[PATCH v2 1/5] wrapper.c: add x{un,}setenv(), and use xsetenv() in environment.c

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add fatal wrappers for setenv() and unsetenv(). In d7ac12b25d3 (Add
set_git_dir() function, 2007-08-01) we started checking its return
value, and since 48988c4d0c3 (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.

We could make these return "void" (as far as I can tell there's no
other x*() wrappers that needed to make that decision before),
i.e. our "return 0" is only to indicate that we didn't error, which we
would have died on. Let's return "int" instead to be consistent with
the C library function signatures, including for any future code that
expects a pointer to a setenv()-like function.

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

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 environment.c     |  3 +--
 git-compat-util.h |  2 ++
 wrapper.c         | 18 ++++++++++++++++++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/environment.c b/environment.c
index d6b22ede7ea..7d8a949285c 100644
--- a/environment.c
+++ b/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);
 }
 
diff --git a/git-compat-util.h b/git-compat-util.h
index b46605300ab..0b0c0305165 100644
--- a/git-compat-util.h
+++ b/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);
+int xsetenv(const char *name, const char *value, int overwrite);
+int 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);
diff --git a/wrapper.c b/wrapper.c
index 7c6586af321..f784904fd67 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -145,6 +145,24 @@ void *xcalloc(size_t nmemb, size_t size)
 	return ret;
 }
 
+int xsetenv(const char *name, const char *value, int overwrite)
+{
+	if (!name)
+		die("xsetenv() got a NULL name, setenv() would return EINVAL");
+	if (setenv(name, value, overwrite))
+		die_errno("setenv(%s, '%s', %d) failed", name, value, overwrite);
+	return 0;
+}
+
+int xunsetenv(const char *name)
+{
+	if (!name)
+		die("xunsetenv() got a NULL name, xunsetenv() would return EINVAL");
+	if (!unsetenv(name))
+		die_errno("unsetenv(%s) failed", name);
+	return 0;
+}
+
 /*
  * 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
-- 
2.33.0.1092.g44c994ea1be