history should not behave like a "two-level store"

[andychu]

The thread over in #320 made me realize what is wrong with bash history. It has two parts:

• the history in the current shell process
• the persisted history in the ~/.bash_history file

When you exit a process, it appends to the history file shopt -s histappend

This is really confusing because most people use multiple shells at once! Exiting a shell shouldn't cause more lines to appear in the history of another shell!

This is the expected model for GNU readline, and OSH inherits it, but shouldn't.

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It's not clear what the solution is, but ideas are:

• History could be stored in a unique file keyed by (PID, timestamp). Then all of them would have to be searched for recent commands?) (History being stored in a separate file for each pwd ameliorates the problem, but doesn't solve it.)
• try doing file system tricks for atomic writes (note that network file systems don't always obey POSIX)
• use an sqlite database (don't want this to be a required dependency)

Probably the answer is to let the user register a history callback, something better than PROMPT_COMMAND. Then the plugins can do what they want, like use an sqlite database or whatever, or use separate files per directory, etc.