Bug: fseg_get_pages_info does not release pages during extent iteration

先阅读 storage/innobase/fsp/fsp0fsp.cc 中的 fseg_get_pages_info()，然后检查 storage/innobase/include/mtr0mtr.h 和 storage/innobase/mtr/mtr0mtr.cc 中 mtr_t 的声明与实现。跟踪 issue 中描述的 CHECK INDEX 路径。完成的标准是：extent 扫描限定所保留的页闩锁，同时保留现有的页面信息结果和 CHECK INDEX 行为。

Bug: fseg_get_pages_info does not release pages during extent iteration

Problem

In fseg_get_pages_info() (storage/innobase/fsp/fsp0fsp.cc), when scanning the FSEG_NOT_FULL and FSEG_FULL extent lists for a segment, the mtr holds latches on every visited page descriptor (xdes) without releasing them until the entire function completes and mtr_commit() is called.

For tables with a large number of extents (large tables or tables with fragmented segments), this can mean holding hundreds or thousands of page latches simultaneously within a single mini-transaction. This causes:

• Excessive buffer pool frame consumption (each latched page pins a buffer pool frame)
• Increased memory pressure and potential OOM
• CHECK INDEX on large tables becomes unreliable (may OOM)

Root Cause

The mtr_t memo stack grows unbounded during extent list traversal in fseg_get_pages_info(). Each call to xdes_lst_get_descriptor() and xdes_mtr_get_bit() adds a page latch to the mtr memo, but these latches are only released at mtr_commit() which happens at the very end of the function.

Fix

Add a new function mtr_t::release_all_after_savepoint() that releases all pages held in the mtr memo after a given savepoint. Then in fseg_get_pages_info(), set a savepoint and release pages every 100 iterations, effectively bounding the number of simultaneously held latches.

Changes

1. storage/innobase/include/mtr0mtr.h: Declare release_all_after_savepoint(ulint savepoint) method and mtr_release_all_after_savepoint() macro.

2. storage/innobase/mtr/mtr0mtr.cc: Implement Release_all_after_savepoint functor and mtr_t::release_all_after_savepoint() method.

3. storage/innobase/fsp/fsp0fsp.cc: In fseg_get_pages_info(), add savepoint and release logic every 100 pages during FSEG_NOT_FULL and FSEG_FULL list scans.

Affected Scenarios

• CHECK INDEX on large tables — ha_innobase::check_index() → innobase_get_index_status() → fseg_get_pages_info() for both leaf and top segments of each index