History entries lost with multiple ksh sessions

[aweeraman]

When several ksh instances start concurrently with the same $HISTFILE (for example, a desktop session restoring many terminals at login), the history file can end up losing entries. The cause appears to be the trimming that happens during history initialization when the file has grown past its size threshold, which has a race on it.

This is a long-standing behavior inherited from the original ksh93, and dates to at least 2013 and likely earlier. It's still present in 1.0.10. In practice it is low severity and only affects shell history, the trigger is fairly specific with simple workarounds to avoid it. I have attached a small reproducer that triggers it reliably.

Version: 1.0.10 / Debian unstable
Reproducer: repro-721986.sh
Debian Bug#721986

[jghub]

an added 'fuzzy' piece of information: I see sporadically complete(?) or at least massive loss of history, as well as growth well beyond HISTSIZE (2**13 in my case in in the absence of any apparent races, i.e. during regular interactive use, switching between of 5-10 shells): a few weeks ago history size was well beyond 10000, then I suddenly noted that I could not ^R-find some command, looked at history and it was down to some 20 entries.... same happened similarly again recently where, when I looked, the history length was about 700 but was several 1000 shortly before. so I suspect what I see here are possibly intermittent complete losses of the histfile? so far I did not open an issue since I do not know how to trigger/reproduce it. but something is fishy for sure.. so just to point out that the issue might not necessarily have to do with such races as you describe (in my case machine runs uninterrupted w/o reboot for many months and I can't recall any x11 window manager crash recently).

am seeing this with Version AJM 93u+m/1.1.0-alpha+4fba84c3 2026-04-22.