chore: improve concurrency and context cancelation

Addressing comments from @alberto-miranda #16 (comment)

cmd/serve.go

@@ -15,6 +15,10 @@
 package cmd
 
 import (
+	"context"
+	"os"
+	"os/signal"
+	"syscall"
 	"time"
 
 	"log/slog"
@@ -26,6 +30,7 @@ import (
 	"github.com/seqeralabs/staticreg/pkg/server"
 	"github.com/seqeralabs/staticreg/pkg/server/staticreg"
 	"github.com/spf13/cobra"
+	"golang.org/x/sync/errgroup"
 )
 
 var (
@@ -40,6 +45,9 @@ var serveCmd = &cobra.Command{
 	Short: "Serves a webserver with an HTML listing of all images and tags in a v2 registry",
 	Run: func(cmd *cobra.Command, args []string) {
 		ctx := cmd.Context()
+		ctx, stop := signal.NotifyContext(ctx, os.Interrupt, syscall.SIGTERM)
+		defer stop()
+
 		log := logger.FromContext(ctx)
 		log.Info("starting server",
 			slog.Duration("cache-duration", cacheDuration),
@@ -50,6 +58,7 @@ var serveCmd = &cobra.Command{
 
 		client := registry.New(rootCfg)
 		asyncClient := async.New(client, refreshInterval)
+		defer asyncClient.Stop(context.Background())
 
 		filler := filler.New(asyncClient, rootCfg.RegistryHostname, "/")
 
@@ -60,24 +69,22 @@ var serveCmd = &cobra.Command{
 			return
 		}
 
-		errCh := make(chan error, 1)
-		go func() {
-			errCh <- srv.Start()
-		}()
+		g, ctx := errgroup.WithContext(ctx)
 
-		go func() {
-			errCh <- asyncClient.Start(ctx)
-		}()
+		g.Go(func() error {
+			return srv.Start(ctx)
+		})
 
-		select {
-		case <-ctx.Done():
-			return
-		case err := <-errCh:
-			if err == nil {
-				slog.Error("operations exited unexpectedly")
-				return
+		g.Go(func() error {
+			return asyncClient.Start(ctx)
+		})
+
+		if err := g.Wait(); err != nil {
+			if ctx.Err() != nil {
+				log.Info("context cancelled, shutting down")
+			} else {
+				slog.Error("unexpected error", logger.ErrAttr(err))
 			}
-			slog.Error("unexpected error", logger.ErrAttr(err))
 			return
 		}
 	},

pkg/registry/async/async_registry.go

@@ -16,12 +16,13 @@ package async
 
 import (
 	"context"
-	"fmt"
+	"errors"
 	"log/slog"
 	"time"
 
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/puzpuzpuz/xsync/v3"
+	"golang.org/x/sync/errgroup"
 
 	"github.com/cenkalti/backoff/v4"
 	"github.com/seqeralabs/staticreg/pkg/observability/logger"
@@ -31,22 +32,35 @@ import (
 const imageInfoRequestsBufSize = 10
 const tagRequestBufferSize = 10
 
+var (
+	ErrNoTagsFound       = errors.New("no tags found")
+	ErrImageInfoNotFound = errors.New("image info not found")
+)
+
 // Async is a struct that wraps an underlying registry.Client
 // to provide asynchronous methods for interacting with a container registry.
 // It continuously syncs data from the registry in a separate goroutine.
 type Async struct {
 	// underlying is the actual registry client that does the registry operations, remember this is just a wrapper!
 	underlying registry.Client
-
+	// refreshInterval represents the time to wait to synchronize repositories again after a successful synchronization
 	refreshInterval time.Duration
 
+	// repos is an in memory list of all the repository names in the registry
 	repos []string
 
+	// repositoryTags represents the list of tags for each repository
 	repositoryTags *xsync.MapOf[string, []string]
 
+	// imageInfo contains the image information indexed by repo name and tag
 	imageInfo *xsync.MapOf[imageInfoKey, imageInfo]
 
+	// repositoryRequestBuffer generates requests for the `handleRepositoryRequest`
+	// handler that is responsible for retrieving the tags for a given image and
+	// scheduling new jobs on `imageInfoRequestsBuffer`
 	repositoryRequestBuffer chan repositoryRequest
+	// imageInfoRequestsBuffer is responsible for feeding `handleImageInfoRequest`
+	// so that image info is retrieved for each <repo,tag> combination
 	imageInfoRequestsBuffer chan imageInfoRequest
 }
 
@@ -68,52 +82,51 @@ type imageInfo struct {
 	reference string
 }
 
+func (c *Async) Stop(ctx context.Context) {
+	close(c.imageInfoRequestsBuffer)
+	close(c.repositoryRequestBuffer)
+}
+
 func (c *Async) Start(ctx context.Context) error {
-	// TODO(fntlnz): maybe instead of errCh use a backoff and retry ops
-	errCh := make(chan error, 1)
+	g, ctx := errgroup.WithContext(ctx)
 
-	go func() {
+	g.Go(func() error {
 		for {
 			err := backoff.Retry(func() error {
 				return c.synchronizeRepositories(ctx)
 			}, backoff.WithContext(newExponentialBackoff(), ctx))
 
 			if err != nil {
-				errCh <- err
+				return err
 			}
 
 			time.Sleep(c.refreshInterval)
 		}
-	}()
+	})
 
-	go func() {
+	g.Go(func() error {
 		for {
 			select {
 			case <-ctx.Done():
-				return
+				return ctx.Err()
 			case req := <-c.repositoryRequestBuffer:
 				c.handleRepositoryRequest(ctx, req)
 			}
 		}
-	}()
+	})
 
-	go func() {
+	g.Go(func() error {
 		for {
 			select {
 			case <-ctx.Done():
-				return
+				return ctx.Err()
 			case req := <-c.imageInfoRequestsBuffer:
 				c.handleImageInfoRequest(ctx, req)
 			}
 		}
-	}()
+	})
 
-	select {
-	case <-ctx.Done():
-		return nil
-	case err := <-errCh:
-		return err
-	}
+	return g.Wait()
 }
 
 func (c *Async) synchronizeRepositories(ctx context.Context) error {
@@ -126,7 +139,14 @@ func (c *Async) synchronizeRepositories(ctx context.Context) error {
 	c.repos = repos
 
 	for _, r := range repos {
-		c.repositoryRequestBuffer <- repositoryRequest{repo: r}
+		if err := ctx.Err(); err != nil {
+			return nil
+		}
+		select {
+		case c.repositoryRequestBuffer <- repositoryRequest{repo: r}:
+		default:
+			return nil
+		}
 	}
 
 	return nil
@@ -146,13 +166,19 @@ func (c *Async) handleRepositoryRequest(ctx context.Context, req repositoryReque
 	c.repositoryTags.Store(req.repo, tags)
 
 	for _, t := range tags {
-		c.imageInfoRequestsBuffer <- imageInfoRequest{
+		if ctx.Err() != nil {
+			return
+		}
+
+		select {
+		case c.imageInfoRequestsBuffer <- imageInfoRequest{
 			repo: req.repo,
 			tag:  t,
+		}:
+		default:
+			return
 		}
 	}
-
-	return
 }
 
 func (c *Async) handleImageInfoRequest(ctx context.Context, req imageInfoRequest) {
@@ -176,10 +202,11 @@ func (c *Async) RepoList(ctx context.Context) ([]string, error) {
 	return c.repos, nil
 }
 
+// TagList contains
 func (c *Async) TagList(ctx context.Context, repo string) ([]string, error) {
 	tags, ok := c.repositoryTags.Load(repo)
 	if !ok {
-		return nil, fmt.Errorf("no tags found") // TODO(fntlnz): make an error var
+		return nil, ErrNoTagsFound
 	}
 	return tags, nil
 }
@@ -191,7 +218,7 @@ func (c *Async) ImageInfo(ctx context.Context, repo string, tag string) (image v
 	}
 	info, ok := c.imageInfo.Load(key)
 	if !ok {
-		return nil, "", fmt.Errorf("image info not found") // TODO(fntlnz): make an error var
+		return nil, "", ErrImageInfoNotFound
 	}
 	return info.image, info.reference, nil
 }

pkg/server/server.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"context"
 	"log/slog"
 	"net/http"
 	"strings"
@@ -9,8 +10,8 @@ import (
 	cache "github.com/chenyahui/gin-cache"
 	"github.com/chenyahui/gin-cache/persist"
 	sloggin "github.com/samber/slog-gin"
+	"golang.org/x/sync/errgroup"
 
-	// "github.com/chenyahui/gin-cache/persist"
 	"github.com/gin-gonic/gin"
 )
 
@@ -78,8 +79,14 @@ func New(
 	}, nil
 }
 
-func (s *Server) Start() error {
-	return s.server.ListenAndServe()
+func (s *Server) Start(ctx context.Context) error {
+	g, ctx := errgroup.WithContext(ctx)
+	g.Go(s.server.ListenAndServe)
+	g.Go(func() error {
+		<-ctx.Done()
+		return s.server.Shutdown(context.Background())
+	})
+	return g.Wait()
 }
 
 func injectLoggerMiddleware(log *slog.Logger) gin.HandlerFunc {