make the transport discovery lk edgecase a EC exclusive workaround

Dont add it to the sdk.

sdk/src/media-backend/livekit/RemotePublishTransport.ts

@@ -8,8 +8,7 @@ Please see LICENSE in the repository root for full details.
 import { type Logger } from "matrix-js-sdk/lib/logger";
 import {
   type CallMembership,
-  getRemoteLivekitPublishCandidates,
-  type LivekitPublishCandidate,
+  isUnstableLivekitTransport,
   type Transport,
 } from "matrix-js-sdk/lib/matrixrtc";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
@@ -30,15 +29,15 @@ export interface RemotePublishTransportParams {
 
 /**
  * The SFU of another member that its homeserver lets us publish on, as the
- * transport to advertise, or null where no member's SFU will do. Candidates
- * are tried oldest membership first, and each one is checked by asking for a
+ * transport to advertise, or null where none does. Candidates are tried
+ * oldest membership first, each SFU once, and each is checked by asking for a
  * publisher token through our homeserver (MSC4195): a homeserver may restrict
  * remote publishers, which shows as a token without the publish grant.
  */
-export async function findRemotePublishTransport({
+export async function findRemotePublishLivekitTransport({
   client,
   memberships,
-  ownMembershipIdentity,
+  ownMembershipIdentity: own,
   roomId,
   slotId,
   logger,
@@ -46,45 +45,51 @@ export async function findRemotePublishTransport({
   logger.info(
     "No local transport available, looking for another member's SFU to publish on",
   );
-  const candidates = getRemoteLivekitPublishCandidates(
-    memberships,
-    ownMembershipIdentity,
-  );
-  for (const candidate of candidates) {
-    const outcome = await tryCandidate(candidate);
-    if (outcome === "usable") return candidate.transport;
-    if (outcome === "noCsApi") return null;
+  const tried = new Set<string>();
+  const byAge = [...memberships].sort((a, b) => a.createdTs() - b.createdTs());
+  for (const membership of byAge) {
+    if (
+      membership.userId === own.userId &&
+      membership.deviceId === own.deviceId
+    )
+      continue;
+    for (const transport of membership.transports) {
+      // Publishing remotely goes through the client-server API, which a JWT
+      // service cannot stand in for, so only a `url` qualifies
+      if (!isUnstableLivekitTransport(transport) || !("url" in transport))
+        continue;
+      const { url } = transport;
+      const serverName = membership.getTransportServerName(transport);
+      const key = JSON.stringify([url, serverName]);
+      if (tried.has(key)) continue;
+      tried.add(key);
+      try {
+        const { jwt } = await csApi.getSFUConfig({
+          client,
+          membership: own,
+          url,
+          serverName,
+          roomId,
+          slotId,
+        });
+        if (decodeSFUJWTPayload(jwt).video.canPublish) {
+          logger.info(`Publishing on remote SFU ${url} of ${serverName}`);
+          return { type: "livekit", url, server_name: serverName };
+        }
+        logger.info(
+          `${serverName} does not let us publish on ${url}, skipping`,
+        );
+      } catch (e) {
+        if (isCsApiUnavailable(e)) {
+          // Without the endpoint no further candidate can work
+          logger.warn(
+            `Cannot use the MSC4195 get_token endpoint (${e}), so no remote SFU can be used`,
+          );
+          return null;
+        }
+        logger.warn(`Failed to get a token for ${url} of ${serverName}:`, e);
+      }
+    }
   }
   return null;
-
-  /** "noCsApi" ends the search: without the endpoint no candidate can work. */
-  async function tryCandidate({
-    url,
-    serverName,
-  }: LivekitPublishCandidate): Promise<"usable" | "unusable" | "noCsApi"> {
-    try {
-      const { jwt } = await csApi.getSFUConfig({
-        client,
-        membership: ownMembershipIdentity,
-        url,
-        serverName,
-        roomId,
-        slotId,
-      });
-      if (decodeSFUJWTPayload(jwt).video.canPublish) {
-        logger.info(`Publishing on remote SFU ${url} of ${serverName}`);
-        return "usable";
-      }
-      logger.info(`${serverName} does not let us publish on ${url}, skipping`);
-    } catch (e) {
-      if (isCsApiUnavailable(e)) {
-        logger.warn(
-          `Cannot use the MSC4195 get_token endpoint (${e}), so no remote SFU can be used`,
-        );
-        return "noCsApi";
-      }
-      logger.warn(`Failed to get a token for ${url} of ${serverName}:`, e);
-    }
-    return "unusable";
-  }
 }