feat: Go versions info disk cache (#455)

* feat: persist play.go.dev Go versions

* feat: add tests

* feat: compute fallback Go versions

cmd/playground/main.go

@@ -15,6 +15,7 @@ import (
 	"github.com/x1unix/go-playground/internal/builder/storage"
 	"github.com/x1unix/go-playground/internal/config"
 	"github.com/x1unix/go-playground/internal/server"
+	"github.com/x1unix/go-playground/internal/server/backendinfo"
 	"github.com/x1unix/go-playground/internal/server/webutil"
 	"github.com/x1unix/go-playground/pkg/goplay"
 	"github.com/x1unix/go-playground/pkg/util/cmdutil"
@@ -77,11 +78,16 @@ func start(logger *zap.Logger, cfg *config.Config) error {
 		go cleanupSvc.Start(ctx)
 	}
 
+	backendsInfoSvc := backendinfo.NewBackendVersionService(zap.L(), playgroundClient, backendinfo.ServiceConfig{
+		CacheFile: filepath.Join(cfg.Build.BuildDir, "go-versions.json"),
+		TTL:       backendinfo.DefaultVersionCacheTTL,
+	})
+
 	// Initialize API endpoints
 	r := mux.NewRouter()
 	apiRouter := r.PathPrefix("/api").Subrouter()
 	svcCfg := server.ServiceConfig{Version: Version}
-	server.NewAPIv1Handler(svcCfg, playgroundClient, buildSvc).
+	server.NewAPIv1Handler(svcCfg, playgroundClient, buildSvc, backendsInfoSvc).
 		Mount(apiRouter)
 
 	apiv2Router := apiRouter.PathPrefix("/v2").Subrouter()
@@ -90,7 +96,6 @@ func start(logger *zap.Logger, cfg *config.Config) error {
 		Builder:      buildSvc,
 		BuildTimeout: cfg.Build.GoBuildTimeout,
 	}).Mount(apiv2Router)
-	//server.NewAPIv2Handler(playgroundClient, buildSvc).Mount(apiv2Router)
 
 	// Web UI routes
 	tplVars := server.TemplateArguments{

internal/server/backendinfo/fallback.go

@@ -0,0 +1,32 @@
+package backendinfo
+
+import (
+	"strconv"
+	"strings"
+)
+
+func prefillFallbacks(info *BackendVersions) {
+	if info.PreviousStable == "" {
+		info.PreviousStable = guessPreviousVersion(info.CurrentStable)
+	}
+
+	if info.Nightly == "" {
+		info.Nightly = "devel"
+	}
+}
+
+func guessPreviousVersion(baseVer string) string {
+	chunks := strings.Split(baseVer, ".")
+	if len(chunks) < 2 {
+		return baseVer
+	}
+
+	minorVer, err := strconv.Atoi(chunks[1])
+	if err != nil {
+		return baseVer
+	}
+
+	minorVer = max(0, minorVer-1)
+	return chunks[0] + "." + strconv.Itoa(minorVer) + ".0"
+}
+

internal/server/backendinfo/provider.go

@@ -0,0 +1,22 @@
+package backendinfo
+
+import "context"
+
+type BackendVersions struct {
+	// CurrentStable is latest stable Go version.
+	CurrentStable string
+
+	// PreviousStable is previous stable Go version.
+	PreviousStable string
+
+	// Nightly is latest unstable Go version (tip) version.
+	Nightly string
+}
+
+type BackendVersionProvider interface {
+	// GetRemoteVersions returns Go version used on remote Go backends.
+	GetRemoteVersions(ctx context.Context) (*BackendVersions, error)
+
+	// ServerVersion returns Go version used on server.
+	ServerVersion() string
+}

internal/server/backendinfo/service.go

@@ -0,0 +1,274 @@
+package backendinfo
+
+import (
+	"context"
+	_ "embed"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"time"
+
+	"github.com/avast/retry-go"
+	"github.com/x1unix/go-playground/pkg/goplay"
+	"go.uber.org/zap"
+	"golang.org/x/sync/errgroup"
+)
+
+const (
+	goVersionRetryAttempts = 3
+	goVersionRetryDelay    = time.Second
+
+	DefaultVersionCacheTTL = 48 * time.Hour
+)
+
+//go:embed resources/version.go.txt
+var versionSnippet []byte
+
+const cacheFileVersion = 1
+
+var _ BackendVersionProvider = (*BackendVersionService)(nil)
+
+type ServiceConfig struct {
+	// Version is cache file version
+	Version int
+
+	// CacheFile is name of a file which will be used to cache Go playground versions.
+	CacheFile string
+
+	// TTL is expiration interval.
+	TTL time.Duration
+}
+
+type cacheEntry struct {
+	Version   int
+	CreatedAt time.Time
+	Data      BackendVersions
+}
+
+// BackendVersionService provides information about used Go versions
+// for all backends.
+type BackendVersionService struct {
+	logger *zap.Logger
+	client *goplay.Client
+	cfg    ServiceConfig
+
+	memCache *cacheEntry
+}
+
+func NewBackendVersionService(logger *zap.Logger, client *goplay.Client, cfg ServiceConfig) *BackendVersionService {
+	return &BackendVersionService{
+		logger: logger,
+		client: client,
+		cfg:    cfg,
+	}
+}
+
+func (svc *BackendVersionService) ServerVersion() string {
+	return normalizeGoVersion(runtime.Version())
+}
+
+func (svc *BackendVersionService) visitCache() (*cacheEntry, error) {
+	if svc.memCache != nil {
+		return svc.memCache, nil
+	}
+
+	if svc.cfg.CacheFile == "" {
+		return nil, fs.ErrNotExist
+	}
+
+	f, err := os.Open(svc.cfg.CacheFile)
+	if err != nil {
+		return nil, err
+	}
+
+	defer f.Close()
+	dst := &cacheEntry{}
+	err = json.NewDecoder(f).Decode(dst)
+
+	return dst, err
+}
+
+// GetVersions provides Go version information for all backends.
+func (svc *BackendVersionService) GetRemoteVersions(ctx context.Context) (*BackendVersions, error) {
+	cached, err := svc.visitCache()
+	if err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			svc.logger.Error("failed to check Go versions cache", zap.Error(err))
+		}
+
+		return svc.populateVersionCache(ctx)
+	}
+
+	if cached.Version != cacheFileVersion {
+		return nil, fs.ErrNotExist
+	}
+
+	dt := time.Now().UTC().Sub(cached.CreatedAt.UTC())
+	if dt >= svc.cfg.TTL {
+		return svc.populateVersionCache(ctx)
+	}
+
+	return &cached.Data, nil
+}
+
+func (svc *BackendVersionService) populateVersionCache(ctx context.Context) (*BackendVersions, error) {
+	versions, err := svc.pullBackendVersions(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := svc.cacheVersions(versions); err != nil {
+		svc.logger.Error("failed to cache Go versions", zap.Error(err))
+	}
+
+	return versions, nil
+}
+
+func (svc *BackendVersionService) cacheVersions(versions *BackendVersions) error {
+	svc.memCache = &cacheEntry{
+		Version:   cacheFileVersion,
+		CreatedAt: time.Now().UTC(),
+		Data:      *versions,
+	}
+
+	if svc.cfg.CacheFile == "" {
+		return nil
+	}
+
+	err := os.MkdirAll(filepath.Dir(svc.cfg.CacheFile), 0755)
+	if err != nil {
+		return fmt.Errorf("MkdirAll failed: %w", err)
+	}
+
+	f, err := os.OpenFile(svc.cfg.CacheFile, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
+	if err != nil {
+		return err
+	}
+
+	defer f.Close()
+	return json.NewEncoder(f).Encode(svc.memCache)
+}
+
+func (svc *BackendVersionService) pullBackendVersions(ctx context.Context) (*BackendVersions, error) {
+	versionInfo := &BackendVersions{}
+	g, gCtx := errgroup.WithContext(ctx)
+
+	mapping := [3]struct {
+		backend string
+		dst     *string
+	}{
+		{
+			backend: goplay.BackendGoCurrent,
+			dst:     &versionInfo.CurrentStable,
+		},
+		{
+			backend: goplay.BackendGoPrev,
+			dst:     &versionInfo.PreviousStable,
+		},
+		{
+			backend: goplay.BackendGoTip,
+			dst:     &versionInfo.Nightly,
+		},
+	}
+
+	for _, e := range mapping {
+		b := e
+		g.Go(func() error {
+			svc.logger.Debug("Fetching go version for backend", zap.String("backend", e.backend))
+			result, err := svc.fetchGoBackendVersionWithRetry(gCtx, e.backend)
+			if err != nil {
+				// Playground "gotip" and "goprev" backends are often broken
+				// and I'm getting tired of seeing 5xx responses if just one of them is dead.
+				//
+				// Throw only if stable version is down. For others - try to figure out fallback values.
+				if e.backend == goplay.BackendGoCurrent {
+					return fmt.Errorf("failed to get Go version from Go playground server for backend %q: %w",
+						b.backend, err)
+				}
+
+				svc.logger.Warn(
+					"can't fetch Go version for backend, will use fallback",
+					zap.String("backend", e.backend), zap.Error(err),
+				)
+				return nil
+			}
+
+			// We don't afraid race condition because each backend is written to a separate address
+			*b.dst = result
+			return nil
+		})
+	}
+
+	if err := g.Wait(); err != nil {
+		return nil, err
+	}
+
+	prefillFallbacks(versionInfo)
+	return versionInfo, nil
+}
+
+func (svc *BackendVersionService) fetchGoBackendVersionWithRetry(ctx context.Context, backend goplay.Backend) (string, error) {
+	var result string
+	err := retry.Do(
+		func() error {
+			version, err := svc.getGoBackendVersion(ctx, backend)
+			if err != nil {
+				return err
+			}
+
+			result = version
+			return nil
+		},
+		retry.Attempts(goVersionRetryAttempts),
+		retry.Delay(goVersionRetryDelay),
+		retry.RetryIf(func(err error) bool {
+			httpErr, ok := goplay.IsHTTPError(err)
+			if !ok {
+				return false
+			}
+
+			// Retry only on server issues
+			return httpErr.StatusCode >= 500
+		}),
+		retry.OnRetry(func(n uint, err error) {
+			svc.logger.Error("failed to get Go version from Go playground, retrying...",
+				zap.Error(err), zap.String("backend", backend), zap.Uint("attempt", n))
+		}),
+	)
+
+	return result, err
+}
+
+func (svc *BackendVersionService) getGoBackendVersion(ctx context.Context, backend goplay.Backend) (string, error) {
+	// Dirty hack to fetch Go version for playground backend by running a simple program
+	// which returns Go version to stdout.
+	result, err := svc.client.Evaluate(ctx, goplay.CompileRequest{
+		Version: goplay.DefaultVersion,
+		WithVet: false,
+		Body:    versionSnippet,
+	}, backend)
+
+	if err != nil {
+		return "", err
+	}
+
+	if result.Errors != "" {
+		return "", fmt.Errorf("probe program returned an error: %s", result.Errors)
+	}
+
+	if len(result.Events) == 0 {
+		return "", errors.New("missing output events from probe program")
+	}
+
+	version := normalizeGoVersion(result.Events[0].Message)
+	return version, nil
+}
+
+func normalizeGoVersion(str string) string {
+	return strings.TrimPrefix(str, "go")
+}