LSM1 Crash on lsm_close() Due to Invalid Forward Pointers in Merged Levels

Issue Overview: LSM1 Crash on lsm_close() with Assertion assert(iBlk!=0) and Invalid Forward Pointers

The core issue revolves around a crash in the LSM1 storage engine when calling lsm_close(). The crash is triggered by an assertion failure assert(iBlk!=0) in the LSM1 codebase, specifically during the cleanup and resource release phase of the database connection. The crash is reproducible and occurs when the database contains multiple levels, with one or more levels having been merged into a single database page without a valid B-tree structure. This merging process leaves behind invalid forward pointers from the previous levels, which are subsequently dereferenced during the lsm_close() operation, leading to the assertion failure.

The call stack provided in the discussion reveals that the crash originates from the fsRedirectBlock function in lsm_file.c, which is called during the lsmFsDbPageNext operation. This function attempts to navigate through the database pages using forward pointers, but encounters an invalid or zero block identifier (iBlk), causing the assertion to fail. The issue is further compounded by the fact that the database in question has five levels, with the fifth level merged into a single page. This merging process invalidates the forward pointers from the fourth level, leading to the loading of unused or freed database pages during the lsm_close() operation.

The problem is not limited to the lsm_close() function but is also indicative of a broader issue with how LSM1 handles merged levels and forward pointers. The invalid forward pointers can cause similar crashes during other operations, such as seeking within the database, as noted by Martijn in the discussion. The issue is particularly problematic because it can go unnoticed until the database is closed, making it difficult to diagnose and debug.

Possible Causes: Invalid Forward Pointers and Merged Levels Without B-Tree Structures

The primary cause of the crash is the presence of invalid forward pointers in the database, which are a result of merging levels without maintaining a valid B-tree structure. When levels are merged in LSM1, the forward pointers from the previous levels should be updated to reflect the new structure. However, in this case, the merging process has left behind invalid forward pointers, which are subsequently dereferenced during the lsm_close() operation.

The issue is exacerbated by the fact that the fifth level of the database has been merged into a single page without a B-tree structure. This merging process invalidates the forward pointers from the fourth level, as they no longer point to valid database pages. When the lsm_close() function attempts to navigate through the database pages using these invalid forward pointers, it encounters a zero block identifier (iBlk), triggering the assertion failure.

Another potential cause of the issue is the lack of proper optimization and maintenance of the database levels. The LSM1 storage engine relies on periodic optimization to merge levels and maintain a consistent structure. If the database is not optimized regularly, it can lead to the accumulation of invalid forward pointers and other inconsistencies, which can cause crashes during operations such as lsm_close().

The issue is also related to the way LSM1 handles the truncation of database files. The dbTruncateFile function, which is called during the lsm_close() operation, attempts to truncate the database file to free up unused space. However, if the database contains invalid forward pointers or other inconsistencies, the truncation process can fail, leading to the assertion failure.

Troubleshooting Steps, Solutions & Fixes: Addressing Invalid Forward Pointers and Optimizing Database Levels

To address the issue of invalid forward pointers and ensure the stability of the LSM1 storage engine, several troubleshooting steps and solutions can be implemented. The first step is to identify and fix the root cause of the invalid forward pointers, which is the merging of levels without maintaining a valid B-tree structure. This can be achieved by modifying the seekInSegment function to handle cases where the forward pointers are invalid or point to unused/freed database pages.

In the seekInSegment function, the condition if (iPtr == 0) should be replaced with if (iPtr < 1 || pPtr->pSeg->iFirst == pPtr->pSeg->iLastPg). This modification ensures that the function does not attempt to dereference invalid forward pointers, preventing the assertion failure. However, this fix may not work in cases where the level has been merged into a single page with cells and multiple overflow pages. In such cases, a more comprehensive solution is required.

A safer and more effective solution is to optimize the database levels using the lsm_work function. The lsm_work function merges all levels into a single, consistent structure, eliminating the need for forward pointers and preventing the accumulation of invalid pointers. The lsm_work function should be called after writing to the database, as follows:

rc = lsm_insert(pDb, "key", 3, "value", 5);
rc = lsm_work(pDb, 1, -1, 0); /* optimize */

This ensures that the database levels are merged and optimized, preventing the occurrence of invalid forward pointers and other inconsistencies. It is important to note that the lsm_work function should only be called after writing to the database, as calling it during read-only operations can lead to unnecessary overhead.

In addition to optimizing the database levels, it is also important to ensure that the database file is properly truncated during the lsm_close() operation. The dbTruncateFile function should be modified to handle cases where the database contains invalid forward pointers or other inconsistencies. This can be achieved by adding additional checks and safeguards to the truncation process, ensuring that it does not attempt to truncate invalid or freed database pages.

Finally, it is recommended to regularly back up the database and monitor its structure for any signs of inconsistencies or invalid forward pointers. Regular optimization and maintenance of the database levels can help prevent the accumulation of invalid pointers and other issues, ensuring the stability and reliability of the LSM1 storage engine.

In conclusion, the crash on lsm_close() with the assertion assert(iBlk!=0) is caused by invalid forward pointers resulting from the merging of levels without maintaining a valid B-tree structure. By modifying the seekInSegment function, optimizing the database levels using lsm_work, and ensuring proper truncation of the database file, the issue can be effectively addressed and prevented. Regular maintenance and monitoring of the database structure are also essential to ensure the long-term stability and reliability of the LSM1 storage engine.