Add rudimentary link+sync support.

libsignal-service/src/main/java/org/whispersystems/signalservice/api/SignalServiceMessageReceiver.java

@@ -7,7 +7,6 @@
 package org.whispersystems.signalservice.api;
 
 import org.signal.core.util.StreamUtil;
-import org.signal.core.util.logging.Log;
 import org.signal.libsignal.protocol.InvalidMessageException;
 import org.signal.libsignal.zkgroup.profiles.ProfileKey;
 import org.whispersystems.signalservice.api.backup.MediaRootBackupKey;
@@ -17,6 +16,7 @@ import org.whispersystems.signalservice.api.crypto.AttachmentCipherStreamUtil;
 import org.whispersystems.signalservice.api.crypto.ProfileCipherInputStream;
 import org.whispersystems.signalservice.api.messages.SignalServiceAttachment.ProgressListener;
 import org.whispersystems.signalservice.api.messages.SignalServiceAttachmentPointer;
+import org.whispersystems.signalservice.api.messages.SignalServiceAttachmentRemoteId;
 import org.whispersystems.signalservice.api.messages.SignalServiceDataMessage;
 import org.whispersystems.signalservice.api.messages.SignalServiceStickerManifest;
 import org.whispersystems.signalservice.api.push.exceptions.MissingConfigurationException;
@@ -38,6 +38,8 @@ import java.util.Map;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
+import kotlin.Unit;
+
 /**
  * The primary interface for receiving Signal Service messages.
  *
@@ -200,6 +202,16 @@ public class SignalServiceMessageReceiver {
     socket.retrieveBackup(cdnNumber, headers, cdnPath, destination, 1_000_000_000L, listener);
   }
 
+  /**
+   * Retrieves a link+sync backup file. The data is written to @{code destination}.
+   */
+  public @Nonnull NetworkResult<Unit> retrieveLinkAndSyncBackup(int cdn, @Nonnull String key, @Nonnull File destination, @Nullable ProgressListener listener) {
+    return NetworkResult.fromFetch(() -> {
+      socket.retrieveAttachment(cdn, Collections.emptyMap(), new SignalServiceAttachmentRemoteId.V4(key), destination, 1_000_000_000L, listener);
+      return Unit.INSTANCE;
+    });
+  }
+
   public @Nonnull ZonedDateTime getCdnLastModifiedTime(int cdnNumber, Map<String, String> headers, String cdnPath) throws MissingConfigurationException, IOException {
     return socket.getCdnLastModifiedTime(cdnNumber, headers, cdnPath);
   }

libsignal-service/src/main/java/org/whispersystems/signalservice/api/link/LinkDeviceApi.kt

@@ -212,4 +212,27 @@ class LinkDeviceApi(
     val request = WebSocketRequestMessage.put("/v1/accounts/name?deviceId=$deviceId", SetDeviceNameRequest(encryptedDeviceName))
     return NetworkResult.fromWebSocketRequest(authWebSocket, request)
   }
+
+  /**
+   * A "long-polling" endpoint that will return once the primary device has successfully sent sync data.
+   *
+   * @param timeout The max amount of time to wait. Capped at 30 seconds.
+   *
+   * GET /v1/devices/transfer_archive?timeout=[timeout]
+   *
+   * - 200: Success, the primary device was sent backup sync data.
+   * - 204: The primary didn't provide data before the max waiting time elapsed.
+   * - 400: Invalid timeout.
+   * - 429: Rate-limited.
+   */
+  fun waitForPrimaryDevice(timeout: Duration = 30.seconds): NetworkResult<TransferArchiveResponse> {
+    val request = WebSocketRequestMessage.get("/v1/devices/transfer_archive?timeout=${timeout.inWholeSeconds}")
+    return NetworkResult
+      .fromWebSocketRequest(
+        signalWebSocket = authWebSocket,
+        request = request,
+        timeout = timeout,
+        webSocketResponseConverter = NetworkResult.LongPollingWebSocketConverter(TransferArchiveResponse::class)
+      )
+  }
 }

libsignal-service/src/main/java/org/whispersystems/signalservice/api/link/TransferArchiveResponse.kt

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.signalservice.api.link
+
+import com.fasterxml.jackson.annotation.JsonCreator
+import com.fasterxml.jackson.annotation.JsonProperty
+
+/**
+ * Data from primary on where to find link+sync backup file.
+ */
+data class TransferArchiveResponse @JsonCreator constructor(
+  @JsonProperty val cdn: Int,
+  @JsonProperty val key: String
+)

libsignal-service/src/main/java/org/whispersystems/signalservice/api/provisioning/ProvisioningSocket.kt

@@ -249,7 +249,7 @@ class ProvisioningSocket<T> private constructor(
     private fun generateProvisioningUrl(deviceAddress: String): String {
       val encodedDeviceId = URLEncoder.encode(deviceAddress, "UTF-8")
       val encodedPubKey: String = URLEncoder.encode(Base64.encodeWithoutPadding(cipher.secondaryDevicePublicKey.serialize()), "UTF-8")
-      return "sgnl://${mode.host}?uuid=$encodedDeviceId&pub_key=$encodedPubKey"
+      return "sgnl://${mode.host}?uuid=$encodedDeviceId&pub_key=$encodedPubKey${mode.params}"
     }
 
     private suspend fun keepAlive(webSocket: WebSocket) {
@@ -288,9 +288,9 @@ class ProvisioningSocket<T> private constructor(
     }
   }
 
-  enum class Mode(val host: String) {
-    REREG("rereg"),
-    LINK("linkdevice")
+  enum class Mode(val host: String, val params: String) {
+    REREG("rereg", ""),
+    LINK("linkdevice", "&capabilities=backup4")
   }
 
   fun interface ProvisioningSocketExceptionHandler {