 1:
 					// incorrect padding
 					return n, false, CorruptInputError(olen - len(src) - 1)
+				case 2:
+					// "==" is expected, the first "=" is already consumed.
+					if len(src) == 0 {
+						// not enough padding
+						return n, false, CorruptInputError(olen)
+					}
+					if src[0] != '=' {
+						// incorrect padding
+						return n, false, CorruptInputError(olen - len(src) - 1)
+					}
+					src = src[1:]
+				}
+				if len(src) > 0 {
+					// trailing garbage
+					return n, false, CorruptInputError(olen - len(src))
 				}
 				dlen, end = j, true
 				break
@@ -247,7 +259,6 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 			if dbuf[j] == 0xFF {
 				return n, false, CorruptInputError(olen - len(src) - 1)
 			}
-			j++
 		}
 
 		// Pack 4x 6-bit source blocks into 3 byte destination

src/pkg/encoding/base64/base64_test.go

@@ -149,9 +149,13 @@ func TestDecodeCorrupt(t *testing.T) {
 	}{
 		{"", -1},
 		{"!!!!", 0},
+		{"====", 0},
 		{"x===", 1},
+		{"=AAA", 0},
+		{"A=AA", 1},
 		{"AA=A", 2},
-		{"AAA=AAAA", 3},
+		{"AA==A", 4},
+		{"AAA=AAAA", 4},
 		{"AAAAA", 4},
 		{"AAAAAA", 4},
 		{"A=", 1},