cmd/internal/obj: remove Biobuf unget

This change applies CL 9365 to the copy of Biobuf in cmd/internal/obj.

In the process I discovered that some of the methods that should have been
checking the unget buffer before reading were not and it was probably just
dumb luck that we handn't hit these issues before; Bungetc is only used in
one place in cmd/internal/gc and only an unlikely code path.

Change-Id: Ifa0c5c08442e9fe951a5078c6e9ec77a8a4dc2ff
Reviewed-on: https://go-review.googlesource.com/9529Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>

src/cmd/internal/gc/lex.go

@@ -1865,20 +1865,21 @@ func getc() int {
 			curio.cp = curio.cp[1:]
 		}
 	} else {
-		var c1 int
-		var c2 int
 	loop:
 		c = obj.Bgetc(curio.bin)
 		if c == 0xef {
-			c1 = obj.Bgetc(curio.bin)
-			c2 = obj.Bgetc(curio.bin)
-			if c1 == 0xbb && c2 == 0xbf {
+			buf, err := curio.bin.Peek(2)
+			if err != nil {
+				log.Fatalf("getc: peeking: %v", err)
+			}
+			if buf[0] == 0xbb && buf[1] == 0xbf {
 				yyerrorl(int(lexlineno), "Unicode (UTF-8) BOM in middle of file")
+
+				// consume BOM bytes
+				obj.Bgetc(curio.bin)
+				obj.Bgetc(curio.bin)
 				goto loop
 			}
-
-			obj.Bungetc(curio.bin)
-			obj.Bungetc(curio.bin)
 		}
 	}
 

src/cmd/internal/obj/util.go

@@ -28,12 +28,10 @@ func Cputime() float64 {
 }
 
 type Biobuf struct {
-	unget    [2]int
-	numUnget int
-	f        *os.File
-	r        *bufio.Reader
-	w        *bufio.Writer
-	linelen  int
+	f       *os.File
+	r       *bufio.Reader
+	w       *bufio.Writer
+	linelen int
 }
 
 func Bopenw(name string) (*Biobuf, error) {
@@ -116,18 +114,11 @@ func Bread(b *Biobuf, p []byte) int {
 }
 
 func Bgetc(b *Biobuf) int {
-	if b.numUnget > 0 {
-		b.numUnget--
-		return int(b.unget[b.numUnget])
-	}
 	c, err := b.r.ReadByte()
-	r := int(c)
 	if err != nil {
-		r = -1
+		return -1
 	}
-	b.unget[1] = b.unget[0]
-	b.unget[0] = r
-	return r
+	return int(c)
 }
 
 func Bgetrune(b *Biobuf) int {
@@ -146,6 +137,10 @@ func (b *Biobuf) Read(p []byte) (int, error) {
 	return b.r.Read(p)
 }
 
+func (b *Biobuf) Peek(n int) ([]byte, error) {
+	return b.r.Peek(n)
+}
+
 func Brdline(b *Biobuf, delim int) string {
 	s, err := b.r.ReadBytes(byte(delim))
 	if err != nil {
@@ -181,10 +176,6 @@ func Blinelen(b *Biobuf) int {
 	return b.linelen
 }
 
-func Bungetc(b *Biobuf) {
-	b.numUnget++
-}
-
 func Bterm(b *Biobuf) error {
 	var err error
 	if b.w != nil {