Server Crash On Login: Bug Report & Fix For Failed Achievements

Experiencing server crashes when players log in can be a nightmare for any game administrator. This article delves into a specific bug report detailing a server crash issue in a game environment, particularly when players with failed achievements attempt to log in. We'll break down the bug, its causes, steps to reproduce it, and potential solutions. Let's dive in and explore this critical issue.

Understanding the Bug: Server Crash on Player Login

At the heart of the matter is a server crash that occurs when certain characters attempt to log in. The key factor triggering this crash appears to be related to failed achievements within the game. The bug report suggests that the issue was either introduced or highlighted by a specific commit (a0d6b00c5308bb81e55cac8268817dec63629145), pointing towards a recent change in the codebase as the potential source. Understanding the root cause is crucial for implementing an effective fix. Our main keyword, server crash, underscores the severity of the problem and its impact on player experience.

The specific location of the bug is pinpointed to line 702 within the AchievementMgr class. The code snippet provided in the bug report reveals the following condition:

if (failTime <= GetPlayer()->GetMap()->GetCurrentClockTime())

This line attempts to access the player's map using GetPlayer()->GetMap(). However, the report highlights that in certain scenarios, the map pointer is null, meaning it hasn't been initialized yet. This leads to a crash when the code tries to dereference a null pointer. The report further suggests a potential fix by referencing a similar check used elsewhere in the file:

Map const* map = GetPlayer()->IsInWorld() ? GetPlayer()->GetMap() : sMapMgr.FindMap(GetPlayer()->GetMapId(), GetPlayer()->GetInstanceId());

This alternative approach checks if the player is already in the world before attempting to access the map, providing a safer way to handle potentially uninitialized map data. It is critical to address such null pointer dereferences as they can lead to unpredictable behavior and system instability. Moreover, the bug report warns that other lines added in the same commit might suffer from a similar issue, indicating a need for thorough code review and testing.

Steps to Reproduce the Server Crash

To effectively address a bug, it's essential to be able to reliably reproduce it. The bug report outlines a simple yet crucial step: login with a character who has a failed achievement. This narrows down the scope of the issue and provides a clear starting point for developers to investigate. By consistently triggering the crash, developers can more easily test potential fixes and ensure they are effective. This step-by-step approach is vital for efficient debugging and resolution.

Expected Behavior: A Stable Login Process

The expected behavior when logging in, regardless of a character's achievement status, is a smooth, crash-free experience. Players should be able to access the game without encountering unexpected server interruptions. This expectation is fundamental to maintaining a positive player experience and preserving the integrity of the game world. When a server crashes, it not only disrupts gameplay but can also lead to data loss and frustration among players. Therefore, ensuring a stable login process is paramount for any online game or application.

Diving into the Crash Log: Unraveling the Technical Details

The crash log is a treasure trove of information for developers trying to diagnose the root cause of a problem. Let's break down the key parts of the provided crash log:

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0)
    at pthread_kill.c:44
#1  0x00007fec75480493 in __pthread_kill_internal (threadid=<optimized out>, signo=6) at pthread_kill.c:89
#2  0x00007fec7542618e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007fec7540d6d0 in __GI_abort () at abort.c:77
#4  0x00007fec7540d639 in __assert_fail_base (fmt=<optimized out>, assertion=<optimized out>, file=<optimized out>,
    line=<optimized out>, function=<optimized out>) at assert.c:118
#5  0x00000000004b2157 in WorldObject::GetMap (this=0x7fec35aac4c0)
    at /home/myuser/mangos/mangos-wotlk/src/game/Entities/Object.h:1181
#6  0x0000000000b8f272 in AchievementMgr::LoadFromDB (this=0x7fec35ab02f0,
    achievementResult=std::unique_ptr<QueryResult> = {...}, criteriaResult=std::unique_ptr<QueryResult> = {...})
    at /home/myuser/mangos/mangos-wotlk/src/game/Achievements/AchievementMgr.cpp:702
#7  0x000000000062929f in Player::LoadFromDB (this=0x7fec35aac4c0, guid=..., holder=0x7fec3b566a20)
    at /home/myuser/mangos/mangos-wotlk/src/game/Entities/Player.cpp:16399
#8  0x0000000000c52b7c in WorldSession::HandlePlayerLogin (this=0x7fec1800fa80, holder=0x7fec3b566a20)
    at /home/myuser/mangos/mangos-wotlk/src/game/Entities/CharacterHandler.cpp:839

The log presents a stack trace, showing the sequence of function calls that led to the crash. Here's a breakdown:

• Lines #0-#4: Indicate a signal (likely SIGABRT) was raised, leading to the program's termination. This is a common result of an assertion failure or a critical error.
• Line #5: Points to the WorldObject::GetMap function as the immediate source of the crash. This confirms the bug report's suspicion that the crash occurs when trying to access a map that might not be valid.
• Line #6: Shows that the GetMap call originates from AchievementMgr::LoadFromDB, specifically line 702, as mentioned in the bug report. This pinpoints the exact location of the issue within the codebase.
• Lines #7-#8: Trace the call stack further up, indicating the crash occurs during the player loading process (Player::LoadFromDB) within the WorldSession::HandlePlayerLogin function. This confirms the context of the crash: it happens when a player is logging in.

By carefully analyzing the crash log, developers can gain a deep understanding of the events leading up to the crash, making it easier to identify and fix the underlying bug. This level of detail is invaluable for efficient debugging and problem-solving.

The Importance of a Suggested Workaround

While the bug report doesn't include a suggested workaround, having one can be incredibly helpful. A workaround is a temporary solution that allows players to continue using the game or application while the underlying bug is being fixed. In this case, a potential workaround might involve temporarily disabling the loading of failed achievements during login. However, this could have other consequences, so a thorough understanding of the system is essential before implementing any workaround. A well-crafted workaround can significantly reduce the impact of a bug on users, providing a smoother experience until a permanent fix is available.

Identifying the Core SHA1 Commit Hash and Database SHA1 Commit Hash

The bug report provides crucial versioning information in the form of SHA1 commit hashes for both the core and the database:

• Core SHA1 Commit Hash: 1a3b897169c969770180c7c9f714cbb84dcb052c
• Database SHA1 Commit Hash: 37ba39442dbf237b72b14ef2d5f43618e4f2aaf1

These hashes are like fingerprints, uniquely identifying specific versions of the codebase and database schema. This information is invaluable for developers as it allows them to:

• Pinpoint the exact version where the bug was introduced: By comparing the commit hash to previous versions, developers can narrow down the timeframe when the bug appeared.
• Replicate the environment: Knowing the specific versions of the core and database ensures that developers are testing the fix in the same environment where the bug was originally reported.
• Track changes: The commit hashes allow developers to trace the history of changes made to the codebase and database, helping them understand how the bug might have been introduced and what other changes might be affected by the fix.

This level of detail is crucial for effective bug tracking and resolution, especially in complex software projects.

The Role of Operating System and Client Version

The bug report also includes information about the operating system and client version:

• Operating System: Fedora 43
• Client Version: 3.3.5a (Wrath of the Lich King)

This information helps to understand if the bug is specific to a particular operating system or client version. Some bugs might only manifest on certain platforms or with specific software configurations. Knowing the operating system and client version allows developers to:

• Identify platform-specific issues: If the bug is only reproducible on Fedora 43, it might indicate a compatibility issue with that operating system.
• Target the fix: The fix might need to be tailored to a specific client version if the bug is related to changes in the client code.
• Prioritize testing: Testing efforts can be focused on the operating systems and client versions where the bug is most prevalent.

This contextual information is essential for a comprehensive understanding of the bug and its potential impact.

Addressing the Server Crash: A Potential Solution

Based on the bug report and crash log analysis, a potential solution involves modifying the AchievementMgr::LoadFromDB function to safely access the player's map. The suggested fix in the bug report provides a good starting point:

Map const* map = GetPlayer()->IsInWorld() ? GetPlayer()->GetMap() : sMapMgr.FindMap(GetPlayer()->GetMapId(), GetPlayer()->GetInstanceId());
if (failTime <= (map ? map->GetCurrentClockTime() : 0))

This code first checks if the player is in the world using GetPlayer()->IsInWorld(). If the player is in the world, it retrieves the map using GetPlayer()->GetMap(). Otherwise, it attempts to find the map using sMapMgr.FindMap(). The added null check (map ? map->GetCurrentClockTime() : 0) before accessing GetCurrentClockTime() handles the case where the map is still null, preventing the crash. This approach ensures that the code only accesses the map if it is valid, preventing the null pointer dereference. It is crucial to test this fix thoroughly to ensure it resolves the issue without introducing new problems.

Conclusion: Resolving Server Crashes for a Better Player Experience

Server crashes can severely impact the player experience, making it critical to address them promptly and effectively. This bug report provides a detailed analysis of a specific server crash issue related to failed achievements during player login. By understanding the bug details, steps to reproduce, crash log, and potential solutions, developers can work towards resolving the issue and providing a more stable gaming environment. Remember, a proactive approach to bug fixing is essential for maintaining a healthy and enjoyable gaming community. For more information on debugging and game development best practices, consider visiting a trusted resource like GameDev.net.