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>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/syntax/scanner_test.go b/src/cmd/compile/internal/syntax/scanner_test.go
index e434db9a91..53995e0c79 100644
--- a/src/cmd/compile/internal/syntax/scanner_test.go
+++ b/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")
+	}
+}