support switching startEvent while waiting for .accept on initiator side

if we get a .start event from the other party and we've also sent one,
the .start event with the sender that is first in sorting order should
be taken, and the other one ignored.

At the point where we will receive it, the verifier has already
been returned from beginKeyVerification, so we'll need to switch
start event internally, and retry the verification, now on the
receiver (sending .accept) side instead of initiator side
(sending .start).

src/crypto/verification/Base.js

@@ -27,6 +27,13 @@ import {newTimeoutError} from "./Error";
 
 const timeoutException = new Error("Verification timed out");
 
+export class SwitchStartEventError extends Error {
+    constructor(startEvent) {
+        super();
+        this.startEvent = startEvent;
+    }
+}
+
 export class VerificationBase extends EventEmitter {
     /**
      * Base class for verification methods.
@@ -69,6 +76,19 @@ export class VerificationBase extends EventEmitter {
         this._transactionTimeoutTimer = null;
     }
 
+    get initiatedByMe() {
+        // if there is no start event yet,
+        // we probably want to send it,
+        // which happens if we initiate
+        if (!this.startEvent) {
+            return true;
+        }
+        const sender = this.startEvent.getSender();
+        const content = this.startEvent.getContent();
+        return sender === this._baseApis.getUserId() &&
+            content.from_device === this._baseApis.getDeviceId();
+    }
+
     _resetTimer() {
         logger.info("Refreshing/starting the verification transaction timeout timer");
         if (this._transactionTimeoutTimer !== null) {
@@ -104,6 +124,22 @@ export class VerificationBase extends EventEmitter {
         });
     }
 
+    canSwitchStartEvent() {
+        return false;
+    }
+
+    switchStartEvent(event) {
+        if (this.canSwitchStartEvent(event)) {
+            if (this._rejectEvent) {
+                const reject = this._rejectEvent;
+                this._rejectEvent = undefined;
+                reject(new SwitchStartEventError(event));
+            } else {
+                this.startEvent = event;
+            }
+        }
+    }
+
     handleEvent(e) {
         if (this._done) {
             return;

src/crypto/verification/SAS.js

@@ -19,7 +19,7 @@ limitations under the License.
  * @module crypto/verification/SAS
  */
 
-import {VerificationBase as Base} from "./Base";
+import {VerificationBase as Base, SwitchStartEventError} from "./Base";
 import anotherjson from 'another-json';
 import {
     errorFactory,
@@ -29,6 +29,8 @@ import {
     newUserCancelledError,
 } from './Error';
 
+const START_TYPE = "m.key.verification.start";
+
 const EVENTS = [
     "m.key.verification.accept",
     "m.key.verification.key",
@@ -201,16 +203,37 @@ export class SAS extends Base {
         // make sure user's keys are downloaded
         await this._baseApis.downloadKeys([this.userId]);
 
-        if (this.startEvent) {
-            return await this._doRespondVerification();
-        } else {
+        let retry = false;
+        do {
+            try {
+                if (this.initiatedByMe) {
                     return await this._doSendVerification();
+                } else {
+                    return await this._doRespondVerification();
                 }
+            } catch (err) {
+                if (err instanceof SwitchStartEventError) {
+                    // this changes what initiatedByMe returns
+                    this.startEvent = err.startEvent;
+                    retry = true;
+                } else {
+                    throw err;
+                }
+            }
+        } while (retry);
     }
 
-    async _doSendVerification() {
-        const type = "m.key.verification.start";
-        const initialMessage = this._channel.completeContent(type, {
+    canSwitchStartEvent(event) {
+        if (event.getType() !== START_TYPE) {
+            return false;
+        }
+        const content = event.getContent();
+        return content && content.method === SAS.NAME &&
+            this._waitingForAccept;
+    }
+
+    async _sendStart() {
+        const startContent = this._channel.completeContent(START_TYPE, {
             method: SAS.NAME,
             from_device: this._baseApis.deviceId,
             key_agreement_protocols: KEY_AGREEMENT_LIST,
@@ -219,11 +242,33 @@ export class SAS extends Base {
             // FIXME: allow app to specify what SAS methods can be used
             short_authentication_string: SAS_LIST,
         });
-        // add the transaction id to the message beforehand because
-        // it needs to be included in the commitment hash later on
-        this._channel.sendCompleted(type, initialMessage);
+        await this._channel.sendCompleted(START_TYPE, startContent);
+        return startContent;
+    }
 
-        let e = await this._waitForEvent("m.key.verification.accept");
+    async _doSendVerification() {
+        this._waitingForAccept = true;
+        let startContent;
+        if (this.startEvent) {
+            startContent = this._channel.completedContentFromEvent(this.startEvent);
+        } else {
+            startContent = await this._sendStart();
+        }
+
+        // we might have switched to a different start event,
+        // but was we didn't call _waitForEvent there was no
+        // call that could throw yet. So check manually that
+        // we're still on the initiator side
+        if (!this.initiatedByMe) {
+            throw new SwitchStartEventError(this.startEvent);
+        }
+
+        let e;
+        try {
+            e = await this._waitForEvent("m.key.verification.accept");
+        } finally {
+            this._waitingForAccept = false;
+        }
         let content = e.getContent();
         const sasMethods
               = intersection(content.short_authentication_string, SAS_SET);
@@ -248,7 +293,7 @@ export class SAS extends Base {
             e = await this._waitForEvent("m.key.verification.key");
             // FIXME: make sure event is properly formed
             content = e.getContent();
-            const commitmentStr = content.key + anotherjson.stringify(initialMessage);
+            const commitmentStr = content.key + anotherjson.stringify(startContent);
             // TODO: use selected hash function (when we support multiple)
             if (olmutil.sha256(commitmentStr) !== hashCommitment) {
                 throw newMismatchedCommitmentError();