package routetable

import (
	"net/netip"
	"runtime"
	"testing"
	"time"
)

// TestPeersDownLetsReadsIn reads the table's counts again and again while a
// peer that carries more than two batches of prefixes goes down. PeersDown lets
// the lock go after every walkBatch prefixes, so a read must get in while the
// peer still carries some of its prefixes but no longer all.
func TestPeersDownLetsReadsIn(t *testing.T) {
	t.Parallel()

	const (
		asn  = 1000
		peer = "192.0.2.1"
		// prefixCount is more prefixes than two of walkBatch.
		prefixCount = 2*walkBatch + 1
	)

	routeTime := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
	table := New(Params{})
	addr := netip.MustParseAddr("1.0.0.0")

	for range prefixCount {
		prefix := netip.PrefixFrom(addr, addr.BitLen())
		table.Announce(prefix, asn, Route{Peer: peer, Time: routeTime})

		addr = addr.Next()
	}

	// Hold a read lock until PeersDown waits for the lock, so that the first
	// read below starts once PeersDown holds it. TryRLock fails once a writer
	// waits.
	table.mu.RLock()

	peerDownDone := make(chan struct{})

	go func() {
		defer close(peerDownDone)

		table.PeersDown([]string{peer})
	}()

	for table.mu.TryRLock() {
		table.mu.RUnlock()
		runtime.Gosched()
	}

	table.mu.RUnlock()

	for {
		routes := table.Counts().IPv4Routes
		if routes > 0 && routes < prefixCount {
			break
		}

		select {
		case <-peerDownDone:
			t.Fatal("no read got in while PeersDown ran")
		default:
		}
	}

	<-peerDownDone
}