Enhancement: add support for unix socket listen

changelog/unreleased/pull-272

@@ -0,0 +1,12 @@
+Enhancement: can now listen on a unix socket
+
+If `--listen unix:/tmp/foo` is passed, the server will listen on a unix socket. This is triggered by the prefix `unix:`.
+
+This is useful in combination with remote port portforwarding to enable remote server to backup locally, e.g.
+
+```bash
+rest-server --listen unix:/tmp/foo &
+ssh -R /tmp/foo:/tmp/foo user@host restic -r rest:http+unix:/tmp/foo:/repo backup
+```
+
+https://github.com/restic/rest-server/pull/272

cmd/rest-server/listener_unix.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"log"
 	"net"
+	"strings"
 
 	"github.com/coreos/go-systemd/v22/activation"
 )
@@ -23,9 +24,20 @@ func findListener(addr string) (listener net.Listener, err error) {
 	switch len(listeners) {
 	case 0:
 		// no listeners found, listen manually
-		listener, err = net.Listen("tcp", addr)
-		if err != nil {
-			return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
+		if strings.HasPrefix(addr, "unix:") { // if we want to listen on a unix socket
+			unixAddr, err := net.ResolveUnixAddr("unix", strings.TrimPrefix(addr, "unix:"))
+			if err != nil {
+				return nil, fmt.Errorf("unable to understand unix address %s: %w", addr, err)
+			}
+			listener, err = net.ListenUnix("unix", unixAddr)
+			if err != nil {
+				return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
+			}
+		} else { // assume tcp
+			listener, err = net.Listen("tcp", addr)
+			if err != nil {
+				return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
+			}
 		}
 
 		log.Printf("start server on %v", listener.Addr())

cmd/rest-server/listener_unix_test.go

@@ -0,0 +1,75 @@
+//go:build !windows
+// +build !windows
+
+package main
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestUnixSocket(t *testing.T) {
+	td := t.TempDir()
+
+	// this is the socket we'll listen on and connect to
+	tempSocket := filepath.Join(td, "sock")
+
+	// create some content and parent dirs
+	if err := os.MkdirAll(filepath.Join(td, "data", "repo1"), 0700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(td, "data", "repo1", "config"), []byte("foo"), 0700); err != nil {
+		t.Fatal(err)
+	}
+
+	// run the following twice, to test that the server will
+	// cleanup its socket file when quitting, which won't happen
+	// if it doesn't exit gracefully
+	for i := 0; i < 2; i++ {
+		err := testServerWithArgs([]string{
+			"--no-auth",
+			"--path", filepath.Join(td, "data"),
+			"--listen", fmt.Sprintf("unix:%s", tempSocket),
+		}, time.Second, func(ctx context.Context, _ *restServerApp) error {
+			// custom client that will talk HTTP to unix socket
+			client := http.Client{
+				Transport: &http.Transport{
+					DialContext: func(_ context.Context, _, _ string) (net.Conn, error) {
+						return net.Dial("unix", tempSocket)
+					},
+				},
+			}
+			for _, test := range []struct {
+				Path       string
+				StatusCode int
+			}{
+				{"/repo1/", http.StatusMethodNotAllowed},
+				{"/repo1/config", http.StatusOK},
+				{"/repo2/config", http.StatusNotFound},
+			} {
+				req, err := http.NewRequestWithContext(ctx, http.MethodGet, "http://ignored"+test.Path, nil)
+				if err != nil {
+					return err
+				}
+				resp, err := client.Do(req)
+				if err != nil {
+					return err
+				}
+				resp.Body.Close()
+				if resp.StatusCode != test.StatusCode {
+					return fmt.Errorf("expected %d from server, instead got %d (path %s)", test.StatusCode, resp.StatusCode, test.Path)
+				}
+			}
+			return nil
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}