Environment variables in dynamic modules

[Spencer Baugh <sbaugh@janestreet.com>]

In Emacs, process-environment (read by Lisp getenv) is distinct from C
environment variables (read by C getenv).

This means that a dynamic module which links against a library which
reads environment variables will not be affected by changes to
process-environment.

For example, if a user calls (setenv "VAR" "value") or binds
process-environment to (cons (cons "VAR" "value" process-environment)),
a getenv("VAR") in the dynamic module library won't return "value".
Likewise, if a dynamic module spawns subprocesses, they will inherit the
environment that the Emacs process started with, not the current
environment in process-environment.

This is usually unexpected, and causes difficult-to-track-down bugs,
especially for dynamic modules that spawn subprocesses or for large
dynamic modules with lots of functionality.

There are a number of possible ways to solve it:

A. Carefully track down every place that a library reads environment
   variables or spawns subprocesses, and pass in the Emacs environment
   instead.
   (but this is intractable in modules which call other libraries)

B. Advise Elisp setenv to also change the C environment
   (but this doesn't work with let-bindings of process-environment)

C. Set all variables in the C environment to match process-environment
   every time we call into the dynamic module
   (but this is slow and hurts performance)

D. Use linker tricks to replace C getenv with a version which calls back
   into Emacs.
   (but this doesn't work on other threads, since we can only call into
   Emacs from the main thread)

None of these are particularly satisfying.  I have implemented D, but
since my module uses multiple threads, it doesn't really solve the
problem for me.

Any suggestions?

Personally, I think some variation on B or C would be nicest, if it
could be done in a performant way which also works with let-bindings of
process-environment.  But I don't know how to do that.