Re: [PATCH 0/6] getenv() timing fixes

[Jeff King <peff@peff.net>]

On Tue, Jan 15, 2019 at 02:41:42PM -0500, Jeff King wrote:

> The more involved one (that doesn't pass along memory ownership) is
> something like:
> 
>   static struct hashmap env_cache;
> 
>   const char *getenv_safe(const char *name)
>   {
> 
> 	if (e = hashmap_get(&env_cache, name))
> 		return e->value;
> 
>         /* need some trickery to make sure xstrdup does not call getenv */
> 	e->value = xstrdup_or_null(getenv(name));
> 	e->name = xstrdup(name);
> 	hashmap_put(&env_cache, e);
> 
> 	return e->value;
>   }
> 
> with a matching setenv_safe() to drop the hashmap entry. Come to think
> of it, this is really pretty equivalent to string-interning, which we
> already have a hashmap for. I think one could argue that string
> interning is basically just a controlled form of memory leaking, but
> it's probably a reasonable compromise in this instance (i.e., we expect
> to ask about a finite number of variables anyway; the important thing is
> just that we don't leak memory for the same variable over and over).

So actually, that's pretty easy to do without writing much code at all.
Something like:

  #define xgetenv(name) strintern(getenv(name))

It means we're effectively storing the environment twice in the worst
case, but that's probably not a big deal. Unless we have a loop which
does repeated setenv()/getenv() calls, the strintern hashmap can't grow
without bound.

-Peff