pixa/internal/imgcache/pending_counts_internal_test.go

package imgcache

import (
	"context"
	"database/sql"
	"errors"
	"testing"
	"time"
)

// pendingCountsWait is how long a test waits for a count to reach the
// database, or for a write to start waiting for the database's connection.
const pendingCountsWait = 5 * time.Second

// holdDatabase takes the one connection of cache's database, so that every
// other query waits for it, and returns the func that frees it.
func holdDatabase(t *testing.T, cache *Cache) func() {
	t.Helper()

	conn, err := cache.db.Conn(t.Context())
	if err != nil {
		t.Fatalf("failed to take the database connection: %v", err)
	}

	return func() { _ = conn.Close() }
}

// waitForConnectionWaits waits until db.Stats().WaitCount, the number of
// times a caller has waited for a connection, reaches waits.
func waitForConnectionWaits(t *testing.T, db *sql.DB, waits int64) {
	t.Helper()

	deadline := time.Now().Add(pendingCountsWait)

	for db.Stats().WaitCount < waits {
		if time.Now().After(deadline) {
			t.Fatalf("callers waited for the database connection %d times, want %d",
				db.Stats().WaitCount, waits)
		}

		time.Sleep(10 * time.Millisecond)
	}
}

// waitForCounters waits until the cache_stats row holds want.
func waitForCounters(t *testing.T, cache *Cache, want cacheStatsCounters) {
	t.Helper()

	deadline := time.Now().Add(pendingCountsWait)

	for {
		got := readCacheStatsCounters(t, cache)
		if got == want {
			return
		}

		if time.Now().After(deadline) {
			t.Fatalf("counters = %+v, want %+v", got, want)
		}

		time.Sleep(10 * time.Millisecond)
	}
}

// TestService_Get_ReturnsByItsDeadlineWhileTheDatabaseIsBusy holds the
// database's one connection while a request whose fetch is held reaches its
// deadline. The request must still return by its deadline with the
// deadline's error, and its miss must reach the database once the
// connection is free.
func TestService_Get_ReturnsByItsDeadlineWhileTheDatabaseIsBusy(t *testing.T) {
	t.Parallel()

	svc, fixtures, fetcher := setupHeldFetchService(t)

	svc.cache.StartPendingCountWrites()
	defer func() { _ = svc.cache.StopPendingCountWrites(t.Context()) }()

	const timeout = 200 * time.Millisecond

	ctx, cancel := context.WithTimeout(t.Context(), timeout)
	defer cancel()

	results := startGet(ctx, svc, photoVariant(fixtures, 85, FitCover))

	// The request has made its database reads by the time it fetches.
	<-fetcher.started

	releaseDatabase := holdDatabase(t, svc.cache)
	defer releaseDatabase()

	select {
	case got := <-results:
		deadline, _ := ctx.Deadline()
		t.Logf("Get() returned %v after its deadline, error = %v",
			time.Since(deadline), got.err)

		if !errors.Is(got.err, context.DeadlineExceeded) {
			t.Errorf("Get() error = %v, want %v", got.err, context.DeadlineExceeded)
		}
	case <-time.After(timeout + time.Second):
		t.Fatal("request did not return by its deadline while the database was busy")
	}

	releaseDatabase()

	waitForCounters(t, svc.cache, cacheStatsCounters{missCount: 1})
}

// TestService_Get_ManyRequestsPastTheirDeadlineLeaveOneWriteWaiting holds the
// database's one connection while many requests whose fetches are held reach
// their deadline. At most one write may then wait for the connection, and
// every miss must reach the database once it is free.
func TestService_Get_ManyRequestsPastTheirDeadlineLeaveOneWriteWaiting(t *testing.T) {
	t.Parallel()

	svc, fixtures, fetcher := setupHeldFetchService(t)

	svc.cache.StartPendingCountWrites()
	defer func() { _ = svc.cache.StopPendingCountWrites(t.Context()) }()

	const (
		requests = 20
		timeout  = 200 * time.Millisecond
	)

	ctx, cancel := context.WithTimeout(t.Context(), timeout)
	defer cancel()

	// Each request asks for a variant of its own, so each fetches.
	results := make([]<-chan getResult, requests)
	for i := range results {
		results[i] = startGet(ctx, svc, photoVariant(fixtures, i+1, FitCover))
	}

	// Each request has made its database reads by the time it fetches.
	for range requests {
		<-fetcher.started
	}

	releaseDatabase := holdDatabase(t, svc.cache)
	defer releaseDatabase()

	waitsBefore := svc.cache.db.Stats().WaitCount

	// Once one write waits for the connection, any others start waiting too.
	waitForConnectionWaits(t, svc.cache.db, waitsBefore+1)
	time.Sleep(arrivalWait)

	waits := svc.cache.db.Stats().WaitCount - waitsBefore
	t.Logf("%d writes waited for the database", waits)

	if waits > 1 {
		t.Errorf("%d writes waited for the database, want at most 1", waits)
	}

	for _, result := range results {
		select {
		case got := <-result:
			if !errors.Is(got.err, context.DeadlineExceeded) {
				t.Errorf("Get() error = %v, want %v", got.err, context.DeadlineExceeded)
			}
		case <-time.After(time.Second):
			t.Fatal("request did not return by its deadline while the database was busy")
		}
	}

	releaseDatabase()

	waitForCounters(t, svc.cache, cacheStatsCounters{missCount: requests})
}

// TestStats_IncludesPendingCounts counts hits and misses whose writes miss
// their deadline, and checks that Stats adds them to the database's counts
// before they are written.
func TestStats_IncludesPendingCounts(t *testing.T) {
	t.Parallel()

	cache, _ := newEvictionTestCache(t, 1<<30)

	_, err := cache.db.ExecContext(t.Context(),
		`UPDATE cache_stats SET hit_count = 75, miss_count = 25 WHERE id = 1`)
	if err != nil {
		t.Fatal(err)
	}

	// Writes given a context past its deadline fail at once.
	ended, cancel := context.WithDeadline(t.Context(), time.Now())
	defer cancel()

	cache.IncrementStats(ended, true, 0)
	cache.IncrementStats(ended, true, 0)
	cache.IncrementStats(ended, false, 0)

	stats, err := cache.Stats(t.Context())
	if err != nil {
		t.Fatalf("Stats() error = %v", err)
	}

	if stats.HitCount != 77 || stats.MissCount != 26 {
		t.Errorf("HitCount = %d, MissCount = %d, want 77 and 26",
			stats.HitCount, stats.MissCount)
	}

	want := cacheStatsCounters{hitCount: 75, missCount: 25}
	if got := readCacheStatsCounters(t, cache); got != want {
		t.Errorf("counters in the database = %+v, want %+v", got, want)
	}
}

// TestStopPendingCountWrites_WritesPendingCounts holds the database's one
// connection while the goroutine that writes the pending counts waits for it,
// then stops that goroutine. The stop must write the counts once the
// connection is free.
func TestStopPendingCountWrites_WritesPendingCounts(t *testing.T) {
	t.Parallel()

	cache, _ := newEvictionTestCache(t, 1<<30)

	cache.StartPendingCountWrites()
	defer func() { _ = cache.StopPendingCountWrites(t.Context()) }()

	releaseDatabase := holdDatabase(t, cache)
	defer releaseDatabase()

	waitsBefore := cache.db.Stats().WaitCount

	// Writes given a context past its deadline fail at once.
	ended, cancel := context.WithDeadline(t.Context(), time.Now())
	defer cancel()

	cache.IncrementStats(ended, true, 0)
	cache.IncrementStats(ended, false, 0)
	cache.IncrementUpstreamFetch(ended, 1024)
	cache.IncrementTransformCount(ended)

	// The goroutine's write waits for the connection.
	waitForConnectionWaits(t, cache.db, waitsBefore+1)

	stopped := make(chan error, 1)

	go func() {
		stopped <- cache.StopPendingCountWrites(t.Context())
	}()

	// The stop's own write waits for it once the goroutine has returned.
	waitForConnectionWaits(t, cache.db, waitsBefore+2)

	releaseDatabase()

	select {
	case err := <-stopped:
		if err != nil {
			t.Fatalf("StopPendingCountWrites() error = %v", err)
		}
	case <-time.After(pendingCountsWait):
		t.Fatal("StopPendingCountWrites() did not return once the database was free")
	}

	want := cacheStatsCounters{1, 1, 1, 1024, 1}
	if got := readCacheStatsCounters(t, cache); got != want {
		t.Errorf("counters = %+v, want %+v", got, want)
	}
}