Should have enclosed a test program. Here: http://folk.uio.no/hbf/testsize.c Output: process #0: open env process #1: open env process #0: write txn, mapsize 204800 process #1: write txn, mapsize 40960 process #2: open env, write txn, mapsize 40960 process #3: open env, write txn, mapsize 204800

Summarizing a reply which got sent privately instead of to ITS:

On 2014-01-24 00:07, Howard Chu wrote:

The doc says the caller of set_mapsize is required to make sure there are no active transactions when it is called. As such, X failed this requirement, and this sequence of events is explicitly unsupported.

No, I'm talking about X changing the mapsize when there is no txn, then afterwards doing a write txn so the mapsize gets written.

If Y doesn't start its write txn until after X finishes, then Y will see the new size.

It doesn't, that's the point. Only txn_commit pays attention, and it checks the size which existed in the wrong metapage.

Come to think of it, set_mapsize(); env_open(); txn begin/end should do the same. Y will either way not pay attention to the new mapsize.