chore: add pprof and debug docs

cmd/finch-daemon/main.go

@@ -17,6 +17,10 @@ import (
 	"syscall"
 	"time"
 
+	// #nosec
+	// register HTTP handler for /debug/pprof on the DefaultServeMux.
+	_ "net/http/pprof"
+
 	"github.com/containerd/containerd"
 	"github.com/containerd/nerdctl/pkg/api/types"
 	"github.com/containerd/nerdctl/pkg/config"
@@ -47,9 +51,10 @@ const (
 )
 
 type DaemonOptions struct {
-	debug       bool
-	socketAddr  string
-	socketOwner int
+	debug        bool
+	socketAddr   string
+	socketOwner  int
+	debugAddress string
 }
 
 var options = new(DaemonOptions)
@@ -65,6 +70,7 @@ func main() {
 	rootCmd.Flags().StringVar(&options.socketAddr, "socket-addr", defaultFinchAddr, "server listening Unix socket address")
 	rootCmd.Flags().BoolVar(&options.debug, "debug", false, "turn on debug log level")
 	rootCmd.Flags().IntVar(&options.socketOwner, "socket-owner", -1, "Uid and Gid of the server socket")
+	rootCmd.Flags().StringVar(&options.debugAddress, "debug-addr", "", "")
 	if err := rootCmd.Execute(); err != nil {
 		log.Printf("got error: %v", err)
 		log.Fatal(err)
@@ -99,6 +105,24 @@ func run(options *DaemonOptions) error {
 	if err := os.Chown(options.socketAddr, options.socketOwner, options.socketOwner); err != nil {
 		return fmt.Errorf("failed to chown the finch-daemon socket: %w", err)
 	}
+
+	if options.debugAddress != "" {
+		logger.Infof("Serving debugging endpoint on %q", options.debugAddress)
+		go func() {
+			debugListener, err := net.Listen("tcp", options.debugAddress)
+			if err != nil {
+				logger.Fatal(err)
+			}
+			debugServer := &http.Server{
+				Handler:           http.DefaultServeMux,
+				ReadHeaderTimeout: 5 * time.Second,
+			}
+			if err := debugServer.Serve(debugListener); err != nil && !errors.Is(err, http.ErrServerClosed) {
+				logger.Fatal(err)
+			}
+		}()
+	}
+
 	server := &http.Server{
 		Handler:           r,
 		ReadHeaderTimeout: 5 * time.Minute,

docs/debug.md

@@ -0,0 +1,40 @@
+# Debugging the finch daemon
+
+This document outlines where to find/access logs and how to configure profiling tools for finch daemon.
+
+## Logs
+
+Logs are the first place to check when you suspect a problem with finch-daemon. If `finch-daemon` was started via `systemd` then you can obtain logs using `journalctl`:
+
+```shell
+sudo journalctl -u finch
+```
+
+> **Note**
+> The command above assumes that you have used the unit file definition [finch.service](../finch.service) we have provided. If you have created your own unit file for `finch-daemon` and replace `finch-daemon` with the one you have made. Amazon Linux distributions of Finch also use the name `finch` for the finch-daemon service.
+
+If you have started `finch-daemon` manually, logs will either be emitted to stderr/stdout.
+
+## CPU Profiling
+
+We can use Golangs `pprof` tool to profile the snapshotter. To enable profiling you must set the `--debug-addr` CLI parameter when invoking `finch-daemon`:
+
+```shell
+./finch-daemon --debug-addr localhost:6060
+```
+
+> **Note**
+> Similarly to adding the command line option for a local run of finch-daemon, any systemd service file can also be modified to include the `--debug-addr` option.
+
+
+Once you have configured the debug address you can send a `GET` to the `/debug/pprof/profile` endpoint to receive a CPU profile of the snapshotter. You can specify an optional argument `seconds` to limit the results to a certain time span:
+
+```shell
+curl http://localhost:6060/debug/pprof/profile?seconds=40 > out.pprof
+```
+
+You can use the `pprof` tool provided by the Go CLI to visualize the data within a web browser:
+
+```shell
+go tool pprof -http=:8080 out.pprof
+```