Run as cache server (#275)

This PR
- adds the `cache-server` command so act_runner can run as a cache server. When running as a cache server, act_runner only processes the requests related to cache and does not run jobs.
- adds the `external_server` configuration for cache. If specified, act_runner will use this URL as the ACTIONS_CACHE_URL instead of starting a cache server itself.

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/275
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-committed-by: Zettat123 <zettat123@gmail.com>

internal/app/cmd/cache-server.go

@@ -0,0 +1,69 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+
+	"gitea.com/gitea/act_runner/internal/pkg/config"
+
+	"github.com/nektos/act/pkg/artifactcache"
+	log "github.com/sirupsen/logrus"
+	"github.com/spf13/cobra"
+)
+
+type cacheServerArgs struct {
+	Dir  string
+	Host string
+	Port uint16
+}
+
+func runCacheServer(ctx context.Context, configFile *string, cacheArgs *cacheServerArgs) func(cmd *cobra.Command, args []string) error {
+	return func(cmd *cobra.Command, args []string) error {
+		cfg, err := config.LoadDefault(*configFile)
+		if err != nil {
+			return fmt.Errorf("invalid configuration: %w", err)
+		}
+
+		initLogging(cfg)
+
+		var (
+			dir  = cfg.Cache.Dir
+			host = cfg.Cache.Host
+			port = cfg.Cache.Port
+		)
+
+		// cacheArgs has higher priority
+		if cacheArgs.Dir != "" {
+			dir = cacheArgs.Dir
+		}
+		if cacheArgs.Host != "" {
+			host = cacheArgs.Host
+		}
+		if cacheArgs.Port != 0 {
+			port = cacheArgs.Port
+		}
+
+		cacheHandler, err := artifactcache.StartHandler(
+			dir,
+			host,
+			port,
+			log.StandardLogger().WithField("module", "cache_request"),
+		)
+		if err != nil {
+			return err
+		}
+
+		log.Infof("cache server is listening on %v", cacheHandler.ExternalURL())
+
+		c := make(chan os.Signal, 1)
+		signal.Notify(c, os.Interrupt)
+		<-c
+
+		return nil
+	}
+}