prevent wrong results in format comparisons or when formats are used as keys (#673)

pkg/format/g722.go

@@ -8,7 +8,12 @@ import (
 
 // G722 is the RTP format for the G722 codec.
 // Specification: https://datatracker.ietf.org/doc/html/rfc3551
-type G722 struct{}
+type G722 struct {
+	// in Go, empty structs share the same pointer,
+	// therefore they cannot be used as map keys
+	// or in equality operations. Prevent this.
+	unused int //nolint:unused
+}
 
 func (f *G722) unmarshal(_ *unmarshalContext) error {
 	return nil

pkg/format/mjpeg.go

@@ -8,7 +8,12 @@ import (
 
 // MJPEG is the RTP format for the Motion-JPEG codec.
 // Specification: https://datatracker.ietf.org/doc/html/rfc2435
-type MJPEG struct{}
+type MJPEG struct {
+	// in Go, empty structs share the same pointer,
+	// therefore they cannot be used as map keys
+	// or in equality operations. Prevent this.
+	unused int //nolint:unused
+}
 
 func (f *MJPEG) unmarshal(_ *unmarshalContext) error {
 	return nil

pkg/format/mpeg1_audio.go

@@ -8,7 +8,12 @@ import (
 
 // MPEG1Audio is the RTP format for a MPEG-1/2 Audio codec.
 // Specification: https://datatracker.ietf.org/doc/html/rfc2250
-type MPEG1Audio struct{}
+type MPEG1Audio struct {
+	// in Go, empty structs share the same pointer,
+	// therefore they cannot be used as map keys
+	// or in equality operations. Prevent this.
+	unused int //nolint:unused
+}
 
 func (f *MPEG1Audio) unmarshal(_ *unmarshalContext) error {
 	return nil

pkg/format/mpeg1_video.go

@@ -8,7 +8,12 @@ import (
 
 // MPEG1Video is the RTP format for a MPEG-1/2 Video codec.
 // Specification: https://datatracker.ietf.org/doc/html/rfc2250
-type MPEG1Video struct{}
+type MPEG1Video struct {
+	// in Go, empty structs share the same pointer,
+	// therefore they cannot be used as map keys
+	// or in equality operations. Prevent this.
+	unused int //nolint:unused
+}
 
 func (f *MPEG1Video) unmarshal(_ *unmarshalContext) error {
 	return nil

pkg/format/mpegts.go

@@ -6,7 +6,12 @@ import (
 
 // MPEGTS is the RTP format for MPEG-TS.
 // Specification: https://datatracker.ietf.org/doc/html/rfc2250
-type MPEGTS struct{}
+type MPEGTS struct {
+	// in Go, empty structs share the same pointer,
+	// therefore they cannot be used as map keys
+	// or in equality operations. Prevent this.
+	unused int //nolint:unused
+}
 
 func (f *MPEGTS) unmarshal(_ *unmarshalContext) error {
 	return nil