support relative fallback paths (#201)

README.md

@@ -414,12 +414,12 @@ paths:
 
 ### Fallback stream
 
-If no one is publishing to the server, readers can be redirected to a fallback URL that is serving a fallback stream:
+If no one is publishing to the server, readers can be redirected to a fallback path or URL that is serving a fallback stream:
 
 ```yml
 paths:
   withfallback:
-    fallback: rtsp://otherurl/otherpath
+    fallback: /otherpath
 ```
 
 ### Start on boot with systemd

internal/conf/path.go

@@ -198,8 +198,8 @@ func (pconf *PathConf) fillAndCheck(name string) error {
 	}
 
 	if pconf.SourceOnDemand {
-		if pconf.Source != "record" {
-			return fmt.Errorf("'sourceOnDemand' is useless when source is not 'record', since the stream is not provided by a publisher, but by a fixed source")
+		if pconf.Source == "record" {
+			return fmt.Errorf("'sourceOnDemand' is useless when source is 'record'")
 		}
 	}
 
@@ -212,18 +212,26 @@ func (pconf *PathConf) fillAndCheck(name string) error {
 	}
 
 	if pconf.Fallback != "" {
+		if strings.HasPrefix(pconf.Fallback, "/") {
+			err := CheckPathName(pconf.Fallback[1:])
+			if err != nil {
+				return fmt.Errorf("'%s': %s", pconf.Fallback, err)
+			}
+
+		} else {
 			_, err := base.ParseURL(pconf.Fallback)
 			if err != nil {
 				return fmt.Errorf("'%s' is not a valid RTSP url", pconf.Fallback)
 			}
 		}
+	}
 
 	if (pconf.PublishUser != "" && pconf.PublishPass == "") || (pconf.PublishUser == "" && pconf.PublishPass != "") {
 		return fmt.Errorf("read username and password must be both filled")
 	}
 	if pconf.PublishUser != "" {
 		if pconf.Source != "record" {
-			return fmt.Errorf("'publishUser' is useless when source is not 'record', since the stream is not provided by a publisher, but by a fixed source")
+			return fmt.Errorf("'publishUser' is useless when source is not 'record'")
 		}
 
 		if !strings.HasPrefix(pconf.PublishUser, "sha256:") && !reUserPass.MatchString(pconf.PublishUser) {

internal/path/path.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/aler9/gortsplib"
+	"github.com/aler9/gortsplib/pkg/base"
 
 	"github.com/aler9/rtsp-simple-server/internal/client"
 	"github.com/aler9/rtsp-simple-server/internal/clientrtsp"
@@ -598,7 +599,19 @@ func (pa *Path) onClientDescribe(req client.DescribeReq) {
 
 	case sourceStateNotReady:
 		if pa.conf.Fallback != "" {
-			req.Res <- client.DescribeRes{nil, pa.conf.Fallback, nil} //nolint:govet
+			fallbackURL := func() string {
+				if strings.HasPrefix(pa.conf.Fallback, "/") {
+					ur := base.URL{
+						Scheme: req.Req.URL.Scheme,
+						User:   req.Req.URL.User,
+						Host:   req.Req.URL.Host,
+						Path:   pa.conf.Fallback,
+					}
+					return ur.String()
+				}
+				return pa.conf.Fallback
+			}()
+			req.Res <- client.DescribeRes{nil, fallbackURL, nil} //nolint:govet
 			return
 		}
 

main_test.go

@@ -851,9 +851,21 @@ func TestRedirect(t *testing.T) {
 }
 
 func TestFallback(t *testing.T) {
+	for _, ca := range []string{
+		"absolute",
+		"relative",
+	} {
+		t.Run(ca, func(t *testing.T) {
+			val := func() string {
+				if ca == "absolute" {
+					return "rtsp://" + ownDockerIP + ":8554/path2"
+				}
+				return "/path2"
+			}()
+
 			p1, ok := testProgram("paths:\n" +
 				"  path1:\n" +
-		"    fallback: rtsp://" + ownDockerIP + ":8554/path2\n" +
+				"    fallback: " + val + "\n" +
 				"  path2:\n")
 			require.Equal(t, true, ok)
 			defer p1.close()
@@ -882,6 +894,8 @@ func TestFallback(t *testing.T) {
 			require.NoError(t, err)
 			defer cnt2.close()
 			require.Equal(t, 0, cnt2.wait())
+		})
+	}
 }
 
 func TestRTMP(t *testing.T) {

rtsp-simple-server.yml

@@ -86,12 +86,12 @@ paths:
     # * redirect -> the stream is provided by another path or server
     source: record
 
-    # if the source is an RTSP url, this is the protocol that will be used to
+    # if the source is an RTSP URL, this is the protocol that will be used to
     # pull the stream. available options are "automatic", "udp", "tcp".
     # the tcp protocol can help to overcome the error "no UDP packets received recently".
     sourceProtocol: automatic
 
-    # if the source is an RTSP or RTMP url, it will be pulled only when at least
+    # if the source is an RTSP or RTMP URL, it will be pulled only when at least
     # one reader is connected, saving bandwidth.
     sourceOnDemand: no
     # if sourceOnDemand is "yes", readers will be put on hold until the source is
@@ -101,11 +101,12 @@ paths:
     # readers connected and this amount of time has passed.
     sourceOnDemandCloseAfter: 10s
 
-    # if the source is "redirect", this is the RTSP url which clients will be
+    # if the source is "redirect", this is the RTSP URL which clients will be
     # redirected to.
     sourceRedirect:
 
-    # fallback url to redirect clients to when nobody is publishing to this path.
+    # fallback stream to redirect clients to when nobody is publishing to this path.
+    # this can be a relative path  (i.e. /otherstream) or an absolute RTSP URL.
     fallback:
 
     # username required to publish.