data-race when publisher catches up with the subscriber

[filipdulic]

Publisher operations during broadcast

1. buffer[wi % size] = item;
2. wi++;

Subscriber operations during try_recv

1. if ri == wi => empty
2. item = buffer[ri % size];
3. loop
   3.a. if wi > ri + size => ri = wi - size;
   3.b. else => ri++, return item

Test Case

wi = 10, ri = 0, size = 10, item = y, buffer[0] = x.

1. pub.1. buffer[10 % 10] = item; <==> buffer[0] = y;
2. sub.1. if 0 == 10 ==> not empty
3. sub.2. item = buffer[0] <==> item = y;
4. sub.3.a. if 10 > 0 + 10 => sub.3.b
5. sub.3.b ri++ => 1, return y.

y is returned even though x should have been, or sub should have moved to next.

[filipdulic]

Possible fix.

Change the check in sub 3.a to if wi >= ri + size => ri = wi - size + 1;

From

if self.wi() > self.ri() + self.size { self.ri.store(self.wi() - self.size, Ordering::Relaxed); }

Into

if self.wi() >= self.ri() + self.size { self.ri.store(self.wi() - self.size + 1, Ordering::Relaxed); }

Test Case

wi == 10, ri == 0, size == 10, item == y, buffer[0] == x, buffer[1] = z;

1. pub.1. buffer[10 % 10] = item; <==> buffer[0] = y;
2. sub.1. if 0 == 10 ==> not empty
3. sub.2. item = buffer[0] <==> item = y;
4. sub.3.a if 10 >= 0 + 10 => ri = 10 -10 +1 = 1; => begin loop again
5. sub.2. item = buffer[1] <==> item = z;
6. sub.3.a if 10 >= 1 + 10 => sub.3.b
7. sub.3.b ri = 2; return z;

next value is returned, even if wi is incremented at any point the check at sub.3.a would catch it.