As far as I can tell, bash deals with multiple shell sessions exactly how I'd expect (and want) it to: an individual terminal doesn't have access to the live session from other terminals, but when you close it it appends it's session to the shared history file, so each session ends up a continuous block in that file.
I don't know what it does exactly, but it always seems to lose history after a reboot. Either the last shell session to close overwrites the others, or they fail to save the history if not closed cleanly enough.
Wouldn't the ideal solution not be a component of a simple file writer, but rather, a buffered writer? A command gets executed, thrown into buffer. A wait process in bash checks buffer, and then writes to .bash_history?
As far as I can tell, bash deals with multiple shell sessions exactly how I'd expect (and want) it to: an individual terminal doesn't have access to the live session from other terminals, but when you close it it appends it's session to the shared history file, so each session ends up a continuous block in that file.