cmd/compile/internal/syntax: fix source buffer refilling

The previous code seems to have an off-by-1 in it somewhere, the
consequence being that we didn't properly preserve all of the old
buffer contents that we intended to.

After spending a while looking at the existing window-shifting logic,
I wasn't able to understand exactly how it was supposed to work or
where the issue was, so I rewrote it to be (at least IMO) more
obviously correct.

Fixes #21938.

Change-Id: I1ed7bbc1e1751a52ab5f7cf0411ae289586dc345
Reviewed-on: https://go-review.googlesource.com/64830
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/syntax/scanner_test.go

@@ -7,6 +7,7 @@ package syntax
 import (
 	"fmt"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -367,3 +368,15 @@ func TestScanErrors(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue21938(t *testing.T) {
+	s := "/*" + strings.Repeat(" ", 4089) + "*/ .5"
+
+	var got scanner
+	got.init(strings.NewReader(s), nil, nil)
+	got.next()
+
+	if got.tok != _Literal || got.lit != ".5" {
+		t.Errorf("got %s %q; want %s %q", got.tok, got.lit, _Literal, ".5")
+	}
+}

src/cmd/compile/internal/syntax/source.go

@@ -164,11 +164,12 @@ func (s *source) fill() {
 			s.lit = append(s.lit, s.buf[s.suf:s.r0]...)
 			s.suf = 1 // == s.r0 after slide below
 		}
-		s.offs += s.r0 - 1
-		r := s.r - s.r0 + 1 // last read char plus one byte
-		s.w = r + copy(s.buf[r:], s.buf[s.r:s.w])
-		s.r = r
-		s.r0 = 1
+		n := s.r0 - 1
+		copy(s.buf[:], s.buf[n:s.w])
+		s.offs += n
+		s.r0 = 1 // eqv: s.r0 -= n
+		s.r -= n
+		s.w -= n
 	}
 
 	// read more data: try a limited number of times