Fix ringbuffer ring size reporting

ringbuf/reader_test.go

@@ -59,7 +59,7 @@ func TestRingbufReader(t *testing.T) {
 			}
 			defer rd.Close()
 
-			if uint32(rd.BufferSize()) != 2*events.MaxEntries() {
+			if uint32(rd.BufferSize()) != events.MaxEntries() {
 				t.Errorf("expected %d BufferSize, got %d", events.MaxEntries(), rd.BufferSize())
 			}
 

ringbuf/ring.go

@@ -77,8 +77,10 @@ func (rr *ringReader) isEmpty() bool {
 	return prod == cons
 }
 
+// The data pages in ring buffers are mapped twice in a single contiguous virtual region
+// Therefore the true size is half the size of the mmaped region
 func (rr *ringReader) size() int {
-	return cap(rr.ring)
+	return cap(rr.ring) / 2
 }
 
 // Read a record from an event ring.