Bot Crashes Rejoining Federated Rooms
Hey guys, let's dive into a super common but annoying issue that many bot developers run into: your bot suddenly crashing when it tries to rejoin a federated Matrix room it was previously kicked from. This isn't just a minor glitch; it can leave your bot stuck in an 'invited' state, completely unable to participate in the conversation. We're talking about that dreaded M_FORBIDDEN error, specifically the one citing "Event <event_id> has duplicate auth_events for ('m.room.member', '<bot_user>')". It's a mouthful, I know, but it points to a fundamental problem in how the Matrix homeserver (usually Synapse in these cases) handles event history, especially when a bot tries to re-establish its presence in a room after being removed. This article will break down exactly what's happening, why it's particularly tricky with federated rooms, and what we can do about it. We'll be looking at scenarios using the popular @vector-im/matrix-bot-sdk, but the underlying principles apply broadly to Matrix bot development.
So, what exactly is this `duplicate auth_events` error? In the Matrix world, every event in a room has a history, a chain of authentication that proves its legitimacy. Think of it like a digital audit trail. When a user joins a room, they generate a series of events (like `m.room.member` to change their state from 'invite' to 'join'). For these events to be valid, they need to be authorized by previous events in the room's history. The `auth_events` are the specific events that authorize the current one. When Synapse, the most common Matrix homeserver, sees that there are two *different* event chains trying to authorize the *same* state change for a specific user (in this case, the bot changing its membership status), it flags it as a security risk and rejects the join. This is often because the server is confused about the bot's true membership state – was it kicked and then rejoined, or is there some lingering state from before? This confusion is amplified in federated rooms because the event history has to be reliably passed between different servers, and sometimes, information can get tangled or duplicated during this process. If your bot is built using the @vector-im/matrix-bot-sdk, and you're encountering this, it's crucial to understand that the problem likely lies in the server's interpretation of the room's event graph, not necessarily a direct bug in your bot's code, although the SDK's handling of the join process can sometimes trigger it. We'll explore a practical scenario and look at how this might manifest, so stick around!
Reproducing the Bot Crash: A Step-by-Step Guide
To really get a handle on this `duplicate auth_events` issue, let's walk through a typical scenario that triggers it. This will help us understand the sequence of events that leads to the bot's failure. We'll use the example provided, which involves a bot built with the @vector-im/matrix-bot-sdk. The core of the problem appears when a bot, after being removed from a room, attempts to rejoin. It's a common enough use case – maybe a bot was temporarily disabled, or its permissions were changed, and now it needs to get back in. But Matrix, especially in a federated environment, can be a bit finicky about state transitions. Imagine you have a bot user, let's call it @mybot3:matrix.borna.golgolniamilad.ir, and you're using a federated server like matrix.org. The process starts innocently enough. You create a new room, let's name it join-test, and send a few messages to populate it. Then, you invite your bot, @mybot3, into this room. At this stage, everything is usually smooth sailing; the bot joins without a hitch. Now comes the critical part: you decide to remove the bot from the room. This could be done via a server admin command or a specific Matrix client action, effectively kicking it out. The bot's membership state in the room changes from 'join' to 'leave' (or potentially 'ban' depending on the action). The problem kicks in when you try to reverse this: you re-invite the *same* bot user, @mybot3, back into the join-test room. The bot receives the new m.room.member invite event, which is expected. However, when it attempts to process this invite and transition its state from 'invite' to 'join', the homeserver (Synapse in this case) throws a M_FORBIDDEN error. The specific error message, `Event "invite", preventing it from participating in the room. This is particularly frustrating because, from the bot developer's perspective, it's a seemingly simple action (rejoining a room) that results in a hard crash and an unrecoverable state without intervention.
This issue isn't isolated to a single user or server setup. It has been observed in federated rooms spanning different accounts and bots, highlighting that it's a systemic problem related to how Matrix handles state reconciliation, especially after a removal and re-invitation. The key takeaway here is that the sequence of removal and subsequent re-invitation, particularly across federated boundaries, seems to be the trigger. The underlying cause is the homeserver's inability to correctly reconstruct the event graph for the bot's membership after it has been removed and then re-invited. This leads to a state where the server believes there are multiple, conflicting valid histories for the bot joining the room, which is a security and integrity violation according to Matrix's protocol. So, while your bot code might be perfectly fine, the interaction with the server's state management, especially in complex federated scenarios, can lead to this disruptive crash. Understanding these reproduction steps is the first crucial step in debugging and finding a viable solution or workaround.
The Expected vs. Actual Behavior: What Should Happen?
Let's clarify what we'd ideally want to see when a bot, or any user for that matter, tries to rejoin a Matrix room after being removed. The expected behavior is straightforward and aligns with the principles of a functional communication platform: the bot should be able to seamlessly re-enter the room. This means that upon receiving a valid `m.room.member` invite event, the bot should successfully process it, and its membership state should transition cleanly from 'invite' to 'join'. There should be no crashes, no obscure error messages, and certainly no 'stuck' states. The Matrix protocol is designed to handle users joining, leaving, and rejoining rooms. While a removal might be a deliberate action, the ability to rejoin should be a standard operation. This clean transition ensures that the bot can resume its duties without interruption and that the room's state remains consistent. Essentially, the server should be able to reconcile the bot's re-entry based on the new invite, irrespective of its previous removal.
However, as we've seen with the `duplicate auth_events` error, the actual behavior is far from ideal. Instead of a smooth re-entry, the bot encounters a hard crash. The join attempt is rejected by the homeserver with the M_FORBIDDEN error, specifically citing the problematic `duplicate auth_events`. This prevents the membership state from ever updating from 'invite' to 'join'. The bot essentially gets stuck in a limbo state, receiving invites but unable to act upon them because the server rejects its attempt to fulfill the invite's conditions. This leads to a broken user experience and operational downtime for the bot. The core issue here is that the server's state reconciliation mechanism fails. It cannot correctly determine the canonical event history for the bot's membership, leading it to believe that the current join attempt is invalid due to conflicting historical data. This failure is particularly prominent in federated rooms, where the complexity of event propagation and state synchronization between different homeservers can exacerbate such issues. The SDK, in its attempt to process the invite and join the room, triggers this server-side validation error, resulting in the crash. It's a critical difference between a functional system and one that's effectively broken for this specific, and arguably common, use case.
Decoding the Log Snippet: The Heart of the Problem
Let's zoom in on the actual error message you might see in your bot's logs. This snippet is the most crucial piece of evidence when diagnosing the `duplicate auth_events` issue. The provided log shows: MatrixError: M_FORBIDDEN: Event $dDSCZSvvUSbiV6luVF2OJ207xjyXzsu91g1dFy9kYsA has duplicate auth_events for ('m.room.member', '@mybot3:matrix.borna.golgolniamilad.ir'): $f75tYW0pWwMNpM7PUUubQWY4vYv9uOQ8Qp1KtS-Zuew and $WVmSrGDOia6JFTs-19Fq1pUDNeVkEKxbaLdCdEnzhZw. This message is incredibly informative, guys. It tells us a few key things. First, the error code is M_FORBIDDEN, meaning the server (Synapse, in this setup) is explicitly denying the request. Second, and most importantly, the reason for denial is "duplicate auth_events". This confirms our suspicion that the server found conflicting authorization paths for the bot's membership event. The part ('m.room.member', '@mybot3:matrix.borna.golgolniamilad.ir') specifies exactly what state change is in conflict: the membership status (`m.room.member`) for a particular user (`@mybot3:matrix.borna.golgolniamilad.ir`). The subsequent event IDs ($f75t... and $WVmS...) are the specific events on the server that are causing the conflict. The server is essentially saying, "I've found two different valid chains of historical events that both lead to the bot's current membership state, and I don't know which one to trust, so I'm rejecting the new one." This situation typically arises when the server's representation of the room's event graph becomes inconsistent, often due to how events are propagated and reconciled, especially across federated servers. The log snippet also notes: "The bot receives an invite, logs `room.invite`, then immediately throws this error on join." This sequence is critical. It means the bot successfully received the invite event itself, but the subsequent action of *accepting* that invite and changing its state to 'join' is where the validation fails on the server. The @vector-im/matrix-bot-sdk, upon receiving the invite, correctly attempts to join the room. However, when it sends this join request to Synapse, Synapse performs its state checks, discovers the `duplicate auth_events`, and rejects the request, causing the SDK to throw the `MatrixError`. This log is your roadmap to understanding that the issue isn't necessarily in how your bot is *receiving* the invite, but in how the server is validating the *state transition* triggered by accepting that invite, particularly in the context of its historical event data.
Understanding the Suspected Cause: Event Graph Corruption
The suspected cause of this frustrating `duplicate auth_events` error boils down to a corrupted or inconsistent event graph on the Matrix homeserver, particularly when dealing with federated rooms. In Matrix, every room is essentially a directed acyclic graph (DAG) of events. Each event is linked to previous events that authorize it, forming a historical chain. When a user joins, leaves, or changes their membership, `m.room.member` events are created, and these events must be authorized by preceding events. The `auth_events` field in an event points to the specific events that prove its legitimacy. Now, imagine a scenario where a bot is removed from a room. The server records this departure. Later, when the bot is re-invited and tries to rejoin, the server needs to establish a new membership event that authorizes its presence. The problem arises if, during this process, the server gets confused about the bot's history. It might end up creating or identifying two *different* sets of `auth_events` that both seem to validly authorize the bot's membership status. This can happen for a variety of reasons:
- Federation inconsistencies: When events travel between different homeservers, there's a complex reconciliation process. If there are network issues, delays, or discrepancies in how servers handle event ordering and validation, it can lead to a state where a single event (or its authorization chain) appears duplicated or conflicting across different servers or even within the same server's memory.
- State resolution failures: Matrix has a robust state resolution mechanism to ensure all clients see a consistent view of the room's state (like who is a member, what the room name is, etc.). If this resolution process falters, especially after disruptive actions like removals, it can leave behind conflicting state information.
- Server-side bugs: While the SDK might be triggering the join attempt, the ultimate rejection comes from the homeserver (Synapse). It's possible that Synapse itself has a bug in how it handles state changes following a user removal and re-invitation, especially in federated contexts. It might not correctly prune or update historical state pointers, leading to the detection of duplicates.
The result is that the remote server, upon receiving the bot's join request, checks the provided `auth_events` and finds them contradictory. It cannot determine a single, unambiguous path through the event history to justify the bot's membership. To maintain data integrity, it rejects the join with the M_FORBIDDEN error. This effectively means the server believes accepting the bot's join would violate the room's history consistency rules. The bot, built with @vector-im/matrix-bot-sdk, faithfully attempts to join, but it's the server's internal state management that is failing. This suspected cause highlights why clearing local storage (like bot-storage.json) might not work, as the problem isn't in the bot's local state but in the server's understanding of the room's historical event graph. It's a deep-seated issue related to state synchronization and event lineage in distributed systems like Matrix.
Workarounds and Potential Solutions
Encountering the `duplicate auth_events` error when a bot tries to rejoin a federated room can be a real headache, leaving you searching for solutions. We've seen that clearing local storage, like the bot's `SimpleFsStorageProvider` files (`bot-storage.json`) and the crypto store, has not resolved the issue in the cases reported. This is a strong indicator that the problem lies server-side, within the homeserver's state management, rather than in the bot's local cache. Since the core issue is suspected to be a tangled event graph or inconsistent state on the homeserver, direct workarounds that can be implemented solely on the bot's side are limited. However, there are a few strategies and avenues to explore:
- Server-Side Intervention (Admin Action): The most effective, albeit often impractical for end-users, solution involves server administration. If you have access to the Synapse homeserver, actions like forcing a state resolution for the specific room or even 'hard-resetting' the room's state (a drastic measure) might clear the conflicting `auth_events`. Sometimes, simply kicking the user *again* and then re-inviting them after a short delay can help the server reconcile the state. However, this requires administrative privileges and can be disruptive.
- Bot Re-initialization (Full State Reset): While clearing `bot-storage.json` didn't fix it, a *complete* tear-down and rebuild of the bot's identity might offer a chance. This involves stopping the bot, deleting *all* its storage files (including crypto) and then starting it anew. If the bot uses a different Matrix user ID upon re-initialization, it would essentially be a
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