core/sync/chan.send: return false if channel is closed while blocked

This commit makes send behave the same as recv: that the call will
return false if the channel is closed while a thread is waiting on the
blocking operation.

Prior logic would have send return true even if the channel was actually
closed rather than read from.

Docs adjusted to make this clear.
Tests added to lock in this behaviour.

tests/core/sync/chan/test_core_sync_chan.odin

@@ -228,6 +228,50 @@ test_full_buffered_closed_chan_deadlock :: proc(t: ^testing.T) {
 	testing.expect(t, !chan.send(ch, 32))
 }
 
+// Ensures that if a thread is doing a blocking send and the channel
+// is closed, it will report false to indicate a failure to complete.
+@test
+test_fail_blocking_send_on_close :: proc(t: ^testing.T) {
+	ch, ch_alloc_err := chan.create(chan.Chan(int), context.allocator)
+	assert(ch_alloc_err == nil, "allocation failed")
+	defer chan.destroy(ch)
+
+	sender := thread.create_and_start_with_poly_data(ch, proc(ch: chan.Chan(int)) {
+		assert(!chan.send(ch, 42))
+	})
+
+	for !chan.can_recv(ch) {
+		thread.yield()
+	}
+
+	testing.expect(t, chan.close(ch))
+	thread.join(sender)
+	thread.destroy(sender)
+}
+
+// Ensures that if a thread is doing a blocking read and the channel
+// is closed, it will report false to indicate a failure to complete.
+@test
+test_fail_blocking_recv_on_close :: proc(t: ^testing.T) {
+	ch, ch_alloc_err := chan.create(chan.Chan(int), context.allocator)
+	assert(ch_alloc_err == nil, "allocation failed")
+	defer chan.destroy(ch)
+
+	reader := thread.create_and_start_with_poly_data(ch, proc(ch: chan.Chan(int)) {
+		v, ok := chan.recv(ch)
+		assert(!ok)
+		assert(v == 0)
+	})
+
+	for !chan.can_send(ch) {
+		thread.yield()
+	}
+
+	testing.expect(t, chan.close(ch))
+	thread.join(reader)
+	thread.destroy(reader)
+}
+
 // Ensures that try_send for unbuffered channels works as expected.
 // If 1 reader of a channel, and 3 try_senders, only one of the senders
 // will succeed and none of them will block.