Implement readonly mode

cmd/go-cache-plugin/commands.go

@@ -32,6 +32,7 @@ var flags struct {
 	MinUploadSize int64         `flag:"min-upload-size,default=$GOCACHE_MIN_SIZE,Minimum object size to upload to S3 (in bytes)"`
 	Concurrency   int           `flag:"c,default=$GOCACHE_CONCURRENCY,Maximum number of concurrent requests"`
 	S3Concurrency int           `flag:"u,default=$GOCACHE_S3_CONCURRENCY,Maximum concurrency for upload to S3"`
+	ReadOnly      bool          `flag:"read-only,default=$GOCACHE_READ_ONLY,Disable uploads to S3"`
 	PrintMetrics  bool          `flag:"metrics,default=$GOCACHE_METRICS,Print summary metrics to stderr at exit"`
 	Expiration    time.Duration `flag:"expiry,default=$GOCACHE_EXPIRY,Cache expiration period (optional)"`
 	Verbose       bool          `flag:"v,default=$GOCACHE_VERBOSE,Enable verbose logging"`

cmd/go-cache-plugin/setup.go

@@ -78,6 +78,7 @@ func initCacheServer(env *command.Env) (*gocache.Server, *s3util.Client, error)
 		S3Client:          client,
 		KeyPrefix:         flags.KeyPrefix,
 		MinUploadSize:     flags.MinUploadSize,
+		ReadOnly:          flags.ReadOnly,
 		UploadConcurrency: flags.S3Concurrency,
 	}
 	cache.SetMetrics(env.Context(), expvar.NewMap("gocache_host"))
@@ -120,6 +121,7 @@ func initModProxy(env *command.Env, s3c *s3util.Client) (_ http.Handler, cleanup
 		Local:       modCachePath,
 		S3Client:    s3c,
 		KeyPrefix:   path.Join(flags.KeyPrefix, "module"),
+		ReadOnly:    flags.ReadOnly,
 		MaxTasks:    flags.S3Concurrency,
 		Logf:        vprintf,
 		LogRequests: flags.DebugLog&debugModProxy != 0,
@@ -202,6 +204,7 @@ func initRevProxy(env *command.Env, s3c *s3util.Client, g *taskgroup.Group) (htt
 		Local:       revCachePath,
 		S3Client:    s3c,
 		KeyPrefix:   path.Join(flags.KeyPrefix, "revproxy"),
+		ReadOnly:    flags.ReadOnly,
 		Logf:        vprintf,
 		LogRequests: flags.DebugLog&debugRevProxy != 0,
 	}

lib/gobuild/gobuild.go

@@ -67,6 +67,10 @@ type S3Cache struct {
 	// which the cache will not write the object to S3.
 	MinUploadSize int64
 
+	// ReadOnly, if true, prevents the cache from writing any data back to S3.
+	// Data is still read from S3 and stored locally.
+	ReadOnly bool
+
 	// UploadConcurrency, if positive, defines the maximum number of concurrent
 	// tasks for writing cache entries to S3.  If zero or negative, it uses
 	// runtime.NumCPU.
@@ -155,6 +159,9 @@ func (s *S3Cache) Put(ctx context.Context, obj gocache.Object) (diskPath string,
 	if err != nil {
 		return "", err // don't bother trying to forward it to the remote
 	}
+	if s.ReadOnly {
+		return diskPath, nil // don't write back to S3
+	}
 	if obj.Size < s.MinUploadSize {
 		s.putSkipSmall.Add(1)
 		return diskPath, nil // don't bother uploading this, it's too small

lib/modproxy/modproxy.go

@@ -61,6 +61,10 @@ type S3Cacher struct {
 	// intervening slash.
 	KeyPrefix string
 
+	// ReadOnly, if true, prevents the cacher from writing any data back to S3.
+	// Data is still read from S3 and stored locally.
+	ReadOnly bool
+
 	// MaxTasks, if positive, limits the number of concurrent tasks that may be
 	// interacting with S3. If zero or negative, the default is
 	// [runtime.NumCPU].
@@ -212,6 +216,9 @@ func (c *S3Cacher) Put(ctx context.Context, name string, data io.ReadSeeker) (oe
 		c.putLocalHit.Add(1)
 		return nil
 	}
+	if c.ReadOnly {
+		return nil
+	}
 
 	// Try to push the object to S3 in the background.
 	f, size, err := openFileSize(path)

lib/revproxy/revproxy.go

@@ -84,6 +84,10 @@ type Server struct {
 	// intervening slash.
 	KeyPrefix string
 
+	// ReadOnly, if true, prevents the server from writing any data back to S3.
+	// Data is still read from S3 and stored locally.
+	ReadOnly bool
+
 	// Logf, if non-nil, is used to write log messages. If nil, logs are
 	// discarded.
 	Logf func(string, ...any)
@@ -274,7 +278,9 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 					} else {
 						s.rspSave.Add(1)
 						s.rspSaveBytes.Add(int64(len(body)))
-						s.start(s.cacheStoreS3(hash, rsp.Header, body))
+						if !s.ReadOnly {
+							s.start(s.cacheStoreS3(hash, rsp.Header, body))
+						}
 					}
 					s.vlogf("rp E H:%s fetch RC:yes B:%d (%v elapsed)", hash, len(body), time.Since(start))
 				}