The transient marker said nothing outside the module points into it once the
call returns - true of its text, silent about its data. A string literal is
emitted into the evaluating module's own image and an expression may store one
anywhere: C-x C-e on (set msg "tuned") left a program global pointing into the
mapping the agent was about to drop. The next thunk can be mapped at the same
address, so what comes back is silent garbage rather than a fault, and nothing
in the compiler refused it.
The third condition is that the module emitted no string constants. Then there
is nothing in its image anyone could still be pointing at. One that did keeps
its mapping, which costs a page and is the bargain every redefinition already
makes.
Found by reading jank, which has met the neighbouring hazard from the other
side: its notes are explicit that nothing is ever unloaded, and the one place
Flan makes an exception is the one place the rule had a hole.
Joseph Ferano
5f0bde8149