Add shared inbox support for ActivityPub delivery (#4755)

* feat(ap): add support for shared inboxes to reduce outbound load

* feat(db): refactor ap followers db into followers repository

* fix(ap): use the updated activity library to pull out the shared inbox endpoint

* chore(deps): point at updated build of owncast/activity

* fix(ap): typeless endpoints

* feat(test): update ActivityPub test to support shared inboxes

* chore(test): remove unused variable

* fix: feedback from review. Guard against SSRF/non-HTTPS/local and handle transaction errors

utils/netutils.go

@@ -2,13 +2,34 @@ package utils
 
 import (
 	"net"
+	"os"
+	"sync"
 
 	log "github.com/sirupsen/logrus"
 )
 
+var (
+	allowInternalFederation     bool
+	allowInternalFederationOnce sync.Once
+)
+
+// AllowInternalFederation returns true if the OWNCAST_ALLOW_INTERNAL_FEDERATION
+// environment variable is set to "true". This is used for testing purposes only.
+func AllowInternalFederation() bool {
+	allowInternalFederationOnce.Do(func() {
+		allowInternalFederation = os.Getenv("OWNCAST_ALLOW_INTERNAL_FEDERATION") == "true"
+	})
+	return allowInternalFederation
+}
+
 // IsHostnameInternal will attempt to determine if the hostname is internal to
 // this server's network or is the loopback address.
+// Returns false if OWNCAST_ALLOW_INTERNAL_FEDERATION is set to "true".
 func IsHostnameInternal(hostname string) bool {
+	// Allow internal federation for testing purposes.
+	if AllowInternalFederation() {
+		return false
+	}
 	// If this is already an IP address don't try to resolve it
 	if ip := net.ParseIP(hostname); ip != nil {
 		return isIPAddressInternal(ip)