runtime: poison the write barrier buffer during flushing

Currently we reset the write barrier buffer before processing the
pointers in it. As a result, if there were any write barriers in the
code that processes the buffer, it would corrupt the write barrier
buffer and cause us to mark objects without later scanning them.

As far as I can tell, this shouldn't be happening, but rather than
relying on hope (and incomplete static analysis), this CL changes
wbBufFlush1 to poison the write barrier buffer while processing it,
and only reset it once it's done.

Updates #27993. (Unlike many of the other changes for this issue,
there's no need to roll back this CL. It's a good change in its own
right.)

Change-Id: I6d2d9f1b69b89438438b9ee624f3fff9f009e29d
Reviewed-on: https://go-review.googlesource.com/c/154537
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mwbbuf.go

@@ -217,14 +217,16 @@ func wbBufFlush1(_p_ *p) {
 	n := (_p_.wbBuf.next - start) / unsafe.Sizeof(_p_.wbBuf.buf[0])
 	ptrs := _p_.wbBuf.buf[:n]
 
-	// Reset the buffer.
-	_p_.wbBuf.reset()
+	// Poison the buffer to make extra sure nothing is enqueued
+	// while we're processing the buffer.
+	_p_.wbBuf.next = 0
 
 	if useCheckmark {
 		// Slow path for checkmark mode.
 		for _, ptr := range ptrs {
 			shade(ptr)
 		}
+		_p_.wbBuf.reset()
 		return
 	}
 
@@ -275,4 +277,6 @@ func wbBufFlush1(_p_ *p) {
 
 	// Enqueue the greyed objects.
 	gcw.putBatch(ptrs[:pos])
+
+	_p_.wbBuf.reset()
 }