[Bug libc/15607] Add threadsafe version of getenv()

["bugdal at aerifal dot cx" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15607

Rich Felker <bugdal at aerifal dot cx> changed:

           What    |Removed                     |Added
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                 CC|                            |bugdal at aerifal dot cx

--- Comment #4 from Rich Felker <bugdal at aerifal dot cx> ---
I agree with Jakub. Programs that are or may be (due to libraries that may use
threads) multi-threaded cannot modify their own environments. There's little
point in modifying your own environment anyway; for executing external
programs, the exec-family functions and posix_spawn both provide interfaces by
which you can specify your own environment for the new process image in a fully
safe manner.

Note that getenv_r could not protect against modifications to the environment
through extern char **environ, nor could setenv_r protect against reads via
environ. Thus, they could only have limited success in making environment
modification "thread-safe".

As far as I'm concerned, this bug report should be filed against
gnome-settings-daemon, not glibc, and it should be fixed by (preferably)
avoiding modification to the environment in the process itself and using the
appropriate exec-type functions, or by generating a completely new environment
and replacing extern char **environ atomically with a pointer to the new
environment.

Note that, if others do end up deeming it desirable to change glibc, the
appropriate change would be having setenv do the above-described atomic
replacement and simply leak the old environment. This would be fully safe with
no locking.

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