3 files changed, 88 insertions, 0 deletions
diff --git a/internal/federation/federator.go b/internal/federation/federator.go
index 016a6fb68..2ee01697f 100644
--- a/internal/federation/federator.go
+++ b/internal/federation/federator.go
@@ -21,6 +21,7 @@ package federation
 import (
 	"net/http"
 	"net/url"
+	"sync"
 
 	"github.com/go-fed/activity/pub"
 	"github.com/sirupsen/logrus"
@@ -54,6 +55,8 @@ type Federator interface {
 	//
 	// If username is an empty string, our instance user's credentials will be used instead.
 	GetTransportForUser(username string) (transport.Transport, error)
+	// Handshaking returns true if the given username is currently in the process of dereferencing the remoteAccountID.
+	Handshaking(username string, remoteAccountID *url.URL) bool
 	pub.CommonBehavior
 	pub.FederatingProtocol
 }
@@ -67,6 +70,8 @@ type federator struct {
 	transportController transport.Controller
 	actor               pub.FederatingActor
 	log                 *logrus.Logger
+	handshakes          map[string][]*url.URL
+	handshakeSync       *sync.Mutex // mutex to lock/unlock when checking or updating the handshakes map
 }
 
 // NewFederator returns a new federator
@@ -81,6 +86,7 @@ func NewFederator(db db.DB, federatingDB federatingdb.DB, transportController tr
 		typeConverter:       typeConverter,
 		transportController: transportController,
 		log:                 log,
+		handshakeSync:       &sync.Mutex{},
 	}
 	actor := newFederatingActor(f, f, federatingDB, clock)
 	f.actor = actor
diff --git a/internal/federation/handshake.go b/internal/federation/handshake.go
new file mode 100644
index 000000000..af720403a
--- /dev/null
+++ b/internal/federation/handshake.go
@@ -0,0 +1,80 @@
+package federation
+
+import "net/url"
+
+func (f *federator) Handshaking(username string, remoteAccountID *url.URL) bool {
+	f.handshakeSync.Lock()
+	defer f.handshakeSync.Unlock()
+
+	if f.handshakes == nil {
+		// handshakes isn't even initialized yet so we can't be handshaking with anyone
+		return false
+	}
+
+	remoteIDs, ok := f.handshakes[username];
+	if !ok {
+		// user isn't handshaking with anyone, bail
+		return false
+	}
+
+	for _, id := range remoteIDs {
+		if id.String() == remoteAccountID.String() {
+			// we are currently handshaking with the remote account, yep
+			return true
+		}
+	}
+
+	// didn't find it which means we're not handshaking
+	return false
+}
+
+func (f *federator) startHandshake(username string, remoteAccountID *url.URL) {
+	f.handshakeSync.Lock()
+	defer f.handshakeSync.Unlock()
+
+	// lazily initialize handshakes
+	if f.handshakes == nil {
+		f.handshakes = make(map[string][]*url.URL)
+	}
+
+	remoteIDs, ok := f.handshakes[username]
+	if !ok {
+		// there was nothing in there yet, so just add this entry and return
+		f.handshakes[username] = []*url.URL{remoteAccountID}
+		return
+	}
+
+	// add the remote ID to the slice
+	remoteIDs = append(remoteIDs, remoteAccountID)
+	f.handshakes[username] = remoteIDs
+}
+
+func (f *federator) stopHandshake(username string, remoteAccountID *url.URL) {
+	f.handshakeSync.Lock()
+	defer f.handshakeSync.Unlock()
+
+	if f.handshakes == nil {
+		return
+	}
+
+	remoteIDs, ok := f.handshakes[username]
+	if !ok {
+		// there was nothing in there yet anyway so just bail
+		return
+	}
+
+	newRemoteIDs := []*url.URL{}
+	for _, id := range remoteIDs {
+		if id.String() != remoteAccountID.String() {
+			newRemoteIDs = append(newRemoteIDs, id)
+		}
+	}
+
+	if len(newRemoteIDs) == 0 {
+		// there are no handshakes so just remove this user entry from the map and save a few bytes
+		delete(f.handshakes, username)
+	} else {
+		// there are still other handshakes ongoing
+		f.handshakes[username] = newRemoteIDs
+	}
+}
diff --git a/internal/federation/util.go b/internal/federation/util.go
index 7be92e13d..9ec0770f6 100644
--- a/internal/federation/util.go
+++ b/internal/federation/util.go
@@ -213,6 +213,8 @@ func (f *federator) AuthenticateFederatedRequest(username string, r *http.Reques
 }
 
 func (f *federator) DereferenceRemoteAccount(username string, remoteAccountID *url.URL) (typeutils.Accountable, error) {
+	f.startHandshake(username, remoteAccountID)
+	defer f.stopHandshake(username, remoteAccountID)
 
 	transport, err := f.GetTransportForUser(username)
 	if err != nil {