Drop start requests from lower precedence parties if sas has already been started.

According to the guide for implementing verification started from
a verification request, both parties should (or at least allowed to)
send a start request when the verification is ready. However, only the
start request from the party with lexicographically smaller user id (or
device id, for device verification, and thus equal user id) is supposed
to be accepted, and the other one ignored.

crates/matrix-sdk-crypto/src/verification/requests.rs

@@ -44,7 +44,7 @@ use super::{
     event_enums::{
         CancelContent, DoneContent, OutgoingContent, ReadyContent, RequestContent, StartContent,
     },
-    CancelInfo, Cancelled, FlowId, VerificationStore,
+    CancelInfo, Cancelled, FlowId, Verification, VerificationStore,
 };
 #[cfg(feature = "qrcode")]
 use super::{
@@ -1126,11 +1126,10 @@ impl RequestState<Ready> {
         };
 
         let identity = self.store.get_user_identity(sender).await?;
-        let own_identity = self
-            .store
-            .get_user_identity(self.store.account.user_id())
-            .await?
-            .and_then(|i| i.into_own());
+        let own_user_id = self.store.account.user_id();
+        let own_device_id = self.store.account.device_id();
+        let own_identity =
+            self.store.get_user_identity(own_user_id).await?.and_then(|i| i.into_own());
 
         match content.method() {
             StartMethod::SasV1(_) => {
@@ -1142,13 +1141,27 @@ impl RequestState<Ready> {
                     we_started,
                     request_handle,
                 ) {
-                    // TODO check if there is already a SAS verification, i.e. we
-                    // already started one before the other side tried to do the
-                    // same; ignore it if we did and we're the lexicographically
-                    // smaller user ID, otherwise auto-accept the newly started one.
                     Ok(s) => {
-                        info!("Started a new SAS verification.");
-                        self.verification_cache.insert_sas(s);
+                        let start_new = if let Some(Verification::SasV1(_sas)) =
+                            self.verification_cache.get(sender, self.flow_id.as_str())
+                        {
+                            // If there is already a SAS verification, i.e. we already started one
+                            // before the other side tried to do the same; ignore it if we did and
+                            // we're the lexicographically smaller user ID (or device ID if equal).
+                            use std::cmp::Ordering;
+                            match (sender.cmp(own_user_id), device.device_id().cmp(own_device_id)) {
+                                (Ordering::Greater, _) | (Ordering::Equal, Ordering::Greater) => {
+                                    false
+                                }
+                                _ => true,
+                            }
+                        } else {
+                            true
+                        };
+                        if start_new {
+                            info!("Started a new SAS verification.");
+                            self.verification_cache.insert_sas(s);
+                        }
                     }
                     Err(c) => {
                         warn!(