patch 8.0.1832: cannot use :unlet for an environment variable

Problem:    Cannot use :unlet for an environment variable.
Solution:   Make it work.  Use unsetenv() if available. (Ken Takata,
            closes #2855)

runtime/doc/eval.txt

@@ -9939,6 +9939,14 @@ This does NOT work: >
 			variables are automatically deleted when the function
 			ends.
 
+:unl[et] ${env-name} ...			*:unlet-environment* *:unlet-$*
+			Remove environment variable {env-name}.
+			Can mix {name} and ${env-name} in one :unlet command.
+			No error message is given for a non-existing
+			variable, also without !.
+			If the system does not support deleting an environment
+			variable, it is made emtpy.
+
 :lockv[ar][!] [depth] {name} ...			*:lockvar* *:lockv*
 			Lock the internal variable {name}.  Locking means that
 			it can no longer be changed (until it is unlocked).

src/auto/configure

@@ -12609,7 +12609,7 @@ for ac_func in fchdir fchown fchmod fsync getcwd getpseudotty \
 	getpgid setpgid setsid sigaltstack sigstack sigset sigsetjmp sigaction \
 	sigprocmask sigvec strcasecmp strerror strftime stricmp strncasecmp \
 	strnicmp strpbrk strtol tgetent towlower towupper iswupper \
-	usleep utime utimes mblen ftruncate
+	usleep utime utimes mblen ftruncate unsetenv
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"

src/config.h.in

@@ -211,6 +211,7 @@
 #undef HAVE_TOWLOWER
 #undef HAVE_TOWUPPER
 #undef HAVE_ISWUPPER
+#undef HAVE_UNSETENV
 #undef HAVE_USLEEP
 #undef HAVE_UTIME
 #undef HAVE_BIND_TEXTDOMAIN_CODESET

src/configure.ac

@@ -3709,7 +3709,7 @@ AC_CHECK_FUNCS(fchdir fchown fchmod fsync getcwd getpseudotty \
 	getpgid setpgid setsid sigaltstack sigstack sigset sigsetjmp sigaction \
 	sigprocmask sigvec strcasecmp strerror strftime stricmp strncasecmp \
 	strnicmp strpbrk strtol tgetent towlower towupper iswupper \
-	usleep utime utimes mblen ftruncate)
+	usleep utime utimes mblen ftruncate unsetenv)
 AC_FUNC_FSEEKO
 
 dnl define _LARGE_FILES, _FILE_OFFSET_BITS and _LARGEFILE_SOURCE when

src/eval.c

@@ -2758,6 +2758,20 @@ ex_unletlock(
 
     do
     {
+	if (*arg == '$')
+	{
+	    char_u    *name = ++arg;
+
+	    if (get_env_len(&arg) == 0)
+	    {
+		EMSG2(_(e_invarg2), name - 1);
+		return;
+	    }
+	    vim_unsetenv(name);
+	    arg = skipwhite(arg);
+	    continue;
+	}
+
 	/* Parse the name and find the end. */
 	name_end = get_lval(arg, NULL, &lv, TRUE, eap->skip || error, 0,
 							     FNE_CHECK_START);

src/misc1.c

@@ -4479,6 +4479,17 @@ remove_tail(char_u *p, char_u *pend, char_u *name)
     return pend;
 }
 
+    void
+vim_unsetenv(char_u *var)
+{
+#ifdef HAVE_UNSETENV
+    unsetenv((char *)var);
+#else
+    mch_setenv((char *)var, "", 0);
+#endif
+}
+
+
 /*
  * Our portable version of setenv.
  */

src/proto/misc1.pro

@@ -60,6 +60,7 @@ void expand_env(char_u *src, char_u *dst, int dstlen);
 void expand_env_esc(char_u *srcp, char_u *dst, int dstlen, int esc, int one, char_u *startstr);
 char_u *vim_getenv(char_u *name, int *mustfree);
 void vim_setenv(char_u *name, char_u *val);
+void vim_unsetenv(char_u *name);
 char_u *get_env_name(expand_T *xp, int idx);
 char_u *get_users(expand_T *xp, int idx);
 int match_user(char_u *name);

src/testdir/test_unlet.vim

@@ -21,3 +21,27 @@ endfunc
 func Test_unlet_fails()
   call assert_fails('unlet v:["count"]', 'E46:')
 endfunc
+
+func Test_unlet_env()
+  let envcmd = has('win32') ? 'set' : 'env'
+
+  let $FOOBAR = 'test'
+  let found = 0
+  for kv in split(system(envcmd), "\r*\n")
+    if kv == 'FOOBAR=test'
+      let found = 1
+    endif
+  endfor
+  call assert_equal(1, found)
+
+  unlet $FOOBAR
+  let found = 0
+  for kv in split(system(envcmd), "\r*\n")
+    if kv == 'FOOBAR=test'
+      let found = 1
+    endif
+  endfor
+  call assert_equal(0, found)
+
+  unlet $MUST_NOT_BE_AN_ERROR
+endfunc

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1832,
 /**/
     1831,
 /**/