add client method for one-time dehydration that can be awaited

src/client.js

@@ -582,6 +582,28 @@ MatrixClient.prototype.setDehydrationKey = async function(
     );
 };
 
+/**
+ * Creates a new dehydrated device (without queuing periodic dehydration)
+ * @param {Uint8Array} key the dehydration key
+ * @param {object} [keyInfo] Information about the key.  Primarily for
+ *     information about how to generate the key from a passphrase.
+ * @param {string} [deviceDisplayName] The device display name for the
+ *     dehydrated device.
+ * @return {Promise<String>} the device id of the newly created dehydrated device
+ */
+MatrixClient.prototype.createDehydratedDevice = async function(
+    key, keyInfo = {}, deviceDisplayName = undefined,
+) {
+    if (!(this._crypto)) {
+        logger.warn('not dehydrating device if crypto is not enabled');
+        return;
+    }
+    await this._crypto._dehydrationManager.setKey(
+        key, keyInfo, deviceDisplayName,
+    );
+    return await this._crypto._dehydrationManager.dehydrateDevice();
+};
+
 MatrixClient.prototype.exportDevice = async function() {
     if (!(this._crypto)) {
         logger.warn('not exporting device if crypto is not enabled');

src/crypto/dehydration.ts

@@ -132,7 +132,9 @@ export class DehydrationManager {
         }
         return matches;
     }
-    private async dehydrateDevice(): Promise<void> {
+
+    /** returns the device id of the newly created dehydrated device */
+    async dehydrateDevice(): Promise<string> {
         if (this.inProgress) {
             logger.log("Dehydration already in progress -- not starting new dehydration");
             return;
@@ -270,6 +272,8 @@ export class DehydrationManager {
             this.timeoutId = global.setTimeout(
                 this.dehydrateDevice.bind(this), oneweek,
             );
+
+            return deviceId;
         } finally {
             this.inProgress = false;
         }