wrapper.c: add x{un,}setenv(), and use xsetenv() in environment.c

Add fatal wrappers for setenv() and unsetenv(). In d7ac12b25d (Add
set_git_dir() function, 2007-08-01) we started checking its return
value, and since 48988c4d0c (set_git_dir: die when setenv() fails,
2018-03-30) we've had set_git_dir_1() die if we couldn't set it.

Let's provide a wrapper for both, this will be useful in many other
places, a subsequent patch will make another use of xsetenv().

The checking of the return value here is over-eager according to
setenv(3) and POSIX. It's documented as returning just -1 or 0, so
perhaps we should be checking -1 explicitly.

Let's just instead die on any non-zero, if our C library is so broken
as to return something else than -1 on error (and perhaps not set
errno?) the worst we'll do is die with a nonsensical errno value, but
we'll want to die in either case.

Let's make these return "void" instead of "int". As far as I can tell
there's no other x*() wrappers that needed to make the decision of
deviating from the signature in the C library, but since their return
value is only used to indicate errors (so we'd die here), we can catch
unreachable code such as

    if (xsetenv(...) < 0)
        [...];

I think it would be OK skip the NULL check of the "name" here for the
calls to die_errno(). Almost all of our setenv() callers are taking a
constant string hardcoded in the source as the first argument, and for
the rest we can probably assume they've done the NULL check
themselves. Even if they didn't, modern C libraries are forgiving
about it (e.g. glibc formatting it as "(null)"), on those that aren't,
well, we were about to die anyway. But let's include the check anyway
for good measure.

1. https://pubs.opengroup.org/onlinepubs/009604499/functions/setenv.html

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -330,8 +330,7 @@ char *get_graft_file(struct repository *r)
 
 static void set_git_dir_1(const char *path)
 {
-	if (setenv(GIT_DIR_ENVIRONMENT, path, 1))
-		die(_("could not set GIT_DIR to '%s'"), path);
+	xsetenv(GIT_DIR_ENVIRONMENT, path, 1);
 	setup_git_env(path);
 }
 

git-compat-util.h

@@ -875,6 +875,8 @@ void *xmemdupz(const void *data, size_t len);
 char *xstrndup(const char *str, size_t len);
 void *xrealloc(void *ptr, size_t size);
 void *xcalloc(size_t nmemb, size_t size);
+void xsetenv(const char *name, const char *value, int overwrite);
+void xunsetenv(const char *name);
 void *xmmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);
 const char *mmap_os_err(void);
 void *xmmap_gently(void *start, size_t length, int prot, int flags, int fd, off_t offset);

wrapper.c

@@ -145,6 +145,18 @@ void *xcalloc(size_t nmemb, size_t size)
 	return ret;
 }
 
+void xsetenv(const char *name, const char *value, int overwrite)
+{
+	if (setenv(name, value, overwrite))
+		die_errno(_("could not setenv '%s'"), name ? name : "(null)");
+}
+
+void xunsetenv(const char *name)
+{
+	if (!unsetenv(name))
+		die_errno(_("could not unsetenv '%s'"), name ? name : "(null)");
+}
+
 /*
  * Limit size of IO chunks, because huge chunks only cause pain.  OS X
  * 64-bit is buggy, returning EINVAL if len >= INT_MAX; and even in